It depends on how flat the battery is, its size and the charger's output. The AA says a full charge typically takes several hours. The RAC says a low-output trickle charger can take up to 48 hours. A smart charger shows when it has finished, so follow its indicator rather than guessing.
What decides the time
- How flat the battery is. A battery that's only a little low needs less time than one that's completely flat.
- Battery size. Bigger batteries hold more charge, so they take longer to fill.
- Charger output. The RAC notes that most trickle chargers only supply one to two amps, which is why they're slow.
Rough guide by charger type
- Smart or automatic charger: usually several hours (AA). It adjusts the rate and stops itself.
- Trickle charger: up to 48 hours for a full charge (RAC).
- Battery conditioner recovering a neglected battery: the RAC says this can take up to four weeks.
A worked example (illustration only)
If a battery is about half flat and the charger supplies a steady current, a rough estimate is the missing amp-hours divided by the charger's amps, plus extra time because charging slows as the battery fills. Your charger's manual will give the real figures.
Is driving enough?
Driving helps, but the RAC says a battery that has been run flat is best fully recharged with a quality charger. See will an alternator charge a dead battery?
Safety
Charge in a well-ventilated place away from sparks and flames. The HSE warns that batteries give off explosive gases while charging. For the step-by-step, see how to charge a car battery.
Frequently asked questions
Can I leave a charger on overnight?
A smart charger with automatic cut-off or maintenance mode is designed for this. A manual charger needs monitoring to avoid overcharging (AA).
How do I know it's fully charged?
Use the charger's indicator. Smart chargers show when they've finished.
Will a 30-minute drive fully charge it?
The RAC suggests driving for around 30 minutes after a jump, but says a full recharge is best done with a charger.