Heat is the warranty

A battery warranty is usually written in cycles. A battery in the field lives in degrees. The two are connected, and the connection is the part most buyers do not see.

What heat does

Lithium cells age faster at higher temperature. The rate is not linear: a pack held a few degrees warmer for a year loses capacity noticeably faster than one kept cooler, even if the cycle count is identical. Heat also increases the chance that one cell group drifts away from the others, which reduces usable capacity before the pack has failed.

Where the heat comes from

• The cells themselves, under charge and discharge.
• The enclosure, which can trap heat if airflow was an afterthought.
• The site: a tin roof, a south-facing wall, a cabinet in direct sun.
• The duty cycle: a fast charge or a heavy load produces more heat in less time.

What good thermal design looks like

Space between cells is not wasted space. A path for air, a case that does not soak in sun and a BMS that derates before the cells are damaged matter more than a headline peak current.

Active cooling helps, but it adds a fan, a filter and a maintenance item. For many packs, passive design done properly is more reliable than active design done cheaply.

The field test

Touch the enclosure at the end of the hardest shift. If it is uncomfortably hot, the cells are hotter. That is the number the warranty is really about.

The buying question

Ask what the pack does at 45°C ambient, not what it does at 25°C. Ask whether the BMS derates on temperature and how it decides. The answer tells you more about service life than the cycle count on the brochure.