How To Calculate The Number Of Cycles Of PV Energy Storage Batteries?
The calculation of the number of cycles of photovoltaic energy storage batteries is directly related to the "depth of discharge (DOD)". When the battery is discharged halfway (such as discharging from 100% SOC to 50%) and then charged, the number of cycles is not calculated as "one complete cycle", but accumulated according to the equivalent full charge and discharge times. The following is a specific analysis:
1. The core definition of the number of cycles: based on "equivalent full charge and discharge"
The definition of a complete cycle
The process of discharging the battery from full charge (100% SOC) to completely empty (0% SOC), and then charging back to full charge, is counted as 1 complete cycle.
Example: 100%→0%→100% = 1 cycle.
Calculation of the number of cycles of partial charge and discharge
When the depth of discharge (DOD) does not reach 100%, the number of cycles is accumulated according to the proportion of the depth of discharge.
Formula: Equivalent number of cycles = Depth of discharge (DOD) ÷ 100%
Example: Discharge 50% (100%→50%), then this operation is 0.5 equivalent cycles; if it is discharged 50% and charged again, the two times are accumulated as 1 complete cycle.
2. Specific scenario calculation example
Scenario 1: Single discharge 50% followed by charging
Discharge process: 100% SOC → 50% SOC (DOD=50%)
Charging process: 50% SOC → 100% SOC
Cycle number calculation:
Equivalent number of cycles = 50% ÷ 100% = 0.5 times, that is, this operation is only counted as half a cycle.
Scenario 2: Multiple partial discharge accumulation
1st time: 100%→70% (DOD=30%)→charge to 100%→accumulated 0.3 cycles
2nd time: 100%→60% (DOD=40%)→charge to 100%→accumulated 0.4 cycles
3rd time: 100%→50% (DOD=50%)→charge to 100%→accumulated 0.5 cycles
Total number of cycles: 0.3+0.4+0.5=1.2 times, which is equivalent to 1.2 complete cycles.
3. Differences in cycle times of different battery types
Battery type Nominal cycle times (80% DOD) Cycle life characteristics under partial discharge.
Lithium iron phosphate battery 3000-6000 times When partially discharged (such as 50% DOD), the cycle times can be doubled to 6000-12000 times.
Ternary lithium battery 1000-2000 times Partial discharge significantly improves cycle life, but decays faster at high temperatures.
Lead-acid battery 300-500 times When partially discharged (such as 50% DOD), the cycle times are about 500-800 times.
Note: The nominal cycle times are usually based on 80% DOD (such as 100%→20%→100%). If the DOD is lower in actual use (such as 50%), the equivalent cycle times need to be converted proportionally, and the battery life will be extended accordingly.
4. Influencing factors in practical applications
Battery management system (BMS) algorithm
Most BMS will calculate the equivalent number of cycles according to the cumulative discharge capacity. For example, if the battery capacity is 10kWh, the cumulative discharge of 5kWh is counted as 0.5 cycles.
Some high-end BMS will dynamically adjust the cycle calculation logic based on parameters such as discharge depth, temperature, and charge and discharge rate.
Temperature and charge and discharge rate
Under high temperature (>40℃) or low temperature (<0℃) environment, even if it is partially discharged, the battery decay rate will accelerate, and the equivalent number of cycles may be "falsely increased".
Fast charging (>1C) will aggravate the internal loss of the battery, and the cycle conversion coefficient may be higher under the same DOD (such as 0.5 cycles The actual loss is equivalent to 0.6 times).
Capacity decay threshold
The termination standard for the number of cycles is usually when the battery capacity decays to 80% of the nominal value. For example: when a 10kWh battery decays to 8kWh, it is considered to have reached the nominal number of cycles.
In partial discharge scenarios, capacity decay is slower, and the actual number of available cycles may exceed the nominal value (e.g., the number of cycles of lithium iron phosphate at 50% DOD can reach 1.5-2 times the nominal value).
5. Suggestions for users: Optimize cycle calculation and battery life
Avoid deep discharge
Try to control the depth of discharge within 50% (e.g., 100%→50%), which can halve the equivalent number of cycles and significantly extend the battery life.
Example: A lithium iron phosphate battery with a nominal 3000 cycles (80% DOD) can actually cycle more than 6000 times if used at 50% DOD.
Use BMS to monitor equivalent cycles
View the "cumulative equivalent cycle number" through the energy storage system APP, and plan the replacement cycle in advance based on the battery capacity decay data (such as the remaining capacity percentage).
Combined with electricity price strategy to balance charging and discharging
If the goal is peak-valley arbitrage (such as charging during valley hours and discharging during peak hours), partial discharge (such as 30%-50% DOD) can be used preferentially to reduce the number of cycles while ensuring profits.
Calculation logic of number of cycles
Core formula: Equivalent number of cycles = Cumulative discharge depth (total DOD) ÷ 100%
Key principle: Partial discharge is accumulated in proportion, the lower the depth, the slower the number of cycles is consumed;
Life optimization: Controlling the discharge depth <50% can make the actual battery life reach 1.5-2 times the nominal value.
Through the above logic, users can accurately evaluate the use loss of photovoltaic energy storage batteries and rationally plan operation and maintenance and replacement strategies.