Cycle life and warranty, honestly
A 6,000-cycle rating is not a promise for six thousand of your cycles. It is a measurement at a defined depth of discharge, temperature and charge rate — and if any of those differ, the number does not transfer.
How cycle life is actually measured
A cycle life figure comes from continuously cycling a cell or pack under defined conditions until its capacity falls to a defined threshold — usually 80 % of the initial rating — and counting the cycles.
The conditions matter as much as the count. The same cell can show 3,000 cycles or 6,000 cycles depending on three variables:
| Variable | Effect on cycle life |
|---|---|
| Depth of discharge | Shallow cycling gives more cycles; deep cycling gives far fewer |
| Temperature | Above 25 °C, life falls; below, it can improve but usable capacity drops |
| Charge and discharge rate | High rates accelerate ageing |
A datasheet that quotes 6,000 cycles without stating these three conditions is not telling you very much.
Depth of discharge is the biggest lever
The relationship is not linear. Roughly, for LiFePO4:
- 100 % depth of discharge — a few thousand cycles at best
- 80 % depth of discharge — the figure most datasheets quote
- 50 % depth of discharge — substantially more cycles, often double
This is why a slightly oversized battery often costs less over ten years than a smaller one cycled deeply. The extra capacity buys shallower cycling, which is where the life comes from.
Practical design rule: size for a maximum of 80 % daily depth and treat 90 % as an occasional event.
Temperature is the variable people ignore
Calendar ageing accelerates at high temperature, and it happens whether or not the battery is cycling. A pack stored at 100 % state of charge in a hot cabinet will lose capacity without ever being used.
Two consequences for specification:
- Where the battery is installed matters as much as what it contains. A garage, a plant room and an outdoor cabinet are three different service lives for the same product.
- Liquid cooling on commercial systems is not a luxury. Holding cell temperature spread tight is what makes a ten-year throughput guarantee defensible rather than optimistic.
Charge profile is where warranty is quietly lost
A charger with an equalisation stage, continuous float at a high voltage, or temperature compensation will shorten a LiFePO4 pack's life without producing an obvious fault. The pack simply underperforms its rating, and by the time capacity retention is measured the cause is two years in the past.
This is the single most common reason a warranty claim is declined, and it is entirely preventable: check that the charger has a genuine LiFePO4 profile with no equalisation, no compensation, and a float at or below 3.35 V per cell — or no float at all.
What a defensible warranty looks like
A warranty that is worth having states three things: the period, the throughput or capacity retention it guarantees, and the conditions under which it applies.
- The period — five years on packs, ten on storage systems.
- Capacity retention — for example not below 80 % of rated capacity within the period, assessed against the end-of-line test log for that serial number.
- The conditions — depth of discharge, temperature range, current limits and the required charge profile.
A warranty without the third item is not enforceable in either direction, because the manufacturer cannot show a breach and the customer cannot show compliance. When you compare suppliers, compare what their warranty states, not how many years it claims.