A car battery that dies overnight is either too weak to hold a charge, not being recharged by the alternator, or being drained by something that stays on after the car is shut off. That last cause, called parasitic drain, is common: a glove box, trunk or dome light that stays on, an aftermarket accessory such as an amplifier, dash camera or remote starter, or a module that never goes to sleep. A parasitic draw test finds it.
Common causes
| Cause | Clues |
|---|---|
| Old or failing battery | Three to five years old or more, slow cranking, fails a load test; see how long car batteries last |
| Parasitic drain | Battery tests good when charged but is flat after sitting |
| Alternator not charging | Battery light, dim lights while driving; see bad alternator symptoms |
| Short trips | Battery never fully recharges between starts |
| Cold weather | Lower capacity and higher starting demand |
| Corroded or loose terminals | White or green crust, intermittent no-start |
New battery dies overnight
A new battery that goes flat overnight points away from the battery and toward a drain or a charging problem. A failing alternator diode can drain the battery with the engine off, and a stuck relay can keep a circuit powered.
How to do a parasitic draw test
- Fully charge the battery and turn everything off; close the doors and latch the hood switch if fitted.
- Connect a multimeter set to amps in series between the negative battery post and the negative cable.
- Wait for the modules to go to sleep, which can take from a few minutes to an hour on modern cars.
- Read the draw. Many vehicles settle at a few tens of milliamps; check the figure for your vehicle. A much higher reading is a drain.
- Pull fuses one at a time and watch for the reading to drop; that circuit holds the drain.
In cold weather
Cold slows the chemistry in a battery, so a battery that is weak but adequate in summer may fail on the first cold morning. Load-test the battery before winter.
What to do now
Jump start the car safely; see how to jump start a car. Then have the battery load-tested and the charging system checked. If the battery light came on while driving, see battery light on.
Preventing it
- Clean and tighten the terminals.
- Use a battery maintainer on cars that sit for weeks.
- Have aftermarket accessories wired through a switched circuit.
If the car clicks but will not crank, see car won’t start, just clicks.
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Trouble codes: P0562 System Voltage Low · P0687 ECM Power Relay Circuit High · P0690 ECM Power Relay Sense High · all OBD2 codes.
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