Battery voltage and OBD2 readings
The OBD2 port has battery power on pin 16, so an adapter can show battery voltage. A fully charged battery reads about 12.6 V at rest, and the reading should rise with the engine running.
Key facts
| Key fact | Detail |
|---|---|
| Engine off, fully charged | About 12.6 V |
| Engine off, below 12.0 V | Likely discharged or weak |
| Engine running | Higher than at rest, roughly 13.2 to 14.7 V depending on the car |
| Running voltage equals resting voltage | The alternator is probably not charging |
| Where it comes from | OBD2 port pin 16 (battery positive) |
An adapter reading is a quick check. For an exact test, measure at the battery terminals with a multimeter.
Where the voltage reading comes from
Pin 16 of the OBD2 connector carries battery positive, and pins 4 and 5 are ground. An ELM327 class adapter is powered from these pins and can report the voltage it sees. OBD Master Pro shows this as battery voltage in its live data.
The reading at the port passes through the car's wiring, so it can differ a little from a reading taken directly at the battery terminals.
Engine off: resting voltage
A fully charged 12 V car battery has a resting voltage of about 12.6 V. Turn off lights and accessories before you read it. If the car has just been driven, the reading can be higher for a while because of surface charge.
A resting voltage around 12.0 V or lower means the battery is discharged or weak and may struggle to start the engine.
Engine running: charging voltage
With the engine running, the alternator should lift the voltage above the resting value. The two references we use give slightly different typical ranges, 13.2 to 14.2 V and 13.7 to 14.7 V, because the exact charging voltage depends on the car.
If the running voltage stays about the same as the resting voltage, the alternator is probably not charging the battery and the car should be checked.
Low voltage and trouble codes
Low battery voltage can disturb communication between modules. Manufacturer bulletins published by NHTSA note that it can store several network communication (U) codes at once, and advise checking the battery and charging system before replacing any module.
See also: P0562 System Voltage Low · U0100 Lost Communication With ECM/PCM · What is a U code?
A simple check with the app
- With the engine off and accessories off, note the battery voltage.
- Start the engine and check that the reading rises.
- If the resting voltage is low or the reading does not rise, confirm with a multimeter at the battery terminals.
Try it on your own car
OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.
Keep reading
Related guides: Check engine light on: what to do · How to read an OBD2 code · Stored, pending and permanent codes · OBD2 readiness monitors explained · ELM327 adapter not connecting · How to clear the check engine light · What is fuel trim · OBD2 live data and PIDs · OBD2 port pinout · What is a U code? · OBD2 glossary
Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak
All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides
Sources
- Wikipedia: On-board diagnostics (OBD-II diagnostic connector and its pinout)
- Elm Electronics: ELM327 datasheet (OBD connector pins, protocols and J1939 support)
- Fluke: How to measure battery voltage with a multimeter
- Hagerty: Make your car's charging system reliable (resting and charging voltage)
- NHTSA: manufacturer service update, low battery voltage stores multiple U codes (2019)
- NHTSA: manufacturer bulletin, U0100 and U0101 with low voltage checks before module replacement (2016)
- NHTSA: manufacturer bulletin, CAN communication codes diagnostic tips and guidelines (2022)
- OBD Master Pro on Google Play (app features, supported connections and protocols)
Published . Updated . General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.