Why does my car battery keep going flat?

Telling short journeys and a parasitic drain apart from a battery at the end of its life, and what actually recharges a battery properly.

Difficulty: beginner
Time: 25 min
Read: 4 min
Safety: warning

Short answer

Most repeat flat batteries are one of three things: journeys too short to replace what starting uses, something drawing current while the car is parked, or a battery over four or five years old that no longer holds charge. A battery test at a garage separates them in minutes and is usually free.

A battery is a buffer, not a fuel tank. Starting the engine takes a large amount out, and only sustained running puts it back. A pattern of short trips can flatten a perfectly healthy battery over a winter.

Safety

Car batteries contain sulphuric acid and give off hydrogen when charging, which is explosive. Wear eye protection, keep sparks and flames away, and never lean over a battery while connecting anything. When jump starting, the final clamp goes to a bare metal earth point on the flat car, well away from the battery — never to the flat battery's negative terminal. Wash any acid contact with plenty of water and seek medical advice.

The order to connect and disconnect jump leads

Step by step

Work out your usage pattern first.If most journeys are under fifteen or twenty minutes, the alternator never fully replaces what starting takes. This is the most common cause by a wide margin and it needs no parts at all.
Have the battery tested.Most garages and parts shops will test the battery and the charging system in a few minutes, usually free. It tells you whether the battery still holds charge, which is the fork in the road.
Note the battery's age.Four to five years is a typical life in the UK, less on a car doing many short trips. There is often a date sticker or a stamped code on the case.
Check the terminals and the earth strap.Corroded or loose terminals produce all the same symptoms as a dead battery. Clean the posts and clamps and refit them tight — negative off first, negative back on last.
Hunt for things left on.Interior lights, boot and glovebox lights, a badly closed door, a dash camera hardwired without a voltage cut-off, an aftermarket alarm or tracker. Any of these will empty a battery over a few days.
Ask for a parasitic drain test if nothing is obvious.A garage measures the current the car draws with everything switched off and everything asleep. A drain above the normal small figure identifies a fault circuit.
Recharge properly rather than jump starting repeatedly.A mains charger, run overnight, restores a battery far better than a jump start plus a short drive. Repeated deep discharge permanently damages a lead-acid battery.
Fit a smart charger or a maintainer if the car sits.A car used once a fortnight benefits enormously from a maintainer on a garage or driveway socket. It is the standard answer for a low-mileage car.
Take one proper run a week if you can.Half an hour of continuous driving, ideally including some at higher revs, does far more for the battery than several short trips totalling the same time.
Replace the battery with the right specification.Cars with stop-start need an AGM or EFB battery, not an ordinary one. Fitting a standard battery to a stop-start car gives it a very short life. Many modern cars also need the new battery registering to the car's electronics.

Tips

Keep a lithium jump starter pack in the boot. It removes the need for a second car and it fits in a glovebox.
Switch off the heated rear screen, heated seats and headlights before switching off the engine — they otherwise all come back on at the next start.
If the battery is under warranty, the garage test result is what you need to claim on it.

Common mistakes

Jump starting again and again — Each deep discharge permanently reduces the capacity of a lead-acid battery. Repeated jump starts turn a recoverable battery into a scrap one and never address the actual cause.
Fitting a standard battery to a stop-start car — Stop-start systems cycle the battery constantly and need AGM or EFB construction. An ordinary battery in that duty typically fails within a year.

If it doesn't work

Flat after standing three or four days

Cause: A parasitic drain — Fix: Look for lights left on or aftermarket electronics first, then have a garage measure the standing current.

Fine in summer, dies every winter

Cause: A tired battery revealed by cold and heavier electrical load — Fix: Have it tested. Cold reduces available capacity while demanding more from it, so a marginal battery fails in November.

Battery light on while driving

Cause: The alternator is not charging — Fix: This is a charging fault, not a battery fault. Head to a garage — the car is running on stored charge and will stop.

New battery flat within weeks

Cause: A drain, a charging fault, or the wrong battery type — Fix: Have both the charging system and the standing drain tested, and confirm the battery specification matches the car.

Questions people ask

How long does a car battery last?

Typically four to five years in the UK, shorter on cars used mainly for short journeys and longer on cars that do regular longer runs. Cold weather is what usually exposes one that is already weak.

Does driving recharge a flat battery?

Partly. An alternator is designed to maintain a battery rather than to bulk charge a deeply flat one. After a jump start, a long run helps, but a mains charger overnight does the job properly.

Is a battery maintainer worth it?

For any car used less than about once a week, yes. They cost little, plug into a normal socket, and prevent the slow damage caused by a battery sitting partly discharged for months.