Battery Maintenance Caring for and testing of batteries

G.KUMARANA.D.S.T.E/BPA

BATTERYGroup of connected cells forms BATTERY.Classification of CELLS

Primary Cells: The chemical materials are used up as electric energy is produced. It is discarded when its active material is depleted and it no longer produces electrical energy.

Secondary Cells: A reversible chemical reaction is used to separate charges so that the cell can be returned to its original chemical state many times

Railway Signalling ApplicationsIn Railway Signalling Applications Secondary

Cells are usually used for situation requiring larger amount of energy at high currents.

In Railway Signalling Application three types of lead acid cells are used

Flooded type Lead Acid Cells IS-1651 (not used) Flooded type low maintenance Lead Acid Cells IRS-

S88/93. Sealed Lead Acid Cells S93/96(For signaling

applications these cells are not used)

Flooded type low maintenance Lead Acid Cells IRS-S88/93.At present in Railway Signalling Applications

Flooded type low maintenance Lead Acid Cells are used.

Advantage of LMLA over Lead Acid Cells.In Grid Alloy composition Low percentage 2%

of antimony is used. This reduces the need for adding water since very little is boiled of during charging.

Ampere-Hour efficiency more than 95%. High Watt-Hour efficiency more than 80%.

Construction details of LMLAIn LMLA number of positive and negative

plates are interleaved and separated by porus rubber sheet. This layering provides greater surface area and current availability. All the positive plates are electrically connected as are all the negative plates. This connections yield a parallel (Higher current) parallel arrangement for single cell developing approximately two volts.

Principle of Cell workingWhen two electrodes of dissimilar metals are

placed in an electrolyte there will be miserable potential difference at their terminals. In the Lead Acid System one electrode of lead peroxide and another of pure sponge Lead or immersed in the electrolyte.

Positive plate is made of Lead Peroxide, Negative plate is made of Sponge Lead

Electrolyte is H2SO4. This above combination in the cell produces about

two volts.

CHEMICAL REACTIONDischarge. In the discharged state both the positive and negative plates become

lead(II) sulfate (PbSO4) and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the positive plate back into the cell at the negative plate.

Charge.In the charged state, each cell contains negative plates of elemental lead (Pb) and positive plates of lead(IV) oxide (PbO2) in an electrolyte of approximately 33.5% v/v (4.2 Molar) sulfuric acid (H2SO4). The charging process is driven by the forcible removal of electrons from the negative plate and the forcible introduction of them to the positive plate.

DISCHARGE

Pb+PbO2+2H2SO4 2PbSO4 + 2H2O+ Electrical EnergyCHARGE

Specific Gravity The State of Charge of Lead Acid Cell can be checked

by determining the specific gravity of the electrolyte. Specific gravity is a ratio of the weight of the given

volume of the Electrolyte to the same volume of the water at temperature 68 degree F, a fully charged cell should have specific gravity of 1.200, a fully discharged cell 1.800 both figures are related to the specific gravity of water which is 1.00 the specific gravity can be measured with hydrometer.

Why the Specific gravity of LMLA cells kept at 1200-1220? If the specific gravity of Acid less than 1200 there

the internal resistance is too high. If the specific gravity is too high the acid damages

the positive and negative plate materials and reduces the cell life hence the specific gravity is in the range of 1200 to 1220. This is specified by different manufactures

Ampere-Hour Capacity. A batteries current rating is usually given in units

of Ampere-Hour Capacity, based on 10hour discharge period. During that period, the cells output voltage must not drop below 1.8V for example 80 AH battery should deliver 8A current for 10Hours without any cell dropping below 1.8V. It is unlikely, however, that the battery could actually deliver 80 Amps per one hours-cells are less efficient at higher discharge currents. In signalling applications the various capacity of batteries are used, 40 AH, 80AH, 120AH, 200AH, 300AH etc.,

Initial Charging. In the absence of manufacturer’s instructions the

following method shall be followed. Mix one par of 1.840 Specific Gravity acid with

five parts of distilled water. Never pour water into acid. The acid explodes and spills over and causes injuries. Always add acid to water for diluting.

Allow the acid to cool down. Pour the cool acid into the cell up to the float

mark. Allow the cells to cool down for not less than 12

hours and not more than 24 hours. Before putting the cells on first charge top up

the acid to proper level if there is fall in the level. Charge the cell at 20-Hr. charge rate for 80

hours.

 

During the charging the cell temperature shall not rise above 50 degree C. If it rises, discontinue charging until the temp. comes down to about 40 degrees. If the temperature crosses 45 degree C, reduce the charging rate to half.

However, the total charge input should be equal to 80hrs X I, where I=20 Hr. charging rate.

While charging, there will be fall in the level of electrolyte due to loss of water by gassing. Restore this at intervals at intervals of say 24 hours by adding required quantity of distilled water.

At the end of charging, the specific gravity of the electrolyte is to be adjusted to 1,200±0.005 at 27 degree C. If the specific gravity at the end of charging is above 1.200 add distilled water and if it is below 1.200 and 1.400 specific gravity acid and charge for a couple hours and check the gravity again.

Allow the cells to cool down. Discharge the cells at one tenth of AH capacity to an

end voltage of 1.85V per cell.

After fully charged cells should not be kept unused. If the cells are continued to keep in ideal condition the cell capacity automatically looses,

Due to the following reasons.Thin layer of Lead Sulphate formed on plates makes the cell plates ineffective.

Stratification of acid leads to higher specific gravity acid reaching the bottom of the cells.

Cells ConnectivitySeries:Connect the positive terminal of one cell into negative terminal of the other cell and so on….V=E1+E2+E3I = Current flowing in one directionTotal EMF = some of the EMF of each cells

Parallel:Connect all the positive wires of the cells to a single wireConnect all the negative wires of the cells to a single wires.Then you will get a battery voltage of a single cell. But the current will be total of all cells.I= I1+I2+I3 and V= V1=V2=V3

Battery Maintenance

Freshening ChargeAfter Initial Charging of Cells if the batteries

are not connected to circuit, the freshening charge rate is 4% of the full capacity to be done (1.6Amps for 40AH).

During the maintenance of batteries if one of the cell found low specific gravity/ low terminal voltage compared to others, the particular sick cell to be removed from bank and should be boost charged until the cell regains its capacity. At any circumstances adding of acid or higher specific gravity electrolyte should not be added.

Negative polarity :- During the course of battery maintenance if the cell is accepting less charge than other cell in the bank, over a period of time the cell slowly loses its capacity and assumes negative voltage if discharged. This negative polarity cell will disconnect the battery voltage to load such cells should be removed from the battery bank and it should be charged separately.

Why Acid cannot be added to working Cell? The specific gravity of the cell is measure of state

of charge of cell. Addition of acid changes the specific gravity without change in the state of charge of cell. Addition of acid of higher specific gravity does not remove the Lead sulphate layer on the cell plates. Only the charging current breaks the sulphate layer.

Equalisation Charge A battery bank requires equilasation charge

periodically. Normally once in 3 months. Why and When Equalisation charge is required?

The individual cells of battery are not identical some cells may not be fully charged when the charging process is completed. During the course of measuring the voltage of each individual cell while the battery is at rest a variation of 0.05V between cells indicates imbalance same way if the variation of 20 points in the specific gravity between cells also indicates imbalance in battery bank.

Both the conditions can be corrected by equalisation charge. Equalising Charge rate 1/10th of AH Capacity of Cell.

Types of ChargingAuto mode of Charging: Depending on the

condition of Cell the charger selects float or boost charging in Auto mode when the maximum charging current falls below 5% the charger goes to float mode.

Float Charge : Charging the cell to reach ultimate terminal voltage of 2.15V. Maximum charging current is limited to 10 hours discharge rate

Boost Charge : Charging the cell to reach ultimate terminal voltage of 2.4V. Maximum charging current is limited to 10 hours discharge rate.

Topping of Distilled Water During battery maintenance if the electrolyte

level found low distilled water should be topped, at any circumstance normal drinking water should not be added to the cell.

If We Add? Normal drinking water even though clear of

gems still have impurities like copper and iron etc., During discharge the plates are covered by copper and iron sulphate which cannot be broken by charging current. It causes permanent damage to the cell and reduces its capacity.

Over Charging/Discharging of Cells

Over Charging: The cells start gassing and water is lost – distilled

water is required to be added more frequently The temperature raises some time may damage the

cell and also it leads to corrosion of positive grid.

Over Discharge: If a cell is discharged at a higher current rate then

recommended the cell plates are likely to be damaged permanently means if we discharge a cell beyond its capacity the cell may not be revived by charging it again.

Both the Cases the cell becomes useless.

Capacity Test During the maintenance, capacity test to be done on

battery bank once in a year.Capacity of Battery in AH= Load Current in Amps x

No. of Hours for eg:- Load Current=5A, original capacity of cell

40AH Time taken for voltage of any cell to fall to 1.8V=4 hrs Capacity in AH= 5A x 4 hrs

= 20AH when the capacity of the battery falls to 50% of the

rated capacity it should be planed for replacement. This type of test is also call as Curative Discharge.

Maintenance Tips Clean the terminals of Sulphation, if

required apply a very thin layer of petroleum jelly.

Maximum depth of discharge permitted is 80%. However for the purpose of design 50% DOD is considered.

All the battery terminals should be tightened during charger ON condition.

Proper cross section cable should be used between charger to battery. For example, 16sqmm copper cable should be used for 200AH batteries.

Battery connecting strap, bolts and nuts should be used as per OEM.

Connecting cable between cell and chargers should be provided with proper size of copper lugs.