Lithium Ferro Phosphate Batteries vs. VRLA Batteries
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 Average Lifecycle subjected to moderate (50%) depth of discharge could be expected to last, on 2500 average, between 400 to 600 cycles. Taking the same VRLA bank and subjecting it to 2250 80% or deeper discharging could shorten 2000 the life of the batteries to as little as 250 to 300 cycles (depending on the manufacturer, 1750 battery construction, etc.) possibly even less in extreme temperature environments. 1500 By contrast, extensive testing indicates 1250 a LiFePO4 battery bank can endure re- peated discharges well beyond 50% and 1000 be expected to last 5000 or more cycles Cycles before it could be considered at the end of 750 its useful life. This capability constitutes an 500 order of magnitude increase in the intervals for scheduled replacement versus the lead 250 acid equivalent. 0
Lead acid LiFePO4
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Lithium Ferro Phosphate Batteries vs. VRLA Batteries
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.
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) - 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.
Average Lifecycle Watts to Weights 110 2500 100 2250 90 2000 80 1750 70 1500 60 Wh / kg 1250 50 1000 Cycles 40 750 30
500 20
250 10
0 0
Lead acid LiFePO4 Lead acid LiFePO4 Cost per kW-h Cost per 2000 cycles
3000 3000
2500 2500
2000 2000
1500 1500 Cost ($) Cost ($)
1000 1000
500 500
0 0
Lead acid LiFePO4 Lead acid LiFePO4
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Lithium Ferro Phosphate Batteries vs. VRLA Batteries
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.
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Lithium Ferro Phosphate Batteries vs. VRLA Batteries
LiFePO4 Performance Graphs (4 kW-h 24V Battery)
Cycle Life LiFePO4 Temperature Performance 110 100 100 90 90 80 80 70 70 60 60 1C Rate 50 50 40 % Capacity 40 30
30 Output (%) Constant Power 20 20 10 10
0 -40 -30 -20 -10 0 25 35 45 55 65 75 500 1000 1500 2000 2500 Min STC Max 80% DOD Cycles Temperature (°C)
Discharge Capacity Characteristics Recharge Characteristics 28.0 28.0
27.2 27.2
26.4 26.4
25.6 25.6
24.8 24.8
24.0 24.0
23.2 C/10 rate 23.2 C/10 rate Voltage Voltage (V) Voltage (V) 22.4 C/5 rate 22.4 C/5 rate C/3 rate C/3 rate 21.6 C/2 rate 21.6 C/2 rate 1C rate 1C rate 20.8 20.8
20.0 20.0 40 80 120 160 200 40 80 120 160 200 Discharge Capacity (Ah) Recharge Capacity (Ah)
Discharge Time Characteristics 29.0
28.0
27.0
26.0
25.0
24.0 Voltage Voltage (V) 23.0
22.0
21.0 C/5 C/10 C/2 C/3 1C 20.0 1 2 3 4 5 6 7 8 9 10 Discharge Time (hours)
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Lithium Ferro Phosphate Batteries vs. VRLA Batteries
Lead Acid Batteries
There 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 Battery Compared 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.
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Lithium Ferro Phosphate Batteries vs. VRLA Batteries
AGM Lead-Acid Performance Graphs
Cycle Life 110 Lead Acid Temperature Performance 100 100 90 90 80 80 70 70 60 60
1C Rate 50 50 40 % Capacity 40 30
30 Output (%) Constant Power 20 20 10 10
0 -40 -30 -20 -10 0 25 35 45 55 65 75 100 250 500 750 1000 Min STC Max 80% DOD Cycles Temperature (°C)
Discharge Capacity Characteristics Recharge Characteristics 28.0 28.0
27.2 27.2
26.4 26.4
25.6 25.6
24.8 24.8
24.0 24.0
23.2 C/10 rate 23.2 C/10 rate Voltage Voltage (V) Voltage (V) 22.4 C/5 rate 22.4 C/5 rate C/3 rate C/3 rate 21.6 C/2 rate 21.6 C/2 rate 1C rate 1C rate 20.8 20.8
20.0 20.0 40 80 120 160 200 40 80 120 160 200 Discharge Capacity (Ah) Recharge Capacity (Ah)
Lead Acid Discharge Rate Characteristics 29.0
28.0
27.0
26.0
25.0
(six cells) 24.0 Voltage Voltage (V) 23.0
22.0
21.0 C/3 C/5 C/10 C/2 1C 20.0 1 2 3 4 5 6 7 8 9 10 Discharge Time (hours)
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