Car batteries fail for a small number of predictable reasons, most of which are accelerated by behaviors that are avoidable once understood. The lead-acid battery that sits in the engine bay of most conventional vehicles is a mature technology with well-documented failure modes and a service life that ranges from two years, in the worst conditions, to six or seven years with appropriate care. The difference is not accidental.

The primary enemy of battery longevity is deep discharge — allowing the battery to drain significantly below its nominal charge before recharging. Lead-acid batteries are not designed for repeated deep discharges. Each significant discharge-recharge cycle causes some sulfation of the lead plates — the buildup of lead sulfate crystals that reduce the plate area available for chemical reaction and permanently reduce the battery’s capacity. A battery that has been deeply discharged multiple times is a battery with reduced capacity that will struggle to start the engine in cold weather and will fail sooner than one that has been maintained at proper charge.