The car battery in winter: the warning signs before you get stranded

Batteries do not die in November: they weaken in summer and you notice in November. Heat speeds up internal wear, the cold only presents the bill: at low temperatures a battery delivers less current, and at the same moment the cold engine asks for more.

The signs, in the order they appear

1. Slower starting in the morning, the kind that lasts half a second longer. It is the first sign, and the one you stop noticing because it gets worse slowly.
2. The lights dimming visibly while the starter turns.
3. The electronics doing strange things: the display restarting, an error message that appears at start-up and disappears. Often it is not an electronic fault: it is the voltage dropping too far.
4. Start-stop no longer working. On many cars the system measures the battery's condition and switches itself off when it is no longer sure it can restart.

Measuring it, with a tester

You need a tester costing a few euros. The values below are indicative for a 12 V lead-acid battery; the exact values for yours are given by the battery manufacturer.

• With the engine off for at least an hour: a charged battery sits around 12.6 V or slightly more. Below 12.4 V it is starting to run down, and leaving it discharged damages it.
• While starting: if the voltage drops a lot, the plates can no longer hold the current, even if the battery looked fine at rest.
• With the engine running: the voltage must rise above the resting value, usually to between 13.8 and 14.6 V. If it stays low, the alternator is not charging; if it is much higher, the regulator is overcharging the battery.

A perfect resting voltage does not mean the battery is healthy. It only means it is charged. The real test is the drop under load, and any auto electrician can do it with a load tester.

Standard, EFB or AGM: not interchangeable

• Standard lead-acid: cars without start-stop.
• EFB: enhanced, for many cars with start-stop.
• AGM: electrolyte absorbed in glass fibre, for start-stop with brake energy recovery.

The right type is specified by the car manufacturer. You can go up (an AGM instead of an EFB), if the manufacturer allows it; you cannot go down: a standard battery in a car with start-stop wears out much sooner, because that system discharges and recharges it constantly.

On many recent cars the new battery has to be registered with the diagnostic tool, stating type and capacity: without that, the system keeps charging it with the old battery's profile.

The two things that wear it out early

• Short trips. A start uses charge that takes a drive of some length to recover. If you only ever drive a few kilometres at a time, you drain it a little every day.
• Months standing still. A parked car still draws current (alarm, control units on standby). If it stands for weeks, it needs a maintainer suited to the battery type, not just any charger.

This advice is general: for the values and procedures for your car, refer to the manufacturer's handbook.