Why a cycle count alone says nothing
A cycle is the turnover of one capacity — charge and discharge. A full cycle corresponds to a complete discharge, a partial cycle to a fraction of it. But what matters for ageing is not the number of charging events, it is the total amount of charge turned over and the conditions under which it is turned over. That is why every cycle figure needs two additional pieces of information to be comparable: the depth of discharge (DoD) — how deeply the cell is discharged per cycle — and the SoH limit at which end of life is defined.
The DoD acts physically: deep discharge means greater mechanical expansion in the electrode lattice and operation closer to the voltage limits, where electrolyte decomposition and SEI growth increase. Shallow cycling stresses the cell less — and thus delivers a multiple of the cycles. The SoH (State of Health) describes the capacity still available compared with the as-new condition; datasheets usually set end of life at 80 % SoH, from which point the cell is considered spent even though it technically keeps running.
The same cell, completely different service life
The following table shows a typical LFP cell under different operating conditions. The numbers are orders of magnitude, not a datasheet guarantee — but they make the leverage visible:
| Depth of discharge (DoD) | End of life (SoH) | achievable cycles |
|---|---|---|
| 100 % | 80 % | ~3,000 |
| 80 % | 80 % | ~4,500 |
| 50 % | 80 % | ~8,000 |
| 50 % | 70 % | ~12,000 |
Two things stand out. First: halving the DoD from 100 to 50 % does not double the cycle count, it raises it roughly 2.5-fold — the relationship is disproportionate. Second: if you also lower the accepted SoH limit from 80 to 70 %, the usable cycle count grows again markedly. The same physical cell therefore “has” anywhere between 3,000 and over 12,000 cycles depending on how it is used. Anyone comparing only “cycles” is comparing apples with oranges.
For the design this means: first the end of use is defined — what residual capacity is still sufficient for operation? — and the DoD is derived from the real load profile. Only then is a cycle figure meaningful.
The battery also ages on the shelf
Alongside cyclic ageing runs calendar ageing: a lithium cell loses capacity even when it is not used. The drivers are temperature and state of charge. Warm storage at a high SoC noticeably accelerates SEI growth; stored cool and at a medium SoC (around 40–60 %), the same cell ages considerably more slowly. Until 80 % SoH is reached, the calendar service life is roughly a decade or more depending on conditions. In applications with few cycles per year, it will not be the cycle count that brings end of life, but the calendar — and then storage temperature matters more than any cycle figure.
The pattern in the proLogistik fleet
Since 2016 we have supplied proLogistik with mobile battery boxes in the Peli Case — several hundred units per year, runtime up to a week per charge. Over the years this produced a large field of identical packs under real load, and that is exactly what makes the pattern visible: units that were rarely deeply discharged in service and not permanently held at 100 % show a noticeably higher residual SoH after many charging events than those that were regularly run fully empty and immediately recharged to full. Not a lab finding, but field reality. We carry the consequence back into the design: wherever the application allows, the usable window is deliberately limited instead of exhausting the full capacity — the later residual SoH is the return on that decision.
Rules for evaluating cycle figures
- –A cycle count without DoD and an SoH limit is worthless — always demand both.
- –For the same cell, the cycle count rises disproportionately as the DoD falls.
- –First define the end of use (what residual capacity is sufficient), then derive the DoD from the load profile, then evaluate the cycles.
- –With few cycles per year, calendar ageing is the determining factor — watch storage temperature and storage SoC.
- –Where possible, limit the usable capacity window instead of fully exhausting it; this pays off in the residual SoH.
If you can state your load profile — cycles per day, typical depth of discharge, required residual runtime at end of life — the realistic service life of a pack can be calculated rather than guessed. That is the basis of any honest design.