Why do car batteries fail in winter here?
Cold does two things at once, and the combination is what kills batteries around Durham Region every January. It slows the chemical reaction inside the battery, so it delivers less current than it would in summer, and it thickens the engine oil, so the starter needs more current to turn the engine over. A battery that was quietly weak through the autumn is often fine until the first genuinely cold morning, then fails with no warning at all. Short winter trips make it worse, because the alternator never gets long enough to replace what starting used.
