Coin cells lose capacity sitting on a shelf even when nobody touches them. The rate depends on chemistry, storage conditions, and the SOC the cell was shipped at. For products with retail-channel inventory cycles of 6 to 18 months, self-discharge is often a bigger constraint than cycle life.
Self-discharge by chemistry
Three numbers worth memorising for the rechargeable coin cells we ship:
| Cell | Self-discharge at 25 °C | Self-discharge at 40 °C | Capacity at year 5 |
|---|---|---|---|
| LIR2032 | 4 to 6% / yr | 10 to 15% / yr | 65 to 75% |
| LIR2450 | 3 to 5% / yr | 8 to 12% / yr | 70 to 80% |
| ML2032 | 1 to 2% / yr | 4 to 6% / yr | 85 to 92% |
| ML2430 | 1 to 2% / yr | 4 to 6% / yr | 85 to 92% |
ML beats LIR substantially on shelf life because the Li-MnO2 cathode is more thermodynamically stable than LCO at the lower cell voltage (3.0 V vs 3.7 V nominal). For long-term storage applications the platform choice almost always lands on ML.
Why temperature is the dominant variable
Self-discharge follows an Arrhenius relationship — every 10 °C increase roughly doubles the rate. A cell shipped at 30% SOC and stored at 25 °C for 5 years holds 75% capacity. The same cell stored at 35 °C for 5 years holds 55%. At 45 °C the same 5 years lands at 35% — below the threshold most products need to function.
Practical implication: warehouse temperature matters more than nameplate self-discharge. If your distribution chain includes summer-month container ships through Suez (where ambient inside the container can exceed 50 °C for weeks), the cell needs to be ML and the inventory rotation plan needs to assume 6 month maximum dwell, not 18.
Why ship-out SOC matters
Cells shipped at 100% SOC age calendar-wise faster than cells shipped at 30% SOC. The cathode is most stable in the middle of its voltage window. Counter-intuitively, the customer who receives a cell at 30% SOC and uses 70% of it before recharging gets longer total service life than the customer who receives at 100% SOC and runs it down to 30% before recharging.
Our standard ship-out SOC for ML and LIR coin cells is 30% ± 5%. We document this on the carton and the test report. Customers who need 100% SOC at receipt (typical for distributors who consumer-package and resell) get a separate SKU with a higher unit price and a shorter inventory rotation requirement.
The 5-year shelf life math, worked
For a smart-meter calibration retention application using ML2430 (110 mAh nominal):
- Ship at 30% SOC = 33 mAh delivered
- Storage 18 months at 30 °C average = -2.7% of capacity = -3 mAh = 30 mAh remaining at install
- Calibration retention current = 0.5 µA average
- Available time = 30 mAh / 0.5 µA = 60,000 hours = 6.8 years
- De-rate by 30% for cathode ageing over 5 years in service = 4.7 effective years
The 4.7 year service life sits below the 10-year product target — the firmware schedules a recharge top-up every 18 months to bring SOC back to 80% during normal calibration cycles. Without that top-up, the device would need a larger cell (ML2450 at 200 mAh) to clear 10 years.
What we recommend
For inventory cycles up to 12 months: LIR is fine, ship at 30% SOC, store below 30 °C.
For inventory cycles 12 to 36 months: ML is the better default, ship at 30% SOC, store below 35 °C.
For inventory cycles > 36 months: ML at 30% SOC, store below 25 °C, plan for inventory rotation every 24 months as a safety margin against high-temperature distribution events you cannot control.