FAQ: Cycle life of LiFePO4 versus Lead-Acid
Question: I talked to a supplier of Lead Acid battery. They told me that they have special stationery lead-acid battery for solar applications that can make 1500 cycles. For me it seems better to buy the cheaper lead-acid battery with 1500 cycles than to buy expensive LiFePO4 with only 2000 cycles.
Answer: Recently some lead-acid battery suppliers have been promoting their products highlighting the number of cycles. However to compare the charge/discharge cycle in real performance additional parameters need to be taken into consideration:
1. The depth of the cycle (also called DOD – the depth of discharge).
The DOD value gives the real energy received from the battery during one cycle. For example the DOD 80% means that 80% of the nominal capacity will be taken: with a battery of a capacity 100h, it means 80AH will be taken from the battery during discharge (without any additional charging).
The LiFePO4 cells support the deep charging. They allow taking 100% of the energy, even though for general application the DOD of 80% is recommended.
The Lead Acid batteries usually do not support deep discharge with many cycles. To reach long cycle life the DOD must be low: 20%, 30%.
2. The charge and discharge speed (also called the C-rating)
The C-rating value gives the time of the charge and discharge. The value of 1C means the battery is discharged (or charged) in 1 hour. The higher the C-rating the faster time of charge and discharge is supported.
The LiFePO4 cells support the high speed charge and discharge. The typical value for LFP technology is 0.5C (or C2) this means the batteries are discharged (or charged) in two hours.
The Lead Acid batteries usually do not allow high speed charging or discharging. The typical discharge rating may be given at C6, C10, or C20 which means recommended discharge is 6 hours, 10 hours or 20 hours. With such type of batteries there is no way to store and release energy quickly.
3. The remaining capacity (also called aging index)
The aging index means the decrease of the capacity during the cycle life. The new battery has 100% of capacity. With more and more cycles the capacity is reducing. The battery is getting weaker and weaker.
The LiFePO4 cells have very low aging index. After 2000 cycles that batteries will stay keep 80% of the nominal capacity. This means the batteries can be used even after the nominal number of cycles was carried out.
The Lead Acid batteries often age gradually. The typical lifespan of 1500 cycles means that after these 1500 cycles the capacity of the battery will be 20% or perhaps less.
4. The effectiveness of the charging cycle (ECC) - also called the effective energy yield
The ECC value means how much energy can be gained from the amount of energy stored. For example the effectiveness of 97% means that for a 100Ah battery you may need 100 Ah energy to charge and you will get 97Ah energy back (in case of 100% DOD).
The LiFePO4 cells have very high effectiveness. The new cells have some 97% of effective energy yield. The old cells (e.g. after 10 years of daily usage) still have some 90% effectiveness..
The Lead Acid batteries have a natural low effectiveness. Because this value is low, it is often never mentioned in the official data sheets. The effectiveness may be between 60 to 70% for high performance batteries. The old batteries may degrade to some 40% or less.
When comparing the LiFePO4 cells with other technology all of these factors need to be taken into consideration. After detailed consideration it is obvious that the LiFePO4 technology is far ahead of the other battery technologies. The other technology is lagging behind (see the animation).