A cycle-life claim is only meaningful with five conditions attached: charge rate, discharge rate, depth of discharge, temperature, and what "end of life" is defined as. "80% capacity after 800 cycles" states one of those five. Without the other four it is not a specification, it is a headline.
The five conditions
| Condition | What it controls |
|---|---|
| Charge rate and cut-off | Fast charging and a high charge cut-off accelerate most degradation mechanisms. |
| Discharge rate or power | A cell cycled at 100 W ages differently from the same cell cycled at 1 C. |
| Depth of discharge | 100% DoD and 80% DoD are not comparable; the shallower cycle usually yields more cycles. |
| Temperature | Cycle life quoted at 25 °C does not describe a pack that runs at 45 °C. |
| End-of-life definition | Capacity retention and runtime retention are different endpoints and give different numbers. |
The end-of-life definition is the one that gets missed
Manufacturers use two different endpoints, and they are not equivalent:
- Capacity retention — the cell still holds X% of its original amp-hours, measured at a low rate. This is the number that goes into a warranty.
- Runtime retention — the cell still delivers X% of the original operating time on a defined power load. Because it is measured under load, it falls faster than capacity on a high-power cell, and it is often the number that matters in a tool or a drone.
A concrete case from documents we hold. Molicel states for the INR18650-P30B: 500 cycles at 100 W discharge with 75% runtime retention. The same manufacturer states for the INR21700-P45B: 500+ cycles at 100 W discharge with 80% capacity retention. Same manufacturer, same discharge power, different endpoint. Read as "500 cycles" and "500 cycles" they look equivalent; they are not measuring the same thing and cannot be compared to each other.
What the silicon anode changed
The INR21700-P50B is stated at 700 to 1400 cycles under a 100 W discharge with a 1 C charge, roughly double the P45B under comparable conditions, attributable to its silicon-carbon anode. That is a genuine generational improvement and it is also a range rather than a number — 700 to 1400 is a wide band, and where a given pack lands in it depends on the four conditions above rather than on the cell alone.
Two other models in the same series, the INR18650-P30S and INR21700-P50S, have no cycle figure in their data sheet. We do not publish a number for them. A cycle-life figure that appears for those models elsewhere is not from the manufacturer document, and it should be treated the same way as any unsourced value.
How to write the requirement
If you are specifying cycle life to a supplier, write it as the test, not as the number:
- Charge: rate, cut-off voltage, cut-off current, temperature.
- Discharge: rate or power, end voltage, temperature.
- Depth of discharge, and whether there is a rest period between cycles.
- End point: capacity retention, measured at what rate — or runtime retention, on what load.
A supplier who can quote against that is quoting a test. One who answers "about 800 cycles" is quoting a number from somewhere, and you will not find out where until the pack is in the field.
For storage duty, where the cycle count is the economic basis of the whole product, the requirement belongs in the enquiry rather than in the datasheet review — see the energy storage cells page for the capacity and rate envelope those packs are usually specified within, and the cell selection guide for where cycle life sits in the ordering of the decision.
Last updated: October 2026
