A lead-acid battery loses roughly a third of its cranking power at freezing point, and a cold engine needs more of it. That is the whole story behind the first frost of the year taking half the street with it.
A battery that has been quietly weak all autumn still starts a warm engine on a mild morning. It has no margin left for a cold one.
Short trips are the real killer
Fifteen minutes of city driving with lights, heated seats and a rear screen on does not replace what starting the car took out. Repeat that for a few weeks and the battery is never fully charged, which shortens its life as well as its reserve.
We test batteries under load, not just at rest — a resting voltage of 12.4 V tells you almost nothing. The test takes two minutes and we do it free with any winter check.
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