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when does a car battery need replacing?

most car batteries last three to five years in southwestern Ontario. some reach seven. some die at two.

the spread is that wide because what kills a battery is not age so much as what happened to it: heat, deep discharges, short trips that never fully recharge it, and a charging system that was quietly overcharging the whole time.

the signs, in the order you usually notice them

  1. slow cranking. the engine turns over lazily before it catches, particularly on the first cold morning of the year. this is the classic warning and most people ignore it.
  2. headlights dimming at idle, then brightening when you rev.
  3. the battery light, which is really a charging system light and can mean the alternator rather than the battery.
  4. electrical oddities. windows slow, the radio resetting, warning lights appearing in combinations that make no sense.
  5. it needs a boost. by this point it is not a warning, it is the failure.

do not wait for it to fail

a battery that dies does it at the least convenient moment, because the least convenient moment is when it is coldest, and cold is what finds a weak battery.

a battery test takes a few minutes and tells you what capacity is left rather than whether it starts the car today. almost any battery starts a car in September. the question is whether it will in January.

get it tested, and get the charging system tested with it

we test both, because they are the same problem twice. an alternator that overcharges cooks a battery, and one that undercharges never fills it. either way you replace the battery, it dies again, and nobody has looked at the thing that caused it.