Operating temperature is the single biggest reason customers pick ML over LIR — and yet most datasheets stop at +60 °C. This article gives the actual capacity and impedance data we measure on a per-batch basis from -40 °C to +85 °C, and the design rules that follow.
What the curves look like
From representative test cells (ML2032, sample size n=30, cycled fresh):
| Temperature | Capacity vs. 25 °C baseline | Internal resistance vs. 25 °C |
|---|---|---|
| -40 °C | 40–55 % | 4–6× |
| -20 °C | 72–82 % | 2.0–2.5× |
| 0 °C | 88–94 % | 1.4× |
| +25 °C | 100 % (baseline) | 1.0× |
| +60 °C | 96–99 % | 0.85× |
| +85 °C | 92–95 % (1st use), accelerated ageing thereafter | 0.75× initially |
For LIR2032 the cold-temperature numbers are roughly 10 % worse, the hot-temperature ageing accelerates more sharply above +60 °C.
Five design rules
Rule 1 — design for end-of-life and the cold corner together. If the device must run at -20 °C after 500 cycles, your headroom factor is 0.8 (cycle ageing) × 0.78 (cold derating) = 0.62. Spec a cell ≥ 1.6× the runtime requirement.
Rule 2 — never charge below 0 °C. Both chemistries lithium-plate aggressively under cold charge. A PTC thermistor in series with the charge path prevents firmware mistakes.
Rule 3 — current pulses get bigger at cold. If your firmware has a 50 mA radio burst, it draws nearly 200 mA equivalent of impedance loss at -40 °C. Your reservoir capacitor needs to be sized for the cold corner, not the room-temperature spec.
Rule 4 — above +60 °C, treat the cell as a calendar-life part. ML at +85 °C ages roughly 3× faster than at +25 °C in calendar terms, even unloaded. Plan for replacement (if accessible) or a shorter product life. We do not honour cycle-life specs above +70 °C average operating temperature.
Rule 5 — pre-condition before measuring. A 12 °C ramp from cold storage to test condition takes ~30 minutes for an ML coin cell to thermalise internally. Capacity tests immediately after cold storage typically read 5–8 % low.
What we ship into automotive and outdoor IoT
Most automotive ECU RTC/event-log applications use ML2032 with an automotive-grade electrolyte rated -40 °C to +85 °C continuous, +125 °C peak (e.g., engine compartment). For outdoor IoT (asset trackers, smart agriculture sensors) we typically recommend ML2430 in a heated micro-compartment driven by the device''s main heater when present.