The short answer is three to five years. The useful answer is that the number on the label is a starting point, and how you actually use the car decides where in that range you land. We have pulled four year old batteries that tested like new and two year old ones that were finished.
What actually wears a battery out
A lead acid battery does not die of old age so much as of shallow cycling. Every time you start the engine you take a big slug of current out, and the alternator puts it back over the following minutes of driving. If the drive is long enough, the battery spends most of its life full. If it is not, the battery sits at a partial charge, and sulphate builds on the plates in a way that does not fully reverse.
That is why the two shortest lived batteries we see belong to opposite drivers. One barely drives at all, so the battery never gets fully recharged. The other does nothing but short hops, which is the same problem wearing a different hat.
Heat is worse than cold
Cold gets the blame because cold is when the battery finally fails, but heat is what does the damage. High under hood temperatures accelerate the chemistry that degrades the plates. A battery that spent two summers baking is a battery that will not survive the first hard freeze, and it fails in January so January takes the blame.
Signs you are near the end
- The engine cranks noticeably slower on cold mornings than it did last winter
- You have needed a jump more than once in a few months
- The headlights dim more than they used to when you turn the key
- The battery is over three years old and has never been tested
None of these is proof on its own. A load test is, because it measures what the battery does under the load of an actual start rather than what it reads sitting still.
What to do about it
If the battery is under three years old and the car has been fine, do nothing. If it is over three and you are heading into winter, get it tested. A test tells you whether you have another season in it, and it costs a great deal less than being stranded on the coldest night of the year.
We test the battery and the charging system on every call, and if yours is healthy we say so.