Batteries function best at room temperature. Operating batteries at an elevated temperature dramatically shortens their life.

Li-ion performs better at high temperatures. Elevated temperatures temporarily counteracts the battery’s internal resistance, which is a result of aging. The energy gain is short-lived because elevated temperature promotes aging by further increasing the internal resistance.

There is one exception to running a battery at high temperature — it is the lithium polymer with dry solid polymer electrolyte, the true ‘plastic battery’. While the commercial Li-ion polymer uses some moist electrolyte to enhance conductivity, the dry solid polymer version depends on heat to enable ion flow. This requires that the battery core be kept at an operation temperature of 60°C to 100°C.

The NiMH chemistry degrades rapidly if cycled at higher ambient temperatures. Optimum battery life and cycle count are achieved at 20°C (68°F). Repeated charging and discharging at higher temperatures will cause irreversible capacity loss.