Car battery in winter: warning signs it is about to fail

A car battery fails more often in winter because in very cold weather it loses up to 20% of its capacity, just when the engine needs more power to start. The warning signs are clear: slow cranking, lights dimming when you turn the key, the battery warning light on and, above all, age (a battery lasts 3 to 5 years). If you notice any of them, test it before a cold morning leaves you stranded.

Why does a car battery fail in the cold?

A car battery runs on a chemical reaction, and cold slows it down. According to the RACE (Spain's largest motoring club), in very low temperatures a battery can lose up to 20% of its capacity. At the same time, the oil is thicker and the starter motor needs more effort to turn the engine. Less energy available and more energy needed: if the battery was already weak, winter is when you find out.

That is why, according to the RACE, the battery is the number one reason for roadside assistance callouts over the early December bank holiday weekend (the puente de diciembre), when much of Spain takes to the roads. The cold does not break it in one go: it exposes wear that was already there.

Don't assume a mild coastal winter spares you. A car that spends cold nights inland or up in the mountains, or one left parked for weeks while you are away, can let you down just the same.

Age matters a lot. The RACE puts a battery's lifespan at 3 to 5 years, and from the fifth year it can fail at any moment, with no further warning. If you do not know when the battery in your car was changed, look for the invoice or check whether the battery itself has a date marked on it.

Warning signs your car battery is about to fail

• Slow cranking. The engine turns over lazily, as if it were struggling, especially on the first start of the morning.
• Dim lights. The headlights or dashboard lights dim when you start the car or with the engine idling.
• Battery warning light on. The battery symbol on the dashboard warns of a charging problem. It may be the battery, but it can also be the alternator or the belt that drives it.
• Odd electrical faults. The clock resets, the electric windows are slow, the radio restarts when you start the car.
• Corroded terminals. White or greenish powder around the terminals.
• Age. A battery more than 4 or 5 years old is suspect even if it has not given any trouble yet.

How to test a car battery with a multimeter

A basic multimeter is cheap and gives you an answer in a minute.

1. Leave the car switched off for a few hours (ideally overnight) so you measure the resting voltage.
2. Set the multimeter to DC voltage (V with a straight line), on the 20 V range.
3. Touch the positive terminal (+, usually red) with the red probe and the negative terminal (−) with the black one.
4. Write down the reading.
5. Start the engine and measure again to see whether the alternator is charging.

Car battery voltage table (a guide only)

These are standard technical values and only a guide: every battery and every car can vary slightly. A battery can show a good resting voltage and still fail to start the car if it is badly worn; a garage can do a load test to settle the question.

How to look after your car battery in winter

The battery recharges while you drive. Anything that uses power without giving it time to recover drains it little by little.

Short trips

Starting the engine is what uses the most power. If your trips are five minutes long, the alternator barely has time to put back what you used to start. Every now and then, take a longer drive of at least half an hour.

A car parked for many days

Even when it is switched off, a car uses a little power (the alarm, the ECU, the remote central locking). If you leave it parked for two or three weeks in winter, it may not start when you get back. This is common if you travel home for Christmas and leave the car in Spain. If you are not going to use it, start it and drive it for a while every week, or use a battery maintenance charger.

Using power with the engine off

Listening to the radio, leaving the sidelights on or charging your phone with the engine off draws directly on the battery. In winter, it is better to avoid it.

Find out before the day it won't start

The trouble with batteries is that they give little warning. One day the car starts slightly worse, the next day the same, and on the third it does not start at all. Nobody tests their battery with a multimeter every week.

The Vehistrack car GPS tracker plugs into the OBD port and, as well as the location, monitors the battery voltage and whether the alternator is charging properly. If the voltage drops below normal, you know before you are stranded on a December morning. We go into more detail in what to look for in a car GPS tracker.

How to jump-start a car step by step

If your battery leaves you stranded, you can jump-start the car with jump leads and another car. Take your time and follow this order.

1. Before you start: both cars switched off, handbrakes on, not touching each other. Check that both batteries are 12 V. If the flat battery is swollen, cracked or leaking, do not jump-start it.
2. Red clamp to the positive (+) terminal of the flat battery.
3. The other red end to the positive (+) terminal of the good battery.
4. Black clamp to the negative (−) terminal of the good battery.
5. The other black end to a bare, unpainted metal part of the engine of the car that won't start, away from the battery. Not to the negative terminal of the flat battery: that way you avoid sparks near it.
6. Start the working car and let it idle for a few minutes.
7. Try to start the car with the flat battery. If it does not start, wait a little before trying again.
8. Remove the clamps in reverse order: first the black one on the bare metal, then the black one on the good battery, then the red one on the good battery and finally the red one on the flat battery.
9. Drive for at least half an hour so the alternator can recharge it a little, or take it straight to be checked.

If your car has an automatic start/stop system or is a hybrid, check the manual first: some have specific connection points under the bonnet.

When should you replace your car battery?

• When it is more than 4 or 5 years old and starting to show symptoms.
• When at rest it does not reach 12.4 V shortly after charging, or goes flat within a few days.
• When you have had to jump-start it more than once in a short time with no clear reason (a light left on does not count).
• When it has visible damage: a swollen case, leaks or badly corroded terminals.

Write down the date and the mileage of the change. Four years from now you will not have to guess how old it is; it is the same logic as a car maintenance schedule by mileage.

Frequently asked questions

How many years does a car battery last?

According to the RACE, 3 to 5 years. From the fifth year it can fail at any moment, and winter is usually when it does.

How long can a car stay parked before the battery goes flat?

It depends on the car and on the condition of the battery. With a battery in good condition, a couple of weeks is not usually a problem; with cold weather and an old battery, much less can be enough.

Is it better to charge the battery or jump-start the car?

Jump leads get you out of trouble, but they do not fully recharge the battery. If it has gone very flat, the best thing is to charge it afterwards with a charger and check its voltage.

Why does a car battery fail more in winter if heat also wears it out?

Heat wears it down over the summer and the cold exposes that wear: at low temperatures it loses capacity and the engine needs more power to start.

Does the battery warning light always mean the battery needs replacing?

No. It warns of a charging problem, which may be in the battery, the alternator or its belt. Have it checked as soon as possible.