Batteries almost always warn you. The problem is that the warnings are easy to dismiss, because the car does still start, right up until the morning it does not.
Slow cranking, especially when cold
The clearest early sign. Compare it against your own memory: if the engine turns over more lazily this winter than last, capacity has gone somewhere. It rarely comes back.
You have needed a jump already
Once, with an obvious cause like a light left on, means very little. Twice in a few months, with no explanation, means the battery is no longer holding what it is given.
Lights that dim when you crank
Some dimming is normal. A pronounced drop, where the dashboard visibly fades as the starter engages, means the battery cannot support both jobs at once.
Electrical oddities at idle
Flickering interior lighting, a stereo that resets, or windows that move more slowly than they should can all point at a battery that is struggling. So can dashboard warnings that appear in groups and then clear.
Age
Three to five years is the normal window. Past three, the battery is on borrowed time and worth testing regardless of symptoms. Past five, it is running on luck.
A swollen or leaking case
A case that bulges at the sides has usually been overheated or overcharged. Any sign of leakage, or a strong smell of rotten eggs, means stop using it and stop charging it. That is not a wait and see situation.
The one test that settles it
A load test measures what the battery does when the starter actually pulls current from it, which is the only question that matters. Resting voltage does not answer it. A battery can read a healthy figure and still fail to turn an engine over.
We test the battery and the charging system on every call. If the numbers say you have another season in it, we tell you that and you keep your money. Selling a battery to somebody who does not need one is a poor way to earn a repeat customer.
Do not wait for winter to ask
The cheapest time to deal with a marginal battery is on a mild afternoon when the car is at home. The most expensive is at eleven at night in January in a car park.