Car battery dies overnight
A battery that is flat every morning but fine after a drive has either lost its ability to hold charge or is being drained while parked. Separate the two with one measurement: resting voltage after the car has sat an hour should be 12.6 V or higher, and the parasitic current after the modules go to sleep (20-40 minutes) should be under 50 mA on most cars. A healthy battery with a drain above 100 mA points at a module, a relay or an accessory that is not switching off.
Urgency
Visit a workshop soon
Possible causes, most likely first
- Battery at end of life
Resting voltage drops below 12.4 V within hours of a full charge and the battery fails a load or conductance test. A battery more than 5-6 years old or one that has been fully drained a few times is the first suspect; replace it rather than chasing a drain. - Parasitic drain from a module that does not sleep
Current draw stays above 100 mA after 40 minutes. Put an ammeter in series or use a clamp meter on the negative lead, then pull fuses one at a time until the current drops; the circuit that fixes it names the culprit, typically infotainment, body control, alarm or an aftermarket unit. - Alternator not fully charging
The battery is never replenished, so it looks like a drain. Check that charging voltage at the battery is 13.8-14.5 V with the engine running and that resting voltage right after a drive is at least 12.7 V. - Leaking alternator diode
A failed diode lets current flow back through the alternator at rest, often 0.5-2 A. Unplug the alternator B+ lead (battery disconnected first) and recheck the drain. - Boot, glovebox or interior light staying on
A faulty switch keeps a lamp lit after the lid closes; a 5 W bulb pulls roughly 0.4 A, enough to flatten a battery in two nights. Close everything and check the lamps with a phone camera through the glass, or feel the lens for warmth. - Aftermarket accessory wired to permanent live
Dash cams, trackers, amplifiers or tow-bar electrics connected to constant power. Disconnect each one and remeasure; a dash cam in parking mode alone can draw 200-300 mA. - Corroded terminals or loose earth
Poor connections cause a weak crank that mimics a flat battery while the battery itself is charged. Check voltage at the terminals versus at the posts while cranking; a drop of more than 0.5 V means clean and retighten.
Advertisement728 × 90
What you can check yourself
- Measure resting voltage after an hour parked: below 12.4 V means the battery is low or weak
- Check that interior, boot and glovebox lights go off when everything is closed
- Unplug dash cams, chargers and any aftermarket devices for one night and see if the problem goes away
- Look at the battery date label and the terminals for corrosion