Lithium Ferro Phosphate Batteries À X sZ> ]

Abstract

The ongoing development and maturation of battery technology as applied in the renewable energy field continue to present new pathways to innovation. Recent advances in cell safety, refinements in energy density, and developments in intelligent on-board battery management systems are driving the growing interest in new technologies. All portable power systems encounter similar challenges in four key areas: power generation, power management, power storage, and the loads placed on the system. In any given scenario, these variables must be considered and the most appropriate technologies employed. While there is a growing selection of equipment to choose from in power generation, the efficient management, storage, and distribution of this energy in a lightweight package remains a formidable challenge.

Utilizing a battery in any electrical circuit ensures operation at the highest level of efficiency. The battery is, quite simply, an electrical storage device that uses a chemical reaction to convert “electrical energy” into “potential chemical energy” and back again.

Introduction

In any highly competitive industry environment, constant innovation and development of new technol-ogy is a requisite. While traditional Lead Acid battery chemistries are both well-understood and well-received, there is undoubtedly a major shift underway in field requirements for portable energy. Baseline predictable performance and resistance to neglect and abuse became preferred characteristics of a battery since its inception. However, the evolving requirements in the 21st Century center on significantly reduced weight and a reduced footprint with more useable power. Lithium-Ion technology addresses these emerging requisites.

A correctly-sized VRLA battery bank subjected to moderate (50%) depth of discharge could be expected to last, on average, between 400 to 600 cycles. Taking the same VRLA bank and subjecting it to 80% or deeper discharging could shorten the life of the batteries to as little as 250 to 300 cycles (depending on the manufacturer, battery construction, etc.) possibly even less in extreme temperature environments. By contrast, extensive testing indicates a LiFePO4 battery bank can endure re-peated discharges well beyond 50% and be expected to last 5000 or more cycles before it could be considered at the end of its useful life. This capability constitutes an order of magnitude increase in the intervals for scheduled replacement versus the lead acid equivalent.

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Lithium Ferro Phosphate Batteries ǾǎΦ ±w[! .ŀǘǘŜǊƛŜǎ

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Battery C-Rate

A charge or discharge current rate of a battery expressed in amps. It is numerically a fraction or a multiple of the rated capacity of the battery expressed in amp-hours.

• Lithium Iron Phosphate (LiFePO4) - when high energy density and light weight are desired. LiFePO4

batteries are used for portable power tools, medical devices, and other mobile applications.

• Lead-Acid - most economical for larger power applications where weight is of little concern. Lead-acid is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS sys-tems. Lead acid is inexpensive and rugged. It serves a unique niche that would be hard to replace with other systems.

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Watts to Weights

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Cost per kW-h

Cost

($)

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Cost per 2000 cycles

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C-Rate Rated Capacity Formula Amps Discharge / Charge Time

10C 100 Ah 10 x 100A 1000A 6 minute

5C 100 Ah 5 x 100A 500A 12 minutes

3C 100 Ah 3 x 100A 300A 20 minutes

2C 100 Ah 2 x 100A 200A 30 minutes

1C 100 Ah 1 x 100A 100A 1 hour

C/2 100 Ah 100A / 2 50A 2 hours

C/3 100 Ah 100A / 3 30A 3 hours

C/5 100 Ah 100A / 5 20A 5 hours

C/10 100 Ah 100A / 10 10A 10 hours



Lithium Iron Phosphate (LiFePO4)The LiFePO4 battery is one of the most promising of the available Lithium-Ion chemistries for the diverse requirements of portable renewable energy systems. Even though the chemistry is relatively young com-pared to lead-acid, LiFePO4 cell banks have thus far shown impressive operational attributes that are often at minimum on par with the much more prevalent lead-acid technologies.

LiFePO4 batteries are constructed in two formats “large” and “small”. The large-format cells are often used in “less-portable” power storage devices where the aggregate battery capacity exceeds 4 kW-h. Small format cells are used in “highly-portable” applications where the total capacity is less than 4 kW-h.

Energy Density, or the measure of “overall unit weight” to “useable energy provided”, is a primary factor under consideration for all portable systems. LiFePO4 batteries have proven to be an excellent solution in regards to Watt-hours per Kg. Although other variations of Lithium-Ion battery chemistries can achieve higher energy densities than LiFePO4, their particular operational characteristics are not necessarily con-sistent with modern portable energy systems.

LiFePO4 Battery Advantages:

• Safety - LiFePO4 batteries will not catch fire or explode during rapid charge/discharge; unlike other Li-ion technologies, LiFePO4 batteries are virtually incombustible due to chemical stability of their Iron Phosphate cathode material.

• Physically light - good “weight-to-energy density” ratio – particularly well suited for mobile applica-tions where light weight is important.

• Long cycle life - can be discharged and recharged many times with minimal performance degrada-tion.

• The self-discharge is among the lowest of all rechargeable battery systems (<2% / month)

• Environmentally-friendly - contains no toxic heavy metals or caustic materials.

• Tolerant of exposure to high temperatures.

LiFePO4 Limitations:

• Higher cost - return on investment is primarily based on long-term deployment.

• HAZ-MAT transport - can only be transported under restricted conditions (set by D.O.T.).

• Less resistant to shock and vibration.

• Requires additional power management devices for integration into traditional applications.

• Relatively young technology - will not operate at full potential using available COTS lead-acid char-gers & power management appliances. Many associated charging/management technologies are more expensive, further driving up the cost to operate.



Discharge Capacity Characteristics

Vol

tage

(V)

22.4

21.6

20.8

20.0

23.2

24.0

24.8

25.6

26.4

27.2

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Discharge Capacity (Ah)

C/10 rate

C/5 rate

C/3 rate

C/2 rate

1C rate

Recharge Characteristics V

olta

ge (V

)

22.4

21.6

20.8

20.0

23.2

24.0

24.8

25.6

26.4

27.2

28.0

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Recharge Capacity (Ah)

C/10 rate

C/5 rate

C/3 rate

C/2 rate

1C rate

Discharge Time Characteristics

Vol

tage

(V)

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29.0

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Discharge Time (hours)

1CC/2 C/3 C/5 C/10

% C

apac

ity

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80% DOD Cycles

Cycle Life

Temperature (°C)

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55 65 75Max

Cons

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t (%

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LiFePO4 Temperature Performance

LiFePO4 Performance Graphs (4 kW-h 24V Battery)



Lead Acid BatteriesThere are many newer battery technologies available in the market-place, however, lead-acid technolo-gies are the most-understood, and widely accepted as the “standard” by which all other batteries are measured. Newer battery technologies often have operational constraints like maximum & minimum operating temperatures and special charging requirements that make them less versatile and usable for the average consumer in everyday applications.

“Deep-cycle” Lead Acid BatteryCompared to other lead acid battery types, the deep-cycle battery is designed for maximum energy storage capacity and high cycle-count (long life).

Absorbed Glass Mat Batteries (AGM)The AGM battery is a newer type sealed lead-acid that uses absorbed glass mats between the plates. It is sealed, maintenance-free, and the plates are rigidly mounted to withstand extensive shock and vibration. AGM batteries feature a thin fiberglass felt that holds the electrolyte in place like a sponge. The AGM battery is used in applications where high performance lead-acid is demanded.

AGM Lead-Acid Advantages:

• Low Cost - Inexpensive & simple to manufacture, provides good return on investment (ROI).

• Unrestricted Transport - D.O.T. approved for all land, sea, and air transport.

• Mature, reliable and well-understood technology - works well with many COTS devices (chargers & power management appliances).

• Durable and provides dependable service.

• The self-discharge is among the lowest of all rechargeable battery systems (~2% / month).

• Usually field-serviceable.

AGM Lead-Acid Limitations:

• Physically Heavy - poor “weight-to-energy density” ratio.

• Cannot be stored in a discharged condition

• Allows only a limited number of full discharge cycles - well suited for standby applications that require only occasional deep discharges.

• Hazardous Disposal – must be properly recycled or disposed of when life cycle is complete.



Discharge Capacity Characteristics

Vol

tage

(V)

22.4

21.6

20.8

20.0

23.2

24.0

24.8

25.6

26.4

27.2

28.0

40 80 120 160 200

Discharge Capacity (Ah)

C/10 rate

C/5 rate

C/3 rate

C/2 rate

1C rate

Recharge Characteristics V

olta

ge (V

)

22.4

21.6

20.8

20.0

23.2

24.0

24.8

25.6

26.4

27.2

28.0

40 80 120 160 200

Recharge Capacity (Ah)

C/10 rate

C/5 rate

C/3 rate

C/2 rate

1C rate

Lead AcidDischarge Rate Characteristics

Vol

tage

(V)

(six

cel

ls)

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Discharge Time (hours)

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Cycle Life

AGM Lead-Acid Performance Graphs

Temperature (°C)

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t (%

)1C

Rat

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Lead Acid Temperature Performance



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Lithium Ferro Phosphate Batteries À X sZ> ]