Starting, battery and electrics
Car battery keeps going flat: causes and what to do
If the battery goes flat while the car is parked, something keeps drawing current, the battery no longer holds its charge, or it is not recharged enough when you drive. Very short trips and plugged-in accessories are frequent causes.
Check without hurryStarting, battery and electrics
Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. It is not dangerous, but it can leave you stranded. Have it checked soon, especially if it keeps happening or the battery light also comes on while driving.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Aging battery: it loses capacity and cannot cope with several days parked.
- Drain while parked: a control unit that does not go to sleep, a trunk or glove box light, an alarm or an accessory.
- Plugged-in accessories: cameras, chargers or devices in the OBD socket or the 12 V socket that keep drawing current with the car off.
- Short trips: the alternator has no time to recover what starting the engine used.
- Insufficient charging: a weak alternator or regulator.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: How can you tell if the 12 V battery is failing? Can an OBD reader damage the car or drain the battery? Why is stop-start not working?
Car system: Electrical system and battery
What you can check yourself
Make sure no light is left on: trunk, glove box, interior or vanity mirrors.
Unplug any accessories you have added and see whether it improves.
Check whether the battery is the original one and how old it is.
See whether the battery light comes on with the engine running.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: system voltage when starting, while driving and after leaving the car parked, and codes from the body control units.
Frequently asked questions
Does an OBD reader left plugged in drain the battery?
Any connected device draws something. A device meant to stay plugged in should go to sleep when the car is parked: the ECO-LITE sleeps and wakes up with vehicle activity or its button.
How long can a car stand unused?
It depends on the car and the battery: modern cars always draw a little. If you leave it for weeks, a battery maintainer helps.
Does charging the battery at home help?
It helps if short trips are the cause, but it does not fix a drain while parked or a worn battery.
Keep reading
- Electrical system and battery: how it works and what fails
- Car will not start and does nothing or just clicks
- Battery warning light on: what it means and what to do
- Belt squeal when starting or turning: what it means
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means