How do you know if your car battery is failing?

The most reliable early sign of a weak battery is an engine that turns over slowly, especially on a cold morning. The starter has to work harder as the battery loses its ability to deliver current, and you may hear the engine cranking with less urgency than it used to.

A battery rarely fails without clues, but the clues are easy to put down to the weather. Knowing what to look for, what else can mimic a bad battery and what a garage will test, helps you avoid being stuck at the worst time.

What are the signs of a dying car battery?

Slow cranking is the classic symptom, but there are others. Headlights that dim when you start the engine, a clicking sound instead of a start, or a dashboard that flickers can all point to low battery power. Electrical items such as windows or the radio may behave oddly too.

A battery warning light on the dashboard needs a little care. It often indicates a charging fault rather than a battery fault, so it should not be assumed that a new battery will cure it.

• Slow or hesitant engine cranking
• Dim headlights or interior lights, especially at start-up
• Clicking noises when you turn the key
• Dashboard lights flickering or resetting
• A battery that is flat after the car has stood for a few days

Why does a car battery fail?

Age is the most common reason. A battery is a consumable part, and how long it lasts depends on the vehicle, the climate and how the car is used. Repeated short journeys are hard on batteries, because the engine does not run long enough to replace the energy used in starting.

Other causes include corroded or loose terminals, a fault that drains the battery when the car is parked, leaving lights on, extreme cold, and a charging system that is not topping the battery up properly. A battery can also be damaged by repeatedly being run flat.

What can I check safely at home?

With the engine off, look at the battery and the terminals. Check that the cable clamps are tight and that there is no white or green crusty corrosion. Look for a cracked case, a bulging or swollen shape, or any signs of leaking. Some batteries have a date code on a sticker, although you may need your handbook to interpret it.

If you need a jump start, check your handbook first, as some modern cars have specific connection points and procedures. Never attempt one on a battery that is cracked, frozen or leaking, and keep sparks and naked flames away, because batteries can release flammable gas.

How does a garage test a car battery?

A technician will test how well the battery holds charge under load, and measure the voltage with the engine off and running. This tells them whether the battery is weak or whether the charging system is the real issue. Checking the alternator output is an essential part of any battery check.

If the battery keeps going flat with no obvious reason, a drain test finds out whether something is using current while the car is parked. At Automaster, a garage at Longcombe Cross near Totnes, we explain whether the battery or something else is at fault before recommending a replacement. The price depends on the type of battery your car needs, and whether fitting it means any programming or resetting of the vehicle's systems, so please ask for a quote.

When should I stop driving with a battery problem?

If the battery warning light comes on while you are driving, the charging system may have stopped working. The car is then running on battery power alone and could stall once that runs out, which may mean losing power steering assistance and lights. Head for a safe place promptly and switch off non-essential electrics.

Stop using the car if you notice a rotten egg smell, a hot or swollen battery, or fluid leaking from it. Do not keep trying to start a car that will not turn over, as repeated attempts can overheat the starter and flatten the battery further.