What is a normal car battery voltage?

A voltmeter reading is the cheapest battery check there is, but only if you know which voltage you are looking at. A car battery has three "normal" voltages: resting (engine off for hours), charging (engine running) and cranking (the brief dip while the starter turns). Each one answers a different question.

Car battery voltage chart (resting, engine off)

This is the open-circuit voltage: the battery measured with nothing charging or drawing from it, after a rest. The Battery Council International glossary describes it as a voltage taken "after a specified stabilizing rest period" and used to estimate charge level.

The figures below are from East Penn Manufacturing's battery care guide, which gives separate columns for flooded and AGM batteries.

Other makers publish slightly different numbers, which is normal because chemistry and temperature shift the curve:

• Optima says its REDTOP starting batteries read about 12.6–12.8 V fully charged, and its YELLOWTOP deep-cycle batteries about 12.8–13.0 V (Optima charging FAQ).
• Varta (Clarios) says to charge a battery to 12.7–12.8 V before storage and to recharge it if it falls to 12.5 V or less (Varta).
• MINI's dealer instructions treat 12.5 V as about 71% state of charge and 12.0 V as about 30% for the batteries in its cars (MINI SI M61 01 02).

The spread is a few tenths of a volt. The pattern is the same everywhere: every 0.1 V at rest matters, and anything below about 12.4 V is worth charging.

How do you measure resting voltage properly?

1. Let the car sit. Switch off, lock up and wait. MINI says an accurate reading is only guaranteed after the battery has rested for at least 4 hours. A battery that was just driven or charged reads high for a while (surface charge).
2. Protect your eyes and avoid sparks. East Penn's guide is blunt: all batteries give off explosive hydrogen gas, and the acid inside burns skin. Wear safety glasses, keep flames and sparks away, and don't lean over the battery.
3. Set a digital multimeter to DC volts (the 20 V range if it isn't auto-ranging).
4. Red probe on the positive (+) post, black on the negative (−). Read the posts themselves, not the cable clamps, if you can.

A plug-in voltmeter or an OBD dongle reads system voltage through a socket or the diagnostic port instead. Wiring between there and the battery means it may not match a meter on the posts exactly (our inference), but it is still useful for spotting trends. See are Bluetooth car battery monitors any good for what logging these readings can and can't tell you.

What should a car battery read with the engine running?

Optima gives about 13.3–15.0 V, "which may differ due to temperature", in its charging FAQ, and about 14–14.8 V at typical temperatures of 10–27 °C (50–80 °F). With the engine running you are measuring the alternator's output, not the battery.

Many newer cars don't hold a steady 14 V. They manage the alternator to save fuel, so the reading moves around. GM's bulletin 07-06-03-009D, written for its 2005–2014 full-size pickups and SUVs, describes a Regulated Voltage Control system that targets 12.6–13.1 V in "Fuel Economy Mode" and 13.9–15.5 V in "Charge Mode", depending on the battery's charge and temperature. GM says the gauge swinging between 12 and 14 V is normal and "NO repairs should be attempted."

So a single running reading of 13.0 V on a modern car is not proof of a bad alternator. What deserves a charging-system test (our inference from the ranges above):

• the running voltage never rises above about 13 V, even with headlights, heater fan and rear defroster on;
• it sits well above 15 V;
• the battery keeps going flat even though you drive regularly.

How far should the voltage dip when you start the car?

In a formal load test, a good 12 V battery stays at or above 9.6 V at 21 °C (70 °F), and at lower limits in the cold (table below). When the starter turns, voltage drops sharply for a second or two. The formal way to judge this is a load test. East Penn's procedure: fully charge the battery, apply a load of half its cold cranking amp (CCA) rating for 15 seconds, and read the voltage at the 15-second mark. A good 12 V battery stays at or above:

Source: East Penn Manufacturing. East Penn notes you can't load test a discharged battery: charge it first if it reads below 12.4 V.

A start in your driveway is not a controlled load test. The load is shorter and depends on the engine, so don't hold your car's start to these numbers exactly. What's useful is the trend: if the dip gets deeper from week to week at similar temperatures, the battery is losing its ability to deliver current. Cold weather makes this worse; see why car batteries die in cold weather.

Can a battery with a "good" voltage still be bad?

Yes. Resting voltage measures charge, not health. A worn battery can recover to 12.6 V overnight and still collapse under the starter's load. That's why East Penn calls a load test "the best way to determine if the battery is delivering adequate electrical performance." Conductance testers, the handheld electronic testers, are also ineffective on a discharged battery, so they need a full charge first too.

What to do at each reading

If a battery keeps showing up low after the car sits, the likely causes are a parasitic drain, too many short trips, or simply too long parked. A discharged or faulty battery is also one of the causes a GM bulletin lists for the network code U0100 (lost communication with the engine computer).

Safety when charging: use a charger suited to your battery type. East Penn warns that charging an AGM battery on a charger that exceeds 15.4 V, even once, can greatly shorten its life. Never charge or jump-start a battery that may be frozen.