The most common causes
A light was left onSidelights can drain the battery in less than 24 hours.
A door or the boot is not closed properlyOn some cars the computers then stay awake and draw power for hours.
A device was left plugged inA dashcam, a phone charger or a sat nav can draw power even when the engine is off.
An alarm, GPS tracker, camera or radio fitted after the factoryThey draw power even while the car is parked.
The alternator is not chargingThe battery is not charged while driving, so it soon goes flat even when it is new.
The battery is oldOver time it holds its charge less and less, so the cold or a single light left on soon drains it.
These causes are common too
- You only drive short distances. The alternator does not have time to top up the battery.
- Corroded terminals or a loose cable. The terminals are the two points on the battery where the cables are connected. A poor connection carries power less well, so the battery does not charge properly.
- A faulty part or a damaged cable. A part that does not stop working when you switch the car off draws power while the car is parked. A cable can be damaged by moisture, rubbing or a marten chewing it.
If even a new battery goes flat overnight
If the battery is new but flat again after a night or a few days, something is drawing power when it should not, or the alternator is not charging it. Without a measurement, batteries get replaced one after another and the fault stays.
A sound car, locked and with a healthy battery, starts even after standing for several weeks. When you lock it, the computers in the car go to sleep after a while and draw very little power.
How it is measured: once the computers are asleep, across the fuses without pulling them, because pulling a fuse and putting it back wakes the computers. After the repair, the draw while parked and the charging while the engine runs are measured again.
Check the charging before a new battery
If the alternator doesn't charge, it will drain a new battery too. More about alternator repair
Signs of a charging fault
- The battery warning light stays on while driving. More about the battery warning light
- The lights flicker.
- The car starts with jump leads and dies straight away. Then the alternator is probably not charging the battery.
A jump start gets the car going, but the cause of the flat battery stays.
Jump leads connected in the wrong order can damage the electronics in the car.
Battery or charging: it depends on the battery's age
- Under 5 years old and the car struggles to start: the cause is usually poor charging, short trips or a fault that drains the battery.
- Over 5 years old: it is more often the battery itself that is worn out. More about battery replacement
Why a battery goes flat in the cold
In the cold a battery gives less power and holds its charge less well. In winter the battery also charges more slowly, while the car uses more, because the heating, heated screens and seats, lights and wipers are on.
An old battery that started the car without trouble in summer can go flat very quickly in the cold. That is why a check before winter matters, especially when the battery is older than 3 years. More about the battery in winter
Going flat often weakens the battery
A battery that often goes flat holds its charge less and less. The longer it stands flat, the greater the chance that it will no longer charge and needs replacing.
How to stop the battery going flat
- Before switching off the engine, turn off the lights, heating, wipers and radio.
- When you get out, check that the lights are off and the doors and boot are properly closed, then lock the car. If a door or the boot is not properly closed, some cars may not start after only 5 days.
- Unplug the dashcam, charger and other devices from the socket.
- Drive regularly, and for at least 30 minutes at a time, so the alternator can top up the battery. A battery that is regularly not fully charged can last half as long.
- Keep a car you rarely drive on a smart charger. It can stay connected for a long time without harming the battery. A car with start-stop, which switches the engine off by itself when you stop at traffic lights, has an EFB or AGM battery. It needs a smart charger.
