Battery and charging voltage
The voltage tells you how charged the battery is with the engine off, and whether it’s being charged with the engine running. The ranges below are rough guidance; your car’s manual or service information has the values for your car.
Updated
Where Read My Car shows it
- Live data, “Battery voltage”
- The supply voltage the engine computer reports, if your car reports it.
- Terminal, ATRV
- The voltage the adapter itself measures at the OBD2 socket. Adapters measure it only roughly.
Both can differ a little from a meter on the battery terminals. Read My Car marks live readings below 12 V or above 15 V.
Engine off
A healthy, fully charged battery that has rested for a while reads roughly 12.6 to 12.8 V; around 12.4 V or less means it needs charging. Measure it after the car has stood for some time: straight after a drive or a charge, it reads higher.
Clearly lower means it’s low on charge or weak. Charge it and have it tested before you judge it.
Engine running
On many cars the charging system shows roughly 13.5 to 14.5 V with the engine running.
Cars with smart charging or start-stop vary the charging on purpose, and can show less at times. A single low reading isn’t proof of a faulty alternator. What matters is whether the voltage rises above the engine-off value, at least just after starting.
Readings well above that range can point to a charging fault, and can damage the battery: see P0563. Read My Car marks them above 15 V.
AGM and EFB batteries
Cars with start-stop usually have an EFB or AGM battery, built for the many restarts. Replace it with the same type: an AGM with an AGM, an EFB with an EFB or an AGM. A conventional battery wears out quickly in a start-stop car.
On many such cars the battery management has to be set up for the new battery with a workshop tool. Read My Car can’t do that.
Signs of low voltage
- Slow cranking, or the engine won’t start.
- Dim lights, or the battery warning on the dashboard.
- Unrelated-looking fault codes from several modules at once.
What to check
- If the battery warning comes on while driving, the alternator may not be charging: the car runs on its battery until it’s flat, and the engine can then cut out. Switch off what you don’t need, drive only as far as you need to, and don’t switch the engine off until you’re there if it may not restart.
- Check the battery clamps and the earth straps: clean, tight and free of corrosion.
- Compare the voltage with the engine off and running.
- Have the battery tested, and charged or replaced if it’s weak.
- Have the alternator and its wiring checked if the voltage doesn’t rise with the engine running.
- Read the codes again once the voltage is right. Codes that low voltage caused often don’t come back.
Questions
My car shows about 13 V while driving. Is the alternator faulty?
Not necessarily. Cars with smart charging or start-stop lower the charging voltage at times. Check whether it rises above the engine-off value just after starting, and look for a battery warning.
Why does Read My Car show a different voltage from my meter?
Live data shows what the engine computer measures, and ATRV what the adapter measures at the socket. Both can differ a little from the battery terminals.
Can a weak battery cause fault codes?
Yes. Low voltage can make control units misbehave and store codes of their own. Fix the voltage first, then read the codes again.