Battery Temperature Derating

Formula

Description

Battery capacity decreases at temperatures below the rated specification (typically 25°C) because the internal chemical reactions slow down and internal resistance increases. At 0°C, lithium-ion batteries deliver roughly 70-80% of their rated capacity, and at -20°C only 40-60%. At elevated temperatures, capacity may slightly increase but cycle life degrades rapidly. The derating factor is obtained from the manufacturer temperature-capacity curves and applied to the nominal capacity to determine the usable capacity at the operating temperature.

Variables

• Ceff — Effective usable capacity at operating temperature (mAh)
• Cnom — Nominal capacity at 25°C (mAh)
• factor — Temperature derating factor (0 to 1)

Practical Notes

Typical derating factors for Li-ion: 0°C = 0.75, -10°C = 0.60, -20°C = 0.45, -30°C = 0.30. Lead-acid batteries have even steeper derating. For outdoor or automotive applications, always size the battery for the minimum expected temperature. Battery heaters are used in EVs and satellite applications to maintain operating temperature.