🚗 Car Problems
How to Fix a Car Battery That Keeps Draining
A battery that keeps going flat points to either a failing battery, a faulty alternator, or a parasitic drain — something drawing power when the car is off.
Likely causes
- Faulty alternator not recharging the battery
- Old battery that can no longer hold charge
- A door, boot, or bonnet light staying on
- Parasitic drain from a faulty module or aftermarket accessory
- Leaving electrical items on when parked
Step-by-step fix
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1
Get the battery tested
Most car parts shops (Halfords, Euro Car Parts) will test your battery for free. A healthy 12V battery should read 12.6V fully charged.
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2
Get the alternator tested
With the engine running, a healthy alternator outputs 13.7–14.7V. Your car parts shop can test this with a multimeter at no charge.
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3
Check for interior lights staying on
Ensure all doors, the boot, and the bonnet are fully closed. An interior light left on overnight will flatten most batteries.
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4
Check for recent accessories
Have any accessories been fitted recently — dashcams, sound systems, trackers? A poorly wired accessory can draw power constantly.
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5
Do a parasitic drain test
A mechanic can use a multimeter to measure current draw with the car off. More than 50mA draw indicates a parasitic fault.
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6
Replace the battery if old
If the battery is over 4 years old and tests poorly, replace it. A new battery from Halfords typically costs £60-120 and includes fitting.