Car battery draining: what to check
LLCAR Editorial Team · updated
If the car struggles to start after sitting, measure the voltage at the terminals with a multimeter after several hours of rest with the ignition off. For a standard 12-volt lead-acid battery, around 12.8 V is a guideline for a good charge; below 12.4 V VARTA recommends charging the battery. For AGM and EFB, refer to the manufacturer's instructions. Resting voltage helps estimate the charge; battery condition and the charging system are checked separately.
How to compare voltage
- Before starting. Record the terminal voltage after several hours parked with the ignition off. Note how long the car was parked and the air temperature.
- During cranking. Note whether the starter turns more slowly than usual, whether the relay clicks, and whether the instrument panel goes dark.
- After starting. Record the voltage and note whether the headlights, heater and other heavy loads were on. Managed charging can change the voltage while the engine is running.
If the parameter is available via OBD-II, LLCAR lets you compare the readings reported by the car. A reading from the control unit and a direct measurement at the terminals may differ. To test the battery, use the procedure from the vehicle manual or have it tested at a service shop.
What a follow-up measurement can tell you
- Starting gets worse after a long time parked. Check the charge, battery condition and possible current draw while parked.
- The problem appears even after a drive. Ask to have the charging system, terminals and connections checked, and then the battery itself.
- The car is often driven on short trips. Compare readings after several similar trips and after a longer time parked.
For AGM and EFB batteries, which are often fitted to cars with a Start-Stop system, ask for the appropriate tester mode to be selected. To check cranking capability, a load test or a suitable battery tester is important.
The battery drains overnight
If the starter barely turns over in the morning, recall what was left on while parked. Check the interior and trunk lights, chargers, and a dash cam in parking mode. A removable OBD-II adapter can also consume charge if the port stays powered. Some connected devices prevent the electronic control units from entering sleep mode.
For comparison, disconnect one removable accessory using its normal method before a regular overnight stop and note how the car starts in the morning. Keep the security systems and factory wiring in working order. One successful start is only a clue. If the drain recurs, ask a service center to check the battery condition, the charging system, and the parasitic draw after the modules go to sleep.
What to pass on to the service shop
Collect the measurement times, readings, temperature, battery age and type, and the circumstances of the difficult start. If there is heat, swelling, smoke or a visible leak, stop attempting to start the car and seek help.