Cold: internal resistance rises
At low temperature the electrolyte and electrode kinetics become sluggish. The lithium ions move more slowly and internal resistance rises. In concrete terms, the extractable capacity at around −20 °C drops, depending on the cell, to roughly 60 to 80 % of the rated value, and under load the voltage sags more sharply. This effect is reversible: as soon as the cell is warm again, the capacity returns. In the cold you have less range, but no lasting damage — as long as you are only discharging.
Charging below 0 °C: the one unforgivable mistake
During charging, cold is no longer a reversible effect. If the cell is too cold, the lithium ions cannot intercalate into the graphite lattice of the anode fast enough. Instead of being intercalated, metallic lithium deposits on the anode surface — lithium plating. This is permanent capacity loss, and the dendrites that form can eventually pierce the separator and trigger an internal short circuit. Hence the iron rule: no charging below 0 °C without cell heating. In this range the battery management system (BMS) must block the charge current or throttle it to a non-critical fraction.
Heat: calendar ageing runs away
Heat accelerates the side reactions in the cell, above all the growth of the SEI layer on the anode, which consumes active lithium in the process. As a rule of thumb the Arrhenius relationship applies: for roughly every additional 10 °C the calendar ageing rate doubles. A cell that is permanently stored at 40 °C instead of 25 °C therefore loses capacity roughly twice as fast — regardless of whether it is used or not. The most damaging condition is the combination of high temperature and a high state of charge. Unlike with cold, this capacity never comes back.
The optimal range in which both effects stay small is around 15 to 25 °C. The further real-world operation deviates from this, the more the thermal concept — not the cell alone — decides service life.
Rules for the design
- Specify the operating window and the charging window separately — they are not identical. Discharging goes far lower than charging.
- Never charge below 0 °C without heating. A BMS charge inhibit plus a heating film / self-heating is mandatory, not optional.
- Cold is reversible, heat is irreversible — plan reserve capacity for the cold, and actively counter heat build-up and a high sustained state of charge in the heat.
- Clarify the real temperature profile before selecting the cell. For wide ranges and high ambient temperatures, LiFePO4 is usually the more robust basis thanks to its more stable chemistry.
- Avoid storage at high temperature — 40 °C at a high SoC is the most expensive state for a battery, even at standstill.
The most common mistake is to assume a single temperature window for charging and discharging. Once you put your real usage profile on the table — lowest charging temperature, highest sustained temperature, required runtime — the thermal management can be designed to fit exactly.