Car battery: how long it lasts, how to check it, and what a new one costs

What the 12V battery does

The battery is a box of lead plates in acid that stores electricity. It has three jobs:

• Starting the engine. The starter motor needs a huge burst of current for a few seconds, and only the battery can deliver it.
• Powering the electronics when the engine is off: the locks, the alarm, the clock and dozens of control units that stay awake while the car is parked.
• Helping out while you drive. Once the engine runs, the alternator produces the electricity and recharges the battery. The battery smooths out peaks, for example when many consumers are on at once.

Start-stop cars ask much more of the battery: it restarts the engine at every red light and powers everything while the engine is off. They use stronger battery types (EFB or AGM, see below).

Electric and hybrid cars have one too. Next to the big drive battery there's a small 12V battery. It powers the locks, the screens and the control units, and it switches on the high-voltage system. If it's flat, the car won't "start", even with a full drive battery.

Why batteries wear out

A battery loses capacity a little with every charge and discharge, and with age. What speeds it up:

• Short trips. Starting takes a lot of energy, and a 10-minute drive doesn't put it all back. Over weeks, the battery runs lower and lower.
• Cold. A cold battery delivers less power, while a cold engine with thick oil needs more to turn over, and heaters, lights and wipers all draw current. That's why batteries fail on the first frosty mornings.
• Heat. Summer heat speeds up the chemical ageing inside, even though the battery then fails in winter.
• Standing still. A parked car slowly drains the battery. After a few weeks without driving, it can be too weak to start.
• Deep discharges. Leaving the lights on until the battery is completely flat damages it for good, and each deep discharge shortens its life.

When to replace it

Batteries don't have a fixed schedule: they're replaced when they test weak or start to struggle. Most last 4 to 6 years. Cheap batteries and cars used for short trips sit at the low end, and premium batteries in cars that make long journeys at the high end.

Kilometres, driving style and your maintenance level don't change this: battery life is counted in years. Track2Drive asks you to check the battery from about 4 years old.

How to check your battery

With a multimeter

A cheap multimeter (under €20) tells you the battery's state of charge:

1. Leave the car parked for at least 6 hours with everything switched off, so the reading isn't distorted by recent driving or charging.
2. Set the multimeter to DC volts (20 V range). Hold the red probe on the battery's + terminal and the black probe on the − terminal.
3. Read the voltage:

A good voltage shows that the battery is charged, not that it's healthy: an old battery can show 12.6 V and still collapse when the starter draws current. Only a load test shows that.

At a workshop

Many workshops and battery shops test the battery for free with a tester that measures how much current it can still deliver. It takes a couple of minutes. Do it before each winter once the battery is about 4 years old.

Which battery does your car need?

The label on your current battery says its type, capacity (Ah) and cold-start current (A). The new one should match them.

• Never fit a weaker type than the original. A standard battery in a start-stop car wears out quickly, and start-stop may stop working.
• Match the size and the terminals (position of + and −), so the cables reach and the battery is held down properly.
• Higher capacity is fine, within reason, if it fits.

Warning signs

• The engine turns over slowly when you start, especially on cold mornings.
• A click but no start, or the dashboard lights flicker when you turn the key.
• Start-stop stops working. Many cars switch it off first when the battery gets weak.
• Dim headlights or interior lights with the engine off.
• The battery warning light comes on while driving: often a charging fault (alternator or belt) rather than the battery itself.
• Electronics act up: warning messages, a radio that resets, a key that isn't recognised.
• The battery is swollen, leaking or corroded around the terminals.

How to jump-start a car safely

You need jump leads (at least 2 m long, 25 mm² thick for most cars) and a donor car with a battery of the same voltage, or a charged booster pack. Check both owner's manuals first: some cars have dedicated jump-start points in the engine bay.

1. 1Park the cars close but not touching, switch both off, and put on the parking brakes. On a road: hazard lights, warning triangle and high-visibility vest.
2. 2Red lead to the flat battery's + terminal.
3. 3Other end of the red lead to the donor battery's + terminal.
4. 4Black lead to the donor battery's − terminal.
5. 5Other end of the black lead to bare metal on the flat car: the engine block or the earth point named in the manual. This still closes the circuit: the flat battery's − terminal is already connected to the engine and body by the car's own earth cable. Don't clamp it to the flat battery's − terminal itself: the last connection can spark, and a battery can give off flammable gas; on start-stop cars it can also damage the battery sensor.
6. 6Start the donor car and let it run for a few minutes.
7. 7Start the flat car. If it doesn't start, wait a minute and try again. After the third failed attempt, get professional help.
8. 8Switch on a few consumers in the jump-started car (fan, rear window heating) to protect the electronics from voltage spikes.
9. 9Remove the leads in reverse order: black from the jump-started car, black from the donor, then the red leads.
10. 10Drive for at least 30 minutes, or charge the battery with a charger, before switching off again.

Electric and hybrid cars

• The 12V battery is the most common breakdown cause for electric cars too. It powers the electronics and switches on the high-voltage system, so a flat one means the car won't drive.
• An electric car can usually receive a jump-start, to the 12V battery or the jump-start points in the manual.
• It usually must not give one. Most manufacturers don't allow using an EV or hybrid to jump-start another car: its small 12V battery and electronics aren't made for it. Check the manual.
• Long periods parked can drain it, especially with the car connected to an app. Follow the manual's advice for longer parking.

What happens if you don't replace it

1. 1Slower starts. The engine turns over a little more slowly, especially in the cold.
2. 2Start-stop switches itself off. The car protects the weak battery, and you lose the fuel saving.
3. 3Electronic faults. Low voltage can trigger warning lights and error messages that look like serious faults.
4. 4The car won't start. Usually on a cold morning, often when you're in a hurry.
5. 5More damage. A battery that's often flat strains the alternator and starter, and deep discharges finish it off for good.

How to make your battery last longer

• Take a longer drive now and then, at least 30 minutes, if you mostly make short trips.
• Use a smart charger if the car is parked for weeks, or on a battery you know is weak. Use one suited to AGM if that's your type.
• Switch things off before you switch off the engine: lights, heated seats, rear window heating.
• Keep the terminals clean and tight. Dirt and corrosion make it harder to charge.
• Park in a garage in winter if you can: a warmer battery starts the engine more easily.
• Have it tested every autumn once it's about 4 years old.

How a battery is replaced

1. 1Check the car's requirements: the right type, size and terminal layout, and whether the new battery has to be registered with a diagnostic tool.
2. 2Keep the settings: some workshops connect a memory saver so the radio, clock and control units don't lose their settings. Have the radio code ready on older cars.
3. 3Switch everything off and remove the key, or keep it away from the car.
4. 4Disconnect the negative (−) terminal first, then the positive (+). This avoids a short circuit if the spanner touches the body.
5. 5Undo the hold-down clamp and lift the battery out. It's heavy (15–25 kg): lift with your legs.
6. 6Fit the new battery, secure the clamp, and connect positive (+) first, then negative (−).
7. 7Register the new battery if the car needs it (many BMW, Mini, VW, Audi, Mercedes and Volvo models, among others), so the car charges it correctly.
8. 8Check that everything works, reset the windows' one-touch function and the clock if needed, and hand in the old battery for recycling.

Many batteries sit under a seat, in the boot or behind panels on newer cars, which makes the job longer.

How much a new battery costs

A standard battery for a typical family car costs about €120, the same everywhere. Fitting takes about 20 minutes, so labour changes the price only a little between countries. Figures are estimates for an independent workshop, VAT included.

Questions people ask