Car battery keeps draining overnight: find the drain
Updated 23 July 2026 · OBDCode UK editorial
Can you keep driving?
Do not repeatedly jump a swollen, frozen, leaking or hot battery. Use the handbook’s connection points and keep sparks away. If the car loses electrical power while driving, treat it as a charging fault and stop safely.
Match what the car is doing
| What you notice | Most likely | How to separate it |
|---|---|---|
| Flat after one night | Large parasitic load or failing battery | Battery health test, then sleep-current test. |
| New battery also goes flat | Drain, undercharging or incorrect battery registration/type | Check alternator strategy, coding and key-off current. |
| Flat after repeated short journeys | Energy deficit rather than one faulty circuit | Fully charge, then monitor use; cold starts consume more than a short run returns. |
| Problem began after dashcam/radio/tracker | Aftermarket wiring keeps circuit awake | Disconnect safely or test that circuit after vehicle sleep. |
Checks to do before buying parts
- Test state of health, not just voltage; use our battery test steps.
- Check running charge behaviour and voltage drop at cables/earths.
- Allow the locked vehicle to enter sleep mode before measuring current; opening a door wakes modules.
- Pull fuses or use voltage-drop-across-fuse methods systematically; do not guess from sparks at the terminal.
Typical UK costs
| Likely job | Typical fitted price |
|---|---|
| Battery charge/health test | £0–£50 |
| Battery fitted/coded | £100–£350 |
| Drain diagnosis | £80–£250+ |
| Alternator | £300–£650 |
Indicative 2026 independent-garage prices for mainstream cars; London, prestige, SUV and seized-part jobs can cost more. Diagnose first.
MOT relevance
Battery capacity is not measured like a breakdown test, but battery security, wiring condition and warning lamps matter. Low voltage can create misleading ABS, airbag and emissions faults.
- TOPDON BT100 battery tester12V battery health and cranking test in 10 seconds
- MotoPower battery testersBattery and charging-system testers for DIY checks
Frequently asked questions
Why does a new battery keep dying?
The original cause may be parasitic drain, undercharging, wrong stop-start battery type or missing battery registration. Test the system rather than replacing it again.
How much parasitic drain is normal?
It varies by vehicle and sleep state; many settled cars are below roughly 50 mA, but use manufacturer data and allow all modules to sleep.
Sources and further reading
General guidance for UK drivers, not professional advice. MOT rules follow the DVSA inspection manual (2026 edition).