Direct answer: A battery degradation budget adds calendar aging and cycle aging, corrected for temperature and state of charge window, then converts the result into a usable capacity floor over the project life. Build it from the real duty cycle, not from a headline cycle life, and tie the result to the performance guarantee so the contract capacity is met at the end of life as well as at commissioning.
Batteries lose deliverable energy as they age, and a commercial and industrial storage project has to keep meeting its contract through that loss. A degradation budget is the financial and engineering bridge between the nameplate capacity and what the system can still deliver years later.
This guide builds the budget step by step with clearly labeled planning assumptions. Every value is illustrative and must be replaced with the manufacturer’s data and the project’s measured conditions.
Separate calendar aging from cycle aging
Calendar aging happens with time regardless of use, while cycle aging happens with each charge and discharge. They are different mechanisms and are estimated separately before being added together.
Calendar aging depends on time, temperature and the state of charge the battery rests at. A battery held at high state of charge in a hot room ages faster even when it is not cycling. Cycle aging depends on the number of cycles, the depth of each cycle and the temperature.
Keep the two estimates separate so a later change in the operating schedule updates the cycle term without hiding it inside a single combined number. A transparent budget is easier to defend in a performance review.
Count cycles from the real duty cycle
The number of cycles per year comes from the operating schedule, not from an assumed daily count. A system doing one full cycle a day accumulates about 365 cycles a year, but a system doing a partial morning peak and a partial evening peak accumulates a different total.
Convert partial cycles into equivalent full cycles using the depth of each cycle. A cycle from 20 to 80 percent state of charge counts as roughly 0.6 of a full cycle for budgeting, although the exact equivalence depends on the cell chemistry and should use the manufacturer’s curve.
Multiply the equivalent full cycles per year by the project years to get the lifetime cycle count, then apply the cycle aging rate per cycle. Using the real schedule avoids both overbuilding and underbuilding.
Apply temperature and SOC window effects
Temperature and the operating state of charge window change both aging rates. High temperature accelerates calendar and cycle aging, while a narrow state of charge window can slow cycle aging at the cost of using less of the battery.
Use the manufacturer’s aging data at the project’s expected temperature, not at a reference temperature. A battery quoted at 25 degrees Celsius ages faster in a 35 degree cabinet, and the budget must reflect that.
Document the state of charge limits the system will actually run between, because the depth and the average resting point both matter. The budget should match the dispatch strategy that the energy management system will enforce.
Convert the budget into usable capacity
The degradation budget reduces the nameplate energy to a usable energy floor. Start with nameplate energy, subtract the depth of discharge limit and the round-trip efficiency, then subtract the accumulated calendar and cycle aging over the project life.
The remaining number is what the system can reliably deliver at the end of life under the stated conditions. That floor, not the nameplate figure, is what the revenue or savings model should use for the later years.
State the floor with the same conditions as the performance guarantee, so the budget and the contract speak the same language. A floor that assumes a different temperature or cycle count than the guarantee is not comparable.
Tie the budget to the performance guarantee
A performance guarantee usually states a retained capacity or energy at a cycle count and an end of life definition. The budget should mirror those terms so the engineering model and the warranty language check each other.
Read the guarantee’s conditions: the cycle count, the depth of discharge, the temperature and the measurement method. A budget built on different conditions will either overstate the remaining energy or understate it, and neither helps the project.
Agree how degradation is measured, usually through a periodic capacity test or a battery management system estimate, and record the baseline at commissioning. Without a measured baseline, there is no later point to compare against.
| Item | Assumption | Effect |
|---|---|---|
| Nameplate energy | 500 kWh | Starting point |
| Depth of discharge limit | 90 percent | 450 kWh usable |
| Round-trip efficiency | 92 percent | 414 kWh delivered |
| Calendar aging | 1.5 percent per year | Subtract over life |
| Cycle aging | 0.02 percent per full cycle | Subtract over life |
This table is an illustrative example only. Replace every value with the manufacturer’s data and the project’s measured duty, and confirm the final floor with engineering review.
Review the assumptions every year
A degradation budget is a forecast, not a guarantee. Review it annually against the measured operating data: the actual cycle count, the temperature history and the state of charge the system actually ran between. Where the real conditions differ from the assumptions, update the budget so the end of life floor stays realistic.
Compare the battery management system’s capacity estimate with a periodic capacity test, and investigate when they diverge. A budget that is updated with evidence is the one that can be defended if the project underperforms its plan.
Frequently asked questions
What is the difference between calendar and cycle aging?
Calendar aging happens with time and temperature regardless of use, while cycle aging happens with each charge and discharge. They are estimated separately and then added into the budget.
How do I count partial cycles in a degradation budget?
Convert partial cycles into equivalent full cycles using the depth of each cycle, following the manufacturer’s curve for the cell chemistry. Multiply the equivalent cycles by the cycle aging rate.
Why does temperature matter so much for aging?
High temperature accelerates both calendar and cycle aging, so a battery ages faster in a hot cabinet than at a reference temperature. The budget must use the project’s expected temperature.
What is the usable energy floor at end of life?
It is the nameplate energy reduced by the depth of discharge limit, the round-trip efficiency and the accumulated aging. That floor, not the nameplate figure, should drive the later-year revenue model.
What to send for a technical review
Share the operating schedule, the expected temperature range, the state of charge limits and the proposed guarantee terms with the AllGrand engineering team. The team can review the lithium battery range and the aging assumptions for the project. Read more BESS engineering guides. AllGrand Group has manufactured batteries since 1987 and supplies energy storage systems to customers in more than 100 countries. The budget must be confirmed by engineering review.
Sources and further reading
These sources were used to verify general battery aging and energy storage concepts. The budget method and worked example are original AllGrand editorial work with clearly labeled assumptions.