Battery and electrics
How can you tell if the 12 V battery is failing?
The 12 V battery starts the engine and powers the control units; signs it is failing are slow cranking, stop-start that stops working, odd electrical warnings or a battery that goes flat after a few days parked. A resting voltage reading gives a first clue, but only a load test or a battery tester tells its real condition.
Reviewed by Daniel Morales, founder of Reprorace SL, on
What voltage is normal?
As a guide, a charged lead-acid battery at rest, several hours after switching off, reads a little over 12.6 V; below about 12.2 V it is fairly discharged.
With the engine running, the alternator charges at a higher voltage, roughly between 13.5 and 14.8 V depending on the car. Many modern cars vary the charge on purpose, so one reading on its own is not enough.
What type of battery does my car have?
Cars with stop-start usually have EFB or AGM batteries, built for many starts. They must be replaced with the same type and capacity.
On some cars, a new battery has to be registered with the control unit using a diagnostic tool, so the charging system treats it as new.
Why does it fail?
It loses capacity over the years, and cold weather makes it obvious. Repeated deep discharges, very short trips that do not recharge it and a higher than normal draw with the car parked also damage it.
A weak battery causes warnings that look like other faults, such as communication codes between control units.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0562System voltage: too low
- P0560System voltage: malfunction
- P0563System voltage: too high
- U0140Lost communication with body control module
Related symptoms
- Battery warning light on: what it means and what to do
- Car battery keeps going flat: causes and what to do
- Car will not start and does nothing or just clicks
Learn more
See what your car is saying
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Frequently asked questions
How long does a car battery last?
There is no fixed figure: it depends on use, climate and type. If it cranks slowly in the cold, have it tested.
Can I replace the battery myself?
On many cars yes, carefully. On others it has to be registered afterward or settings are lost: check the manual.
Does jump-starting damage anything?
Done as the manual says, no. Communication codes may be left stored: read them and clear them if they do not return.