An autonomous power supply is not requiredif an extension cord of sufficient length isavailable. Likewise, a downhole tool doesnot need batteries when run on an electric-line cable, which provides, in addition tomechanical conveyance, a direct link to thesurface for power transmission and signaltelemetry. Wireline operations, however,require a relatively unobstructed vertical pathfrom surface to the measurement depth inopenhole or cased wellbore, but many wellstoday are inclined, high-angle, extended-reach or horizontal. Other wireline limita-tions involve gauges that need to be placedbelow valves in production tubulars or toolsthat must be rotated. The first downhole bat-teries were developed for pressure buildup,production logging, measurements-while-drilling (MWD) and logging-while-drilling(LWD) tools that have no way of receivingpower from the surface.

Until the early 1980s, most production log-ging tools recorded pressure, temperatureand flow rate solely with mechanicallydriven components. These devices contained

a stylus to scratch marks on a carbon-coatedmetallic cylinder that turned at a constantslow rate. Specially trained field personnelread the excursion marks with a microscopeand plotted digitized values versus elapsedtime on paper. Results were entered manu-ally into proprietary computing software toanalyze formation pressures, drive mecha-nisms and reservoir boundaries.

This mechanical tool, the J200, wasreplaced by a battery-powered version, theJ300, which was a predecessor of solid-statedownhole processor (SSDP) models. Witheach new-generation device, data stability,precision and accuracy as well as tool auton-omy were improved by energy-efficient elec-tronics and high-performance batteries.Through better information and improvedanalysis, formation evaluation and reservoircharacterization were also enhanced by theevolution of battery technology. To appreci-ate the role of batteries in oilfield services,the unique demands of subsurface environ-ments need to be defined.

The Evolution of Oilfield Batteries

Don HensleyMarvin MilewitsWenlin ZhangRosharon, Texas, USA

Well logging, measurements-while-drilling and seismic acquisition

require custom power sources. A dedicated development effort that

began in 1984 as a small project to meet short-term needs has grown

into a specialized design and manufacturing team that produces

primary and rechargeable batteries, and fuel cells for E&P services.

42 Oilfield Review

For help in preparation of this article, thanks to Bill Jones,Bic Nguyen, Chris Spring, Henry Stevenson and TonyVeneruso, Schlumberger Perforating & Testing Center,Rosharon, Texas, USA; and Ting Lau, Schlumberger Sugar Land Product Center, Sugar Land, Texas.ARC5 (Array Resistivity Compensated), IRIS (IntelligentRemote Implementation System), MSRT (MultiSensorRecorder/Transmitter), RAB (Resistivity-at-the-Bit) andUNIGAGE are marks of Schlumberger.

Autumn 1998 43

Temperatures—Logging tools are testedat the surface before a trip into the well,so batteries must function from arctic toextreme downhole conditions. Well tem-peratures routinely range from 70 to 200°F[20 to 100° C], but can get as hot as 400°F[200°C] or higher.

Limited space—Tool diameters have to besmaller than the borehole and lengths needto be minimized to facilitate handling.Batteries must fit into the available shapeand size of a tool compartment.

Shock and vibration—Tools and batteriesare subjected to the same conditions andmust meet the same standards. However,rugged cell designs also have to be cost-effective and disposable.

Power and safety—A battery is designedto provide only the power required to oper-ate a tool. Excess power increases the dan-ger and chance of a leak or failure. For thisreason, commercially available spiral-wrap

cells with liquid cathodes are not used bySchlumberger in oilfield services (see“Inside a Lithium Cell,” page 46).

Operating life—Battery life requirementsrange from several days to a year. Longerlasting power is achieved with larger cellsand energy efficient, or battery friendly, elec-tronics. Chemistry, cell composition, toolloads and temperature also affect battery life.

Shelf life—Global oil and gas operationsdictate that batteries often spend weeks intransit to job locations. Once on site, bat-teries may be stored for long periods untilwork begins, so shelf life must be longenough to provide reliable power undermost conditions and standby situations.

Production logging and formation evalua-tion tools must be accurate and reliable andbatteries are essential in meeting theserequirements. This article reviews the historyof battery technology, and discusses currentstate-of-the-art power source development atthe Schlumberger Perforating & Testing (SPT)Center in Rosharon, Texas, USA.

Basic Electrochemical CellsA review of conventional electrochemical,or galvanic, cells sets the stage for under-standing downhole batteries (see “BatteryTerminology,” page 45). The anode andcathode—the electrodes—of conventionalcells are composed of solid materials sur-rounded by liquid electrolyte. A porousseparator isolates the electrodes mechani-cally to prevent an internal short circuit, butallows ion flow, or diffusion. When a con-ductive path or electronic device is con-nected to the battery, electrons releasedfrom the anode in a continuous oxidationprocess flow through this external load, per-forming work by electric potential.

Inside the cell, in a process called oxida-tion-reduction, ions are released from boththe anode and cathode as charge neutral-ity is maintained (below). The cell pro-duces electric power as long as the anodesupplies electrons and the cathode acceptselectrons. Electron flow (current) stops oris limited if the anode or cathode are con-sumed, ions cannot reach the cathode, theexternal current path is interrupted, theanode contacts the cathode or the ion dif-fusion limit is reached.

Separator

Electrolyte

Load

Ano

de

e

Cat

hod

e-+

Electron flow

■■Basic electrochemical cells. A conventionalbattery is a can of chemicals that performswork by virtue of electrical potential differ-ence between the anode and cathode. Spe-cific electrochemical reactions produce elec-tron flow, or current, to the external load,providing power for tools or devices.

44 Oilfield Review

An Early Downhole Power SourceOptimal battery designs address electro-chemistry, tool power needs, packaging anddiverse considerations such as cost, shelflife, shipping regulations, remaining lifeand disposal. These factors were not alwaysconsidered prior to the 1980s when batter-ies were a last-minute tool addendum, usu-ally obtained from outside vendors.

In 1984, a Flopetrol-Johnston group inMelun, France, requested that engineeringcounterparts in the USA develop anothersource for the SSDP pressure-recording toolbattery. Prior to that time, there were threeSSDP batteries, each with differentchemistries and temperature ranges (aboveleft). Because each cell chemistry had lim-ited operating temperatures, field engineershad to plan each job carefully. One unusualprejob procedure required lithium copperoxyphosphate packs to be short circuitedintentionally to warm them up above 122°F[50°C]. Wireline operators then had to lowerthe tool into the hole quickly before the bat-teries cooled down. Operations in coldweather were problematic.

The original SSDP battery was a one-piecepack, consisting of a welded metal tube thatcontained the cells and had two solid endpieces for connectors (left). Individual electro-chemical cells, typically AA size, were held inplace by epoxy. These packs were difficult todispose of and expensive.

Batteries for reliable oilfield servicerequire cell components, processes andpackaging to be addressed in a systematicfashion. Beginning with basic electro-chemical technology, a second SSDP bat-tery with one chemistry for the entire rangeof operating temperatures was developed.To ensure safe and cost-effective batteryoperation, both packaging and cell electro-chemistry were reevaluated.

A Specialized Downhole Power Source In the search for a new battery supply, elec-trochemistry was scrutinized first. Downholetools are relatively small, but increasingly need more operating power. High-voltagechemistries require fewer cells to meet these power requirements, fit the space avail-able inside tools more easily and reduce cost(left). Lithium thionyl chloride [LiSOCl2]appeared to be best for oilfield service in partbecause only one battery type was needed formost temperatures, simplifying field inventory.

■■Early SSDP Battery Chemistries.

Chemistry Temperature operating range

Alkaline –22 to 176° F [–30 to 80° C]Lithium copper oxide –22 to 257° F [–30 to 125° C]Lithium copper oxyphosphate 122 to 347° F [50 to 175° C]

■■The early SSDP battery pack. In this one-piece, all-metal, welded housing, individualcells were locked in place with epoxy resin. This design was costly and made the packsa disposal problem.

0.6

0.8

1.0

1.2

2.0

2.6

2.8

3.0

3.2

3.4

3.6

[LiSOCI2]

[LiSO2]

[LiFeS2] [ZnAg2O]

[MgMnO2]

10 20 30 40 50 60 70 80 90 100

1.4

1.6

1.8

[LiMnO2]

[CdHgO][Zn-carbon]

[Zn-air][Alkaline-MnO2]

[ZnHg0]

Discharged capacity, %

Cel

l vol

tag

e, v

olts

(V

)

■■Voltage discharge for primary battery chemistries. High-voltage lithium chemistriesincluding lithium thionyl chloride [LiSOCl2], lithium sulfur dioxide [LiSO2] and lithiummanganese dioxide [LiMnO2] were evaluated. Other electrochemistries include magnesium manganese dioxide [MgMnO2], lithium iron disulfide [LiFeS2], zinc mercury oxide [ZnAg2O], zinc silver oxide [ZnHgO], zinc air [Zn-air], alkaline man-ganese dioxide [alkaline-MnO2], heavy-duty zinc carbon [Zn-carbon] and mercad, or cadmium silver oxide [CdHgO].

1. Gabano JP (ed): Lithium Batteries. London, England:Academic Press, 1983.

Autumn 1998 45

Lithium thionyl chloride chemistry was dis-covered almost by accident. In 1969, Jean-Paul Gabano, a French chemist, wasdeveloping a rechargeable lithium batterycomprised of a lithium cell containingthionyl chloride electrolyte with dissolvedchlorine acting as the cathode. The celldemonstrated high-current rechargeable per-formance and, surprisingly, continued toproduce current even after the chlorinedepleted. An evaluation determined thatthionyl chloride could also serve as the cath-ode and the LiSOCl2 battery was born.1

Theoretically, lithium thionyl chloride bat-teries should not exist. Normally, when ananode touches a cathode, oxidation andreduction begin immediately and continue inan abrupt short-circuit reaction. During cellmanufacture, however, when liquid thionylchloride is poured into the lithium metal cell,there are no violent reactions because of theinstantaneous formation of a LiCl layer on thefreshly wetted lithium metal surface. This pas-sivation layer seals the surface from furtherdirect contact with the liquid cathode andprevents dangerous reactions.

This chemistry, with one of the highestenergy densities of practical cells, has somelimitations and intrinsic safety issues.Lithium metal mixed with water isflammable and explosive, the liquid elec-trolyte is corrosive and toxic, and high-rateelectrode structures are susceptible toexplosions when shorted. Low-rate elec-trodes have the same hazards at high tem-peratures. Explosions can also result fromforced over-discharge and cell charging.This chemistry also exhibits a voltage delay,or passivation, with load onset, which is afunction of storage time and temperature.As a result, lithium batteries must be pre-discharged—depassivated—before use.

Lithium thionyl chloride chemistry was notwidely used at first and had been availablecommercially for fewer than 10 years whenthis chemical reevaluation began. Therewere initial concerns and problems relatedto premature market introduction, includingleaks and failures in airline emergency light-ing, an early use of these batteries. Inresponse, strict shipping regulations wereput into effect. Batteries with liquid cathodesand more than 0.5, but less than 12, gramsof lithium had to pass specific tests and beplaced in special containers before beingshipped on cargo aircraft. By 1984, five ven-dors were making this type of cell and mostof the associated problems were understoodand had been addressed.

Ampere (A): A coulomb per second, the basic international standard unit of electric current.

Anode: The electrode where oxidation, or lossof electrons, occurs in electrochemical cells.

Aqueous: Substances that are like water or water based.

Battery pack: A single galvanic cell or group of cells connected through series or parallel circuits and housed in a modular enclosure to provide power to electronic devices.

Cathode: The electrode where reduction, orelectron gain, occurs in electrochemical cells.

Cell charging: Electric current flowing into acell from an external source or other cells.

Conductivity: The material property of con-ducting or transmitting electric current.

Coulomb: The quantity of electricity trans-ported in one second by a one-ampere current, the basic international standard unit of electric charge.

Current: The transfer, or flow, rate of electricenergy, or electricity, measured in amperes(A), or one coulomb per second.

Deep discharging: Depletion of a batterybelow the normal end-of-life voltage, but still above zero volts (V).

Electrode: An electronic conductor that actsas an electron source or sink, usually madeof metal and immersed in electrolyte solution.

Electrolyte: An ionic solution capable of conducting electric current.

Energy density: Electrical energy of a unit mass or volume expressed in W-hr/kg or W-hr/L.

Galvanic cell: An arrangement consisting oftwo electrodes and an electrolyte that pro-duces electric current from a spontaneouschemical reaction when the electrodes areconnected externally.

Gravimetric energy density: Electric energyin a unit mass expressed in W-hr/kg.

Hermetic seal: A gas-tight and nonconductiveexternal barrier that allows an electrical con-nection with an internal cell electrode.

Ion: A positive or negative charged atomthrough either gaining or losing electrons.

Ion diffusion rate: A measure of the movementor travel of ions from the anode to the cathode.

Overdischarging: Depletion of a battery below zero volts (V) and into voltage reversal.

Oxidation: The process of electrons beingremoved at the anode in the electrochemicalreaction of a galvanic cell.

Parallel circuit: An electrical path formed byconnecting positive terminals through oneconductor and negative terminals throughanother conductor, so current can flow fromeach cell to the external load.

Passivation: The formation of solid productson electrode surfaces.

Reduction: The process of electrons beingdonated at the cathode in the electrochemicalreaction of a galvanic cell.

Series circuit: An electrical path formed byconnecting battery cell positive terminals tonegative terminals in a sequence, so currentcan flow through each cell in succession tothe external load.

Short circuit: A very conductive path placedacross the terminals of a battery (externalshort circuit) or contact between anode andcathode (internal short circuit) that generateslarge currents and damages subsequent cell performance.

Volt (V): The difference in electric potentialthat makes a 1-amp current flow through a1-ohm resistance, the basic internationalstandard unit of electromotive force.

Voltage delay: The immediate drop in voltagebelow normal operating values caused by pas-sivation. Voltage recovery may occur slowly ornot at all, depending on passivation severity.

Volumetric energy density: Electric energy in a unit volume expressed in W-hr/L.

Watt (W): A 1-ampere current under 1-volt of electric potential; 1 joule per second or 1/746 horsepower; the basic international unit of electric power.

Battery Terminology1

1. Hibbert DB and James AM: Dictionary ofElectrochemistry, Second Edition. New York, New York, USA: Wiley-Interscience, 1984.

46 Oilfield Review

The next concern in developing a new bat-tery source was packaging. The previousmethod of enclosing cells in a metallic tubewas costly and did not enhance safety.Instead, a new design concept separated thebattery into two parts (right). Rather than dis-pose of the entire depleted battery pack, onlythe section containing cells is removed. Theremaining housing, consisting of the metaltube and end pieces, is reused. Because cellscan leak, the reusable battery package is gas-tight to protect tools against corrosive leak-age from the encapsulated cells. Pressurerelief valves are built into the end-caps toallow controlled venting.

The result of this new chemistry and pack-aging was a 50% reduction in SSDP batterycost. This backup source soon became theprimary source and the first stage of new-generation battery developments was com-plete. Currently, the hazards and limitationsof lithium-based batteries cannot be elimi-nated completely, but must be understoodand addressed by battery developers, tooldesigners and end-users. Schlumberger con-siders battery safety issues to be of paramountimportance and addresses them in detail dur-ing the evaluation and design process.

Inside a Lithium Cell There are four major components of primary,or nonrechargable, lithium thionyl chloridecells—anode, cathode, electrolyte salt andseparator (right). The negative terminal islithium metal foil or lithium alloy, whichserves as the anode. In this chemistry, thecathode and electrolyte are the same and arecalled “catholyte.” The liquid thionyl chlo-ride [SOCl2] cathode reaction takes place ona high-surface-area carbon electrode, whichserves as the positive terminal. A commonelectrolyte salt, lithium aluminum chlorate[LiAlCl4], is dissolved in the thionyl chlorideto increase ion conductivity and improvecurrent rates. A layer of nonwoven fiberglassmaterial physically and electrically isolatesthe lithium anode from the positive terminal.

■■A new pack design. A novel approach replaced expensive one-piece, metal batteries bybuilding packs in sections. A removable cell holder is disposable, but the housing can bereused. The end-caps have integral valves to relieve internal pressure in the event of anaccidental cell leak.

Glass-to-metalseal (GMS)

Liquid cathode

Liquid cathode

Anode

Anode

Carbon electrode

Carbon electrode

Separator

Separator

+ -

4LiCl + S + SO2

Overall reaction: 4Li + 2SOCI2(solid) (liquid) (solid) (solid) (gas)

Can

Can

Anode reaction:4Li 4Li++4e-

Cathode reaction:2SOCI2+4e- S+SO2+4CI-

LoadElectron flow

■■Idealized lithium cell. Major components of a non-rechargable, or primary, lithium thionyl chloride bat-tery are the anode, cathode, electrolytic salt and aseparator. For this chemistry, the cathode and elec-trolyte are the same. Electrons from the lithium anodeflow toward the positive terminal by way of the exter-nal circuit, or load. Lithium ions diffuse through thecatholyte—combined cathode and electrolyte—toward the positive terminal. Thionyl chloride at thepositive terminal meets returning electrons, which arereduced to form chloride ions, sulfur and sulfur diox-ide. Chloride ions combine with lithium ions to formlithium chloride, a solid that is deposited on the car-bon electrode reaction site.

Autumn 1998 47

Cell chemistry—Lithium, with optimalelectrochemical potential (3-volt) and elec-trochemical equivalence (3.86-A-hr/g), isthe best choice for downhole batteryanodes. Combined with nonaqueousthionyl chloride, it has one of the highestvoltages and energy densities of practicalbattery systems. During discharge, lithiumoxidation occurs at the negative anode ter-minal and thionyl chloride reduction takesplace at the positive carbon electrode.

Thionyl chloride serves as both electrolyteand cathode reactant. The carbon electrodeis not part of the cell reaction, but providesactive sites where reactions take place. Thereare six modes of capacity loss and currentstoppage. If the anode is consumed, there isno electron source and current ceases. If thecatholyte is consumed, there are no reac-tants to receive electrons. If a cell leaks orhas no active reaction sites, current stops. Ifthe load, or external circuit, is disconnected,electrons cannot move to the positive termi-nal. If the anode and carbon electrode touch,current will not flow. The ion diffusion limitis reached when loads are too high for theavailable electrode surface area and cellscannot support external current due toexcessive ion flow that results in depressedvoltage or current.

Premature depletion of most primarylithium batteries with liquid cathodes occurswhen carbon electrode sites becomeblocked by lithium chloride or other soliddischarge products, even if lithium andthionyl chloride remain. This is called cath-ode passivation. For safety, lithium cells haveexcess thionyl chloride with respect to theanode material. This stops dendritic growth,microfingers of lithium metal, on wet-dryelectrode interfaces, which can cause aninternal short circuit. As a result, at the endof battery life there is thionyl chlorideremaining and minimum wet-dry interfaces.

Cell structure—In a typical cylindrical celldesign, the solid components form concen-tric shells (above right). For an anode,lithium foil is swaged against the inner wallof a stainless steel tube, making the con-tainer a negative terminal. Nonwoven glasspaper is placed against the inside diameter(ID) of the lithium foil as a separator and ahighly porous carbon plug is placed insidethe separator shell. In the center of this car-bon plug, a nickel or stainless steel screenserves as current collector with a connectionto the top-mounted, glass-to-metal seal (GMS)for outside termination, which is the positivecell terminal. Liquid thionyl chloride withdissolved electrolyte salt fills most cell voids.

The bobbin design with limited anode andcathode surface area for a given cell size issuitable for low discharge rates. The chal-lenge is to design electrodes with suffi-ciently high surface area to satisfy tool loadrequirements without decreasing safety.This can be achieved with appropriatesafety features using the more popular spi-ral-wrap cell (below).2

Cells are designed to have intrinsic safety atambient conditions for battery assemblersand end users in the field. Failures occuronly if there is a short circuit at hot down-hole temperatures. In addition to designingsafe, high-performance cells, the issue ofpreventing anodes and cathodes from touch-ing during high-shock and vibration applica-tions like MWD and LWD was addressed bySPT. Proprietary designs for cylindrical andannular cells were implemented.

Hermeticallywelded seam

Cover

Glass-to-metalseal (GMS)

Separator

Anode

Insulating sleeve

Spot-weldednegative terminal

Can

Positive terminal

Top insulator

Currentcollector

Cathode

Bottom insulator

■■Limited anode-cathode surface. Bobbin-type cylindrical cells are designed for low powerand moderate to low current discharge rates. The solid internal components form concentricshells. The anode is lithium foil and nonwoven glass paper is used as a separator. A porouscarbon plug forms the interior ring. This plug has a steel screen in the center to collect cur-rent and is connected to the positive terminal. Liquid thionyl chloride with dissolved elec-trolytic salt fills the void spaces. [Adapted from Linden, reference 2.]

■■High anode-cathode surface area. Tightly-wrapped, spiral cylindrical cells are commer-cially available for high power and high cur-rent discharge rates. [Adapted from Linden,reference 2.]

2. Linden D (ed): Handbook of Batteries. New York,New York, USA: McGraw-Hill, Inc., 1995.

48 Oilfield Review

Lithium Battery PerformanceAnode passivation, self-discharge, carbonpore blocking (cathode passivation) and fail-ure mechanisms influence battery perfor-mance and longevity. Designers mustaddress these factors during development ofdownhole power sources.

Anode passivation—When a lithium anodecomes in contact with thionyl chloride elec-trolyte, while filling a cell for example, a solidelectrolyte interface (SEI), or passivation layer,is generated immediately on the lithium sur-face. This thin layer protects the anode surfacefrom further chemical reaction. However, forelevated temperature and long storage peri-ods, continuous passivation layer growthresults in capacity loss. Lithium thionyl chlo-ride batteries would not exist without a SEIlayer. The cell reaction would proceed uncon-trollably upon initial catholyte filling.

Because of SEI microdefects, the chemicalreaction proceeds at a low rate, but neverstops, resulting in slow layer growth. Lesspassivation occurs at low temperatures. Thereaction is faster at high temperatures. Forprolonged storage under no load, especiallyat high temperature, the SEI layer gets quitethick and causes observed voltage delayswhen loads are placed on a battery. Voltagedrops below normal operating values ini-tially, but recovers. For severe passivation,voltage drops farther and takes longer torecover, if at all. The voltage delay is worsefor batteries stored at high temperatures withno load for long periods and then dischargedin cold conditions.

To reduce and prevent voltage delay,lithium batteries should be stored in con-trolled environments at about 70°F [25°C].Batteries need to be depassivated before useby applying a temporary load, which ishigher than the required tool current, untilvoltage recovers. If passivation is heavy, it isbest to discharge the pack with a low currentfor an extended time before applying highercurrents to address the difference betweenthin and thick passivation layers. At SPT, aspecial depassivation box was designed toprovide a timed constant current, which turnsoff when the proper voltage is met or the bat-tery fails to depassivate in a given time.

Self-discharge—Anode passivation reducesa self-discharge condition. If a lithium anodeis fully passivated, the rate of self-discharge isdecreased dramatically. Self-discharge alsotakes place during normal battery depletion.When a cell is activated, the SEI is disruptedcontinuously by ion flow from the anode sur-face beneath, exposing more lithium tothionyl chloride. This process occurs in par-allel with the normal electrochemical reac-tion, resulting in two discharge reactions: an

Short circuitsCauses:

External—conductivity across terminals is toohigh; can occur accidentally during pack fabrica-tion or battery operation.Internal—accidental anode and cathode contact;can occur during high shock and vibration.

Mechanisms: Heat and gas are produced in an acceleratedchemical reaction. Solid discharge products formon the carbon electrode, block surface reactionsites and gradually decrease current output. Anexplosion may occur if cell temperature risesabove 80°C.

Cures:Design low-rate cells with optimum electrodesurface area.Confirm safety at ambient and higher tempera-tures with low resistance, short circuit andmechanical tests.Use correct safety fuse at pack level.

Temperature buildupCauses:

Environmental—increase in external temperature.Internal—discharge current is too high and poorheat dissipation.

Mechanisms: At temperatures above 356°F [180°C], a lithiumanode melts. A short circuit can develop dependingon cell orientation and mechanical shock environ-ment. At temperatures below 180°C, hydrostaticbursting and leaking welds occur due to thionylchloride’s [SOCl2] high thermal expansion coeffi-cient. If external temperature is low, because it is anexothermic process, high battery temperature canresult from a discharge current that is too high.

Cures:Use lithium alloy anode to increase meltingtemperature.Leave sufficient void volume when filling cellwith SOCl2.Assure that battery is right size for tool load.Design pack to have good thermal conductivity.

Cell chargingCauses:

Battery with single string of cells has an externalcharging current.A charging current flows into a battery when thepositive and negative terminals of an externalpower source are connected to the correspondingbattery terminals.A battery with multiple strings has a chargingcondition when one or more strings with a highervoltage imparts a charging current into a stringwith lower voltage.

Mechanisms: Charging current forces lithium ions to flow back-wards towards anode.Ions plate back onto lithium anode to form finedendrites, or metal “fingers.”High surface area lithium dendrites are unstable in SOCl2.

Cures:Use a series diode in each battery string.If required, use series diode even in a single-battery string to avoid inadvertent charging byan external power source.

Forced overdischargeCauses:

Occurs when using two or more cells in a pack.Less likely to occur if cells have uniform capacity.Environmental factors like shocks may cause cellsto deplete differentially.Most likely to occur near the end of battery life.May take a long time, since driving currents aresmall.Usually does not occur during short periods oftool operation because cells are relatively fresh.

Mechanisms: An active cell depletes available lithium and turnsfrom power source to a sink.When an external current flows through the cell,the terminals reverse polarity.Lithium ions flow back toward anode and form dendrites.Dendrites tend to form more along wet-dryelectrolyte interfaces.Surface dendrites form slowly with low driving currents.

Cures:Do not deplete batteries to 0 volts.Design tool to stop high load at a 2-volt per cellpack equivalent. For example, 4 volt end-of-life fora two cell-pack.Treat packs with an open circuit voltage (OCV) of less than 3.6-volt per cell pack equivalencewith caution.

Cell leaksCauses:

Each cell is a miniature pressure vessel with fourweld rings on cylindrical top and bottom lids,which can become problem areas.High internal pressure, even on good welds, pro-duces leaks.Nominal internal pressure on defective welds canproduce leaks.

Mechanisms: Escaping SOCl2 liquid combines with moisture toform hydrochloric acid and sulfur dioxide gas.Hydrochloric acid corrodes container exterior,forming pits and further damaging the tool hous-ing and electronics, and the battery. Batteries mayalso build up internal pressure during thermalcycling.

Cures:Fabricate welds with good penetration into the can material.Do not overfill cells with thionyl chloride.Operate batteries within their temperature rating.Do not subject batteries to repeated thermal cycles.

Troubleshooting Batteries

Autumn 1998 49

external current and a parasitic internal cur-rent. Capacity losses can be appreciable. Athigh temperatures for long periods, self-dis-charge has a more severe effect. The effect isdecreased without sacrificing power outputby minimizing the lithium surface area that isopen to electrolyte fluid.

Carbon pore blocking—During lithium bat-tery discharge, solids form inside and outsideof the carbon electrode. Solids precipitate incarbon electrode pores and block access tounused reaction sites, resulting in less capac-ity and lower voltage. This carbon passivationoccurs late in battery life. Unlike anode pas-sivation, when solid products form on theanode surface, it is not reversible. Carbonplugging is more severe for high dischargerates at low to moderate temperatures.

During discharge, chloride ions, a cath-ode reaction product, form inside the car-bon electrode. Lithium ions, an anodereaction product, dissolve from the lithiumanode, diffuse toward the carbon cathodeand combine with chloride ions to formsolid lithium chlorine [LiCl]. At low dis-charge rates, LiCl is uniformly distributed inthe carbon electrode. At higher dischargerates, Li ions cannot diffuse deep into car-bon cathodes because of ion-diffusion ratelimits. Precipitation of LiCl occurs primarilyon the outer carbon-electrode surface,which plugs the pores and results in ineffi-cient use of interior reaction sites. Highertemperatures and lower currents increaseion diffusivity and promote recovery ofsome carbon volume. Cathode blocking ismore persistent and much harder to removethan anode passivation.

Failure mechanisms—Lithium thionyl chlo-ride batteries are ideal for oilfield service;however, caution must be exercised whenhandling or using this chemistry due to highenergy content. Failure modes, which nor-mally cause hot cells or annoying leaks in lesspotent batteries may create venting or run-away reactions. The reasons for this differenceare inherent high energy density and her-metic construction of lithium cells. If a shortcircuit occurs, it lasts longer, gets hotter andbuilds up higher internal pressure, creating agreater hazard than lower energy systems likenonhermetic, alkaline cells. There are fivefailure modes, which can cause cell or tooldamage from resulting leaks (see“Troubleshooting Batteries,” previous page).

Custom Cell DevelopmentAfter a second SSDP battery was successfullyintroduced in 1985, the next pack to bedeveloped was for the MSRT MultiSensorRecorder/Transmitter tool. The MSRT pack fitsin the tool annular battery compartment andhouses AA-size cells in a carousel configura-tion (right). The pack is an inexpensive com-bination of phenolic tubes and end-caps withcells connected in series and parallel.

The next achievement in battery technol-ogy came in 1987 with development of thefirst LWD tool. Until this time, Schlumbergerbuilt the packaging, but used commercialcell vendors. The problems with thisapproach were confidentiality, limited con-trol, delivery uncertainty and lack of differ-entiation from competitors. The key toadvancing to the next stage of battery devel-opment was control of cell manufacturing.

■■The MSRT MultiSensor Recorder/Trans-mitter tool. Built using inexpensive phenolictubes and end-caps, the MSRT annular bat-tery pack holds AA-size, series and parallelconnected cells in a carousel configuration.

50 Oilfield Review

Packaging was important, but rugged cellswith custom sizes and shapes are essential tobattery success.

A vendor was secured to develop and pro-duce standard and custom lithium cellsquickly, economically and with utmost con-fidentiality. This move allowed developmentof a special lithium cell for LWD tools.Because LWD tools are part of the drillstring,the tool and battery pack must endure hun-dreds of hours of drilling without failures. Inaddition, since batteries are a throw-awayitem, the LWD battery not only had to usecost-effective packaging, but also had to relyon intrinsically rugged cells to help ensurean overall inexpensive pack (left). Afterdeveloping the first custom cylindrical cells,battery evolution at SPT accelerated. A rangeof cylindrical cells of varying diameters,lengths and electrode structures was devel-oped. The next customized battery develop-ment was the unique annular cell.

Downhole tools used inside or in conjunc-tion with oilfield tubulars are cylindrical orannular. Depending on the tool, batterycompartments are also annular, so the bat-tery and enclosed cells can be annular aswell (below). The advantages of annular bat-teries are fewer cells, higher capacity, thin-ner cross sections, lower usage cost andimproved reliability. A large annular cellreplacing an ensemble of small cells in acarousel array requires fewer connections.

By using annular shapes, it is easier to max-imize cell life and minimize the tool volumethat batteries take up. The annular battery alsohas a relatively large ID, which minimizesmud or hydrocarbon flowing pressure drop.

These benefits were verified in two diversebattery types previously composed of cylin-drical cells—the Schlumberger Wireline &Testing IRIS Intelligent Remote Implementa-tion System tool and Anadrill LWD batteries.After annular batteries for these tools wereintroduced in 1992, other custom cells fol-lowed quickly. The ARC5 Array ResistivityCompensated and RAB Resistivity-at-the-Bittools have the same annular design, but dif-fer in ID, OD, length and electrode structure.

Fuse

Bore

Bore

Top

Bottom

Metallic tube

Section A-A

A A

Cells

Cell

Stiffeningelements

Each cell containsproprietary structuresfor extra ruggedness.

■■Batteries for drilling. Because MWD and LWD tools are subjected to extended shocksand vibration, power sources for these applications rely on rugged cell and battery packaging to meet performance and reliability requirements.

Annular Pack Designs

Cylindrical cells Annular cells

■■Annular cell designs. For annular shapes it is easier to maximize battery life and minimize required volume compared to carousel-style cylindrical cells. Annular cellpacks also have fewer electrical connections and are more robust.

Autumn 1998 51

Design Considerations Past unrealistic tool designer and field end-user expectations highlight confusion andmisunderstandings that exist about batteries.Cell specifications have included a widerange of requirements in unique and some-times conflicting combinations—small,lightweight, waterproof, long-lasting, neverdeplete prematurely, work at any temperaturewith all tools, easy to install and remove,reusable and rechargeable in or out of thetool for an unlimited number of times, mea-sure remaining power capacity, long shelf lifeand no shipping or disposal restrictions.

Tool requirements are dictated by endusers in the field. There are, however, manyspecifications that should be defined by tooldesigners and battery-specific considera-tions that should be addressed early in thedevelopment process. Ideally, these issuesshould be dealt with before tool electronicsare planned and designed in order todecrease battery costs and make tools more“battery friendly.”

Costs can be reduced by minimizing toolpower and voltage requirements. Lower volt-age means fewer cells and less expensivepacks. By making packs with larger annularcells, fewer cells are needed to increasecapacity, more power and longer life areavailable per cell and assembly costs areless. Future rechargeable cell designs couldalso reduce cost.

Batteries can be sized for single jobs, orpacks with larger custom cells can be usedmultiple times. Tools should be able tofunction through the end of battery life andthen shut down safely. Innovative manage-ment and depletion techniques will helpbetter utilize battery capacity and allowmaximum power to be squeezed out ofeach pack. Remaining battery life andavailable power can be measured bydevices in the pack or the tool can trackcumulative hours of operation.

Tools can be made battery friendly by usinga constant or slowly varying load, notexceeding battery power limits and avoidingno-load conditions under high-temperatureoperation. Battery designs should considerthe electrical load hierarchy (easy to hard) forbatteries—constant resistor, constant currentand power, and short “on” (low-duty) or long“on” (high-duty) cycles.

Tools need to be fault-tolerant (immune toshort periods of low voltage), function at lowvoltage and have a low-power delay for sur-face check-out or long service. Currentrequirements should be reduced as voltagegoes down for benign end-of-life loads andcumulative amp-hours as well as tempera-ture should be recorded to efficiently man-age tool power and battery life.

To make batteries easy and safe to use,there should be external voltage access anda disconnect mechanism when battery packsare installed. Concentric connector termi-nals should be used if possible, and batteriesshould not be a force- or load-bearing toolcomponent. A pressure seal between the bat-tery compartment and tool electronics isdesirable, and an external leak indicator isalso needed.

In addition to SPT Power Source ProductsGroup experience and technical expertise,other less visible factors like a systems designapproach positively influence battery tech-nology implementation. Consolidating andprioritizing user expectations are done earlyin battery development, ideally before toolelectronics are designed.

The Design ProcessWhen a battery is requested, many parame-ters need to be specified by both battery andtool developers before cell design and devel-opment are initiated (below). The goal is toestablish mutually agreed, achievable objec-tives based on realistic input and constraints,so user expectations can be met. Using bat-teries or cells developed and tested previouslyis also an option. Cells from a proven batterycan be adapted to meet new requirements—more or less cells or different packaging.

Battery specifications need to detail deliv-erables, schedules and testing. To initiate thedesign process, a project resumé stating therequirements that batteries must fulfill is pre-pared (see “LWD Battery Project Resumé,next page). This record, which also helpsdesigners optimize tool electronics to usebatteries efficiently, includes operating tem-peratures, tool-specific power consumption,pack and minimum operating voltage, shelfand operating life at various temperatures,physical dimensions, mechanical shock,vibration and safety testing, and transporta-tion certification. The project resumé, ini-tially written by tool designers for batterydevelopers and subsequently modified byboth parties, documents the design process.

After completion of a project resumé, engi-neers design and test prototype batteries. Thenext step is pilot testing. Batteries are built andrun on actual jobs in the field under the super-vision of tool designers. After a predeterminednumber of successful jobs, the batteries andtools go into the commercialization phase.

■■Battery Design Input: A System-Level Approach.

Input Responsibility

Specifications Battery Tool End userdeveloper designer

Chemistry XBattery shape XCell design XTool load XBattery-tool interface X XUse environment X XSafety requirement X X XBattery cost X XShipping and disposal X X

52 Oilfield Review

Qualification Testing The battery qualification program was opti-mized after years of testing and analysis.Upon completion of safety, electrical andmechanical qualification testing at SPT, bat-teries are manufactured for field tests. Tool,field and SPT battery engineers work closelyduring testing. Any issues that arise from fieldtests are addressed quickly to obtain a qualityproduct that meets or exceeds expectations.Staffed with a research team, electrical andmechanical design engineers, and techni-cians, SPT fully supports battery sustainingissues anywhere in the world.

A blast-resistant building was built at SPTspecifically to perform qualification tests andtroubleshooting. The Battery Electrical andShock Testing (BEST) facility has 10 explo-sion-resistant bays with ovens, speciallydesigned shock machines, and a controlroom in which tests in each bay are moni-tored by computers (next page, left). Testingcapability of the BEST facility is beingexpanded due to the increased number oftools and new business opportunities requir-ing special batteries. Battery manufacturingand engineering at SPT achieved ISO 9001certification in August 1996.

Tests are sometimes conducted to deter-mine failure modes and severity, and toensure that batteries are not operated nearfailure. Cells in various stages of depletionare placed in a pressure vessel and subjectedto 15,000 psi [103 MPa] at temperatures upto 302°F [150°C] to simulate flooding of abattery housing. Battery packs are alsoplaced in a tool and heated until theyexplode to verify that tools can be recoveredin the event of a downhole failure. Cells withvarious types of lithium chemistry are testedto determine the temperatures that can betolerated without affecting battery perfor-mance or compromising safety. Cells haveeven been crushed hydraulically to simulatebeing dropped or physically damaged.

A short circuit can occur accidentally dur-ing manufacturing and assembly or in severefield conditions. Safety is of utmost impor-tance, so tests are performed to determine ifthe cells can withstand a direct short withoutleaking or venting. New batteries undergoshort-circuit testing at room temperature and122°F [50°C] to confirm that cells are safe atall ambient temperature conditions.

To assess battery autonomy, electrical testsare performed at the cell and pack levels.Initial testing is performed on the cells with astatic resistive or constant current load todetermine cell performance at the maximumrated temperature. Results are analyzed and, ifnecessary, the cell is redesigned. Next, a sim-

ObjectiveDevelop custom lithium battery to power LWD tool. Tool will be subjected to unusual shock and vibrationstimuli at temperatures exceeding 150° C.

DescriptionThe pack will fit inside the LWD tool battery compartment (see dimensions below) and provide N wattsof tool power for Y days at 175° C.

Technical and performance advantagesThis battery must provide uninterrupted power for continuous tool operation during drilling. Previoustools suffered intermittent power outages due to mechanically induced battery short circuits. High powercapacity is required to ensure tool operation during the total life of a drill bit.

General specificationsDescription Target Current VerificationDimensions

Length, in. 24 28 C, LOD, in. 2.5 2.8 C, LConnector termination TBD – L

PowerOpen circuit voltage (OCV) 29 Same LMinimum voltage 20 Same LNominal current, amps 0.3 TBD L, F

Independent operation, days 14 TBD L, FTemperature range 0 to 175° C TBD L, FMechanical shock TBD TBD L, FPrice, $USD <500 TBD ELegend

C: Design calculations E: Experience from previous study F: Field testingL: Laboratory testing TBD: To be determined

Feasibility status and development plansIn this section, battery users and designers list concerns, specific technical challenges and issues thataffect timely development of the proposed battery. Previous work that may serve as a guide is typicallyincluded in this section.

As projects progress, this section is used to document changes to original specifications or schedules.Updates are performed at specific milestones during product development reviews as indicated in theschedule. Changes and input are made as needed and communicated to users or battery designers whenthey occur.

SchedulePlan Actual

Project launch First quarter 1998 First quarter 1998Feasibility review Second quarter 1998 Second quarter 1998Field prototype available Third quarter 1998 Fourth quarter 1998Field test conclusion First quarter 1999 TBDCommercialization Second quarter 1999 TBD

Project teamThese groups represent the core team that is responsible for the development of specific batteries.Typically, each battery developer has three to five projects, as well as field support responsibility for batteries developed previously.

Battery developer Tool designerAssigned engineer Project leaderMechanical engineer Mechanical engineerElectrical engineer Electrical engineerMechanical or electrical technicianManufacturing engineerBuyer

LWD Battery Project Resumé

Autumn 1998 53

ulated pack consisting of the proper numberof cells is tested at the maximum rated tem-perature under simulated tool load. Onceautonomy as defined by the project resumé isconfirmed, mechanical qualification begins.

One of the toughest environments for abattery in the oil field is MWD service, whichoften positions the batteries directly abovethe drill bit. The MWD batteries undergo rig-orous mechanical and electrical qualifica-tion tests to ensure that they can withstanddrilling shocks and vibrations while supply-ing continuous power to operate the tool.

A battery pack is manufactured formechanical testing. New batteries are placedin mock-up housings and mandrels with thesame electrical connections as actual tools.By using a custom shock machine and spec-ifications similar to those used for qualifyingMWD and LWD tools, every lithium batterydesign is shock and vibration tested prior tofield testing. During testing, battery voltage ismonitored to check for packaging damage,cell leaks and electrical integrity (above).

Once shock testing is complete, the batteryis placed in a test bay and cycled from max-imum rated temperature to room tempera-ture while being depleted. Thermal cycling isrepeated until the end-of-life voltage isreached. These tests are needed because bat-teries are used multiple times during theirservice life. The battery is checked for physi-cal and electrical damage to determine ifthere are problems with swelling, internalwiring or cell venting. The battery is thendepleted and thermal cycled again to deter-mine deep discharge effects.

Battery packs manufactured at SPT undergoqualification tests to meet US and interna-tional certification for transportation oflithium batteries in cargo aircraft. In 1997, the

■■Testing. The Schlumberger Battery Electrical and Shock Testing (BEST)facility (top) in Rosharon, Texas, is equipped with 10 blast-resistant testbays with ovens (middle), a specialized shock and vibration apparatus,and control room with computers to monitor each bay (bottom). Thisfacility is being used for both internal Schlumberger battery qualifica-tions and outside testing services.

■■Battery shock and vibration testing.

54 Oilfield Review

US Department of Transportation (DOT)revised testing requirements to incorporatemore stringent testing than was previouslydone for shipping regulation compliance. Sixseries of tests are now required for lithiumbatteries—altitude simulation, extreme tem-perature and short circuit; vibration, shockand short circuit; vibration, shock andcharge; internal short circuit; vibration, shockand low-capacity cell; and forced discharge.

In some instances, vibration testing isrequired to make sure that the resonant fre-quency of a battery pack is not reached,which could result in failure during trans-portation. A cumulative database that iskept with each Project Resumé includesperformance and DOT test results, designdrawings and communications, key docu-ments, and e-mails or messages that affectproject milestones.

Oilfield Applications Dozens of battery types are manufactured by SPT for three service sectors—wirelinelogging and well testing, measurements-while-drilling and seismic surveys. Eachbattery has different requirements and spe-cifications (below).

Logging, monitoring and testing wells—Tools to monitor flow, pressure and tempera-ture are typically small and mandrel-shaped,and fit inside production tubing. Run timecan be a few hours or more than two weeks.Well conditions are often below 150°C,although temperatures as high as 200°C areencountered in limited severe service. Powerconsumption is usually less than a watt.

Semipermanent monitors are small—about1.2-in. OD—tools placed on tubing hangersto record pressure and time for long-termmonitoring of hydrocarbon flow in produc-ing fields. Operations can last up to 90 days,but temperatures are usually below 320°F[160°C]. Power requirements are low, typi-cally less than an eighth of a watt.

Drillstem testing (DST) involves temporaryproduction of formation intervals. There aretwo types of battery-operated DST tools. The

first type opens and closes valves to controldownhole flow remotely. The second toolrecords pressure during the period of flow.The first requires short power pulsesrepeated up to 30 times, while the second,like production logging tools, requires con-stant low power.

Controlling direction and measuring whiledrilling—Drilling control and measure-ments-while-drilling are the most demand-ing and critical of all battery applications. Inthese applications, tools are located justabove the drill bit and subjected to tremen-dous shock and vibration, and rugged cus-tom battery designs offer distinct advantages.Batteries need to last for the life of a drill bit,up to two weeks. Temperatures of most jobsare 212°F [100°C], but in some cases theyclimb above 150°C when mud circulationstops. These batteries are usually either shortwith large diameters or long with smalldiameters depending on the tool.

Acquiring seismic data—In explorationapplications, tools on streamers behind spe-cially equipped vessels or in land equipmentrequire power to record seismic surveys.Marine batteries power a clamp-on tool thatgives streamers lift for depth control and pro-vide compass location information. Normally,marine tools use nonrechargeable and landtools use rechargeable batteries. Operatingtemperatures range from 0 to 50°C. Criticalfactors for marine batteries are operating livesmeasured in months and safe handling char-acteristics. Marine batteries are typicallyshorter than 24 inches, about 2 inches indiameter, and weigh less than 5 pounds.

Battery disposal—In the past, disposal oflithium batteries was left for field organiza-tions to handle. The Power Source Productsgroup recently took over responsibility fordisposing of lithium cells manufactured bySPT. For no additional cost, field locationsworldwide can now transport used batteriesto a facility in Cuyanosa, Texas for safe andefficient disposal. This uniform process canminimize user costs, hassles, adverse envi-ronmental effects and liability.

Longer Life: New Power Source Directions There are several ways to extend the usablelife of batteries. The first, managing electricalloads, is essential in a total power sourcedesign and development program. Two otherkeys to improving battery service are thecapability to measure and track remainingpower, and innovative methods to get maxi-mum capacity from a battery. Each methodensures that batteries are fully depletedbefore discarding.

Battery-life indicator—One way to achievemaximum battery consumption is to measureelapsed A-hr. A battery can be retired when amajority—80% to allow for safety margin—ofthe capacity is consumed. The best place torecord A-hr is in the tool, rather than in thebattery. In this way, electronics can be easilyaltered to record time and tool load, and adatabase of battery performance can be gen-erated. Adding electronics to batteriesincreases cost in addition to reducing reliabil-ity and independent operation.

A major disadvantage of tool-resident elec-tronics is that if a battery is removed from thetool, the A-hr record is decoupled from thepack unless field personnel make a note onthe pack. Either way, a mark must be madeon the battery to record remaining life. Thebest method is one that is integral to the bat-tery and does not depend on recordingelapsed parameters. Ideally, a battery-lifeindicator should be compatible with existingtools and associated directly with the battery.

Battery-life indication for lithium thionylchloride chemistry is difficult because mostmeasurable parameters, like voltage andinternal resistance, do not vary appreciablywith discharge until near the end of batterylife.3 In addition, general lithium battery volt-age performance at ambient temperaturesdepends greatly on the growth of LiCl sur-face layers at the internal electrodes, whichresults in cell passivation. At elevated tem-peratures, passivation effects are significantlydiminished. Electronic interrogation of bat-teries to determine remaining life is alsocomplicated by additional internal batteryresistance, which depends on thermal andload history as well as temperature.

■■Oilfield Battery Applications.

Typical Typical Voltage Energybattery battery potential, capacity,

Service type OD, in. length, in. V A-hr

Wireline logging 1 to 1.5 15 7 12 to 28Drilling measurements 1.5 to 4.5 25 to 100 20 to 60 28 to 30Seismic data acquisition 2 15 7 34

3. Milewits M: “A Novel Method to Determine LithiumBattery State of Charge,” in Savadogo O and RobergePR (eds): Proceedings, Second InternationalSymposium on New Materials for Fuel Cell andModern Battery Systems. Montreal, Quebec, Canada:Ecole Polytechnique de Montreal (1997): 358-367.Milewits M: “Intrinsic Method to Determine LithiumThionyl Chloride Battery Capacity,” presented at the38th Power Sources Conference, Cherry Hill, NewJersey, USA, June 8-11, 1998: 65-68.

Autumn 1998 55

The anode electrode structure can bealtered to overcome these restrictions andgive an indication of the depth of dischargeupon application of a defined load at ambi-ent temperatures. Typically, for oilfield appli-cations, depletion is at high temperature, butinterrogation occurs between jobs at ambi-ent conditions. Battery-life indication wasdemonstrated in the field with the UNIGAGEwell-testing battery for pressure and temper-ature recording. This battery is scheduled tobe commercial in late 1998.

Sequential depletion—The need for con-stant power on MWD jobs prompted a newmethod of salvaging batteries and extendingoperating life. To ensure that tools would notrun out of power during a drilling job, fieldengineers put in new battery packs when atool was tripped out of a well. This procedurewas followed even if a battery was only par-tially depleted. As a result, there was a large

backlog of partially used batteries in the fieldthat engineers were reluctant to re-use.

To overcome this problem, during aplanned upgrade of tool sensors, the powerelectronics were modified to allow more effi-cient battery usage. This “sequential deple-tion” method was conceived andimplemented by Anadrill engineering. Itrequires the use of two packs, one at a time,in temporal sequence. While the first batteryprovides primary power, the second batteryis on standby with a small background cur-rent load to minimize passivation.

Switching and replacement occur at thesurface. A two-battery configuration allowsmaximum power to be extracted from eachpack and there is a fresh battery in case ofan extended run with a partially depletedpack (above).

RechargeablesRechargeable, or secondary batteries, areused in automobiles, power backup systems,energy storage and consumer electronics.Recharge capability depends on the electro-chemical reaction. Primary battery reactionsare irreversible. When the active materialsare consumed during discharge, the batteryis completely depleted. For rechargeables,reactions at the anode and cathode arereversible. Active materials on both elec-trodes can almost be fully recovered by elec-tronic recharging.

Although a secondary battery is typicallyless costly than the corresponding number ofrequired primary batteries, there are disad-vantages. Rechargeables typically have athird to a quarter of the energy density of pri-mary batteries. Secondary cells have higherself-discharge rates, their capacity varieswith each recharge cycle, and special equip-ment and training are required to properlyrecharge the battery. There are three major

Position 2

Position 1

Tool run 1

Pack A(100 hr)

Pack B(100 hr)

Pack A(50 hr)

Pack B(98 hr)

Pack A(0 hr)

Pack B(96 hr)

Start Logging Finish

Position 2

Position 1

Tool run 2

Pack B(96 hr)

Pack C(100 hr)

Pack B(46 hr)

Pack C(98 hr)

Pack B(0 hr)

Pack C(96 hr)

Start Logging Finish

Switch

pack

■■Sequential MWD and LWD battery depletion. The battery pack in first position is removed from the tool at the surface. The pack fromsecond position, which was operating on standby current and as a backup power source, is moved into first position. A new battery isplaced in second position. This procedure ensures that a “fresh” battery pack is available if a drill-bit run is longer than anticipated.

56 Oilfield Review

rechargeable chemistries—lead-acid, alka-line with nickel cathode and nonaqueouswith lithium anode (above).

Lead-acid—Cells based on lead-acidchemistry have been around since GastonPlante developed the first practical battery in1859, and are familiar for starting and light-ing vehicles, and uninterrupted stand-bypower. Heavy weight, and low gravimetricand volumetric energy density limit portableelectronic applications.

Alkaline with nickel cathode—Alkalinebatteries are currently the most popularrechargeables for portable electronics. These batteries include nickel-cadmium [Ni-Cd], nickel-metal hydride [Ni-MH],nickel-zinc [Ni-Zn] and nickel-iron [Ni-Fe]cells. The cathode is a nickel electrode[NiOOH/Ni(OH)2] and potassium hydroxide[KOH] is the common electrolyte. The Ni-Febattery with heavier iron anode and lessgravimetric energy density is not widelyused. The Ni-Zn battery, due to the cycle lim-itation of a Zn anode, is still in development.The Ni-MH battery with higher energy den-sity and similar discharge performance, butfewer environmental concerns, is replacingthe Ni-Cd battery in portable electronics.

Nonaqueous with lithium anode—Theseadvanced batteries include lithium-ion andlithium-metal polymer chemistries. Lithium-ion batteries, with the highest energy densityof rechargeables, are produced at a rate ofseveral million per month for consumer elec-tronics. Lithiated carbon [LiC6] is the anode,lithium metal oxide is the cathode and theelectrolyte is a nonaqueous organic solution.By replacing liquid electrolyte with a solidpolymer electrolyte, the lithium polymer bat-tery achieves the versatility and safety of anall-solid design. The dry polymer electrolyteplays a dual role as ionic conductor for a

current pathway and electronic insulator—separator—inside the battery. Solid-statelithium polymer cells for industrial applica-tions are under development.

For surface application at ambient temper-ature, commercial Ni-MH, lithium-ion andlithium metal rechargeable batteries maypotentially replace lead-acid batteries. Thehigh energy density could reduce the sizeand weight of battery packs. Presently, how-ever, these advanced batteries are limited bytheir relatively small size. The largest com-mercially available size is only about 5 A-hrcompared with 50 to 100 A-hr (typical carbattery), which are normally found in lead-acid batteries. To put the same energy into abattery pack, larger numbers of cells need tobe connected in series or parallel. The result-ing battery management becomes morecomplex during charge and discharge. Inaddition, designing a rugged mechanicalpackage that houses many small cells andsurvives high shock is challenging.Designing the largest cell size possible isclearly the way to proceed for optimal per-formance and reliability of both primary andsecondary cells.

For downhole application, aqueous alka-line and lithium-ion cells are not suited forhigh temperature because of the low boilingpoint of liquid electrolytes—aqueous andnonaqueous. However, a lithium-metal poly-mer rechargeable battery with solid elec-trolyte adds little vapor pressure to the batteryand is less prone to volatile reaction with thelithium anode. In addition, the plastic char-acteristics of the polymer electrolyte allowthin laminates to be placed on the lithiummetal surface to enhance thermal and electri-cal conductivity, and uniform current distri-bution. Manufacturing involves winding atight spiral wrap of three thin electrodes—lithium foil laminate, lithium ion-conducting

polymer electrolyte and a layer of cathodematerial. This geometry minimizes ion diffu-sion losses across the polymer film andenhances charge and discharge efficiency.

It is possible that a high-temperature sta-ble polymer will be developed for down-hole use. Obstacles like thermal stability,manufacturing feasibility and cost are cur-rently being addressed. In addition, otherrechargeable chemistries are being moni-tored and evaluated for potential use inhigh-temperature markets.

Fuel Cells Fuel cells (FC) also generate power from elec-trochemical reactions, but unlike batteries,which have a finite amount of chemicals, fuelis stored separately from the reaction zone.Theoretically, a fuel cell will run forever iffuel and oxidizer are supplied continuously.

Small fuel cells (less than 500 watts) canreplace batteries in some ambient surfaceapplications. If power is required for longperiods of time, fuel cells have an advantageover batteries because the fuel source isexpandable and renewable. Fuel cells canalso have a higher energy density than bat-teries with resident fixed chemical supplies ifdesigned properly. Tripling battery liferequires a cell that is three times larger, butonly the fuel tank size needs to be tripled fora fuel cell. Three types of fuel cells are appli-cable at low or medium temperatures.

Proton exchange membrane (PEM) fuelcells use an ion exchange membrane elec-trolyte. A polymer film with good acid func-tionality provides higher proton mobility forhigh power density. The typical PEMFCoperating temperature is from 68 to 212°F[20 to 100°C]. These fuel cells can be sizedfor milliwatt to kilowatt power. In smallersizes, the systems are simple with few mov-ing parts, start quickly and can be throttledup and down rapidly. Presently, high costlimits wide application.

No fuel cell is right for all applications. Forsmall 10-W to 1-kW portable applications,the PEMFC offers the most advantages and agreat deal of flexibility, ranging from passivesystems with no moving parts for low-power applications to systems with thecomplexity and power output of internalcombustion engines.

These systems can use hydrogen ormethanol directly as fuel. For surface appli-cations, an air-breathing system is usedinstead of pure oxygen to supply oxidizer. Adirect methanol PEMFC permits the conve-nient use of a liquid fuel that is easy to trans-port. Refueling is like filling the gas tank ona lawn mower. A hydrogen-fueled PEMFC is

Pb

-A

cid Ni-C

d

Ni-M

H

Li-I

on

500 20 40 60 80 100 120 140 160

100

150

200

250

300

350

400

Volu

met

ric e

nerg

y d

ensi

ty, W

-hr/

kg

Gravimetric energy density, W-hr/kg

Smaller

Lighter

■■Energy densities ofcommon secondarybatteries. Chemistrieswith the highest vol-umetric and grav-metric energy con-tent make thesmallest and lightestrechargeable cells,respectively. How-ever, the energydensities of mostpractical secondarybatteries are far lessthan primary elec-trochemistries likelithium thionyl chloride.

Autumn 1998 57

Alkaline fuel cells, which are used in thespace shuttle, employ aqueous potassiumhydroxide [KOH] as the ionic conductingelectrolyte. The operating temperature rangeis 68 to 248°F [20 to 120°C]. A fundamentalproblem limiting alkaline FC use is sensitiv-ity of the basic electrolyte (aqueous KOH) tocarbon dioxide [CO2], which forms potas-sium carbonate [K2CO3] and precipitates.These systems use only pure oxygen, makingthem impractical in most applications.

Fuel cells generate waste heat as a result ofchemical inefficiency. The key reaction inmost fuel cells is hydrogen oxidation. In afuel cell, this process is split into two half-cell reactions that are carried out separately.In an acid-type, PEMFC for example, hydro-gen is oxidized to produce protons and elec-trons at the anode. Protons pass through anacid electrolyte to the cathode, where oxy-gen is reduced to form water. The electrolyteis an electronic insulator, so electrons cannotpass to the cathode; instead they flow to theexternal circuit, providing electric current.Fuel cells use pure oxygen or collect air fromthe atmosphere (above left).

The full range of fuel cell applications isbeing studied at SPT. Not only are physicaland mechanical constraints—confininggeometry, high shock, wide temperature

ranges—tighter, but costs are also higherthan for batteries. If extended life and safehigh power are to be realized, these obsta-cles must be overcome.

A Systems Engineering Approach By utilizing modern manufacturing and test-ing facilities, and an expanding base of knowledge, expertise and experience,Schlumberger Perforating & Testing develops,designs and manufactures battery packs for avariety of oilfield services and measurementapplications (above). These developmentsinclude custom mandrel packs made withcylindrical and annular cells, which currentlyuse liquid cathode lithium chemistries andunique, proprietary electrode constructions.Higher temperature cells for greater than200°C, solid-cathode rechargeable cells andother new power-source technologies are cur-rently being pursued for oilfield services andother applications outside of the industry.

As in the case of primary batteries, the keyto successful, cost-effective, high-perfor-mance secondary batteries and fuel cells isconcurrent engineering of mechanical, elec-trical and operational tool factors—a systemsapproach. This approach helps battery engi-neers, tool designers and end users meet thechallenges of supplying power to advanceddownhole and surface tools. —MET

(H2)

Hydrogen fuel

Anode

(H2O)

(O2)

Proton conducting membrane

H+

e-

LoadElectron flow

Anode reaction:H2 2H++2e-

Cathode reaction:O2+4H++4e- 2H2O

Water andwaste heat

Air supply

Cathode

■■Typical fuel cell process.

■■Battery manufacturing, packing and shipping at SPT.

refueled by pressurized tank. Because hydro-gen oxidation electrocatalysts support cur-rent densities an order of magnitude greaterthan methanol oxidation electrocatalysts andoxidize hydrogen at a higher cell potentialthan methanol can be oxidized, a hydrogen-fueled PEMFC operates at higher power den-sity. This leads to smaller systems to meetgiven power requirements. Safer gas storage,such as metal hydride hydrogen devices, areavailable. Hydrogen stored as a metalhydride is not under high pressure. If the tankruptures, release is slow and hydrogen dissi-pates quickly instead of pooling like propaneor butane because it is lighter than air andhas a rapid diffusion rate.

The phosphoric acid fuel cell uses liquidphosphoric acid in an inorganic matrix asthe electrolyte and must operate at 302 to428°F [150 to 220°C] due to the poor acidionic conductivity at low temperatures. Thephosphoric acid FC is the most advancedsystem for commercial power generation.High operating temperatures and low powerdensity—the major disadvantage—make thissystem suitable only for multikilowatt sta-tionary applications, but it is efficient in thisrole. Most phosphoric acid FCs operate onnatural gas from pipelines or on-site storage.