Not sure if your car needs a new battery or a new alternator? Measure three voltages with a multimeter, enter them in the free tool below, and it tells you what the numbers mean. You can fill in one reading or all three. More readings give a more reliable answer.
Car Battery Health Checker
Enter the voltage readings from your multimeter. Fill in one, two or all three, but each extra reading gives a more accurate answer. Works for standard 12V car batteries.
Estimates only. Readings shift with temperature, battery type (AGM, flooded, EFB) and meter accuracy. A load test at an auto parts store is the final word on a weak battery. Working near a car battery: remove jewelry, avoid sparks and wear eye protection.
How to Take the Three Readings
You need a digital multimeter set to DC volts (the 20V range is ideal). Wear safety glasses, remove metal jewelry, and keep the probes away from each other so they never touch while connected to the battery.
1. Resting voltage
Turn the engine off and wait at least 15 minutes so the surface charge settles. Touch the red probe to the positive (+) terminal and the black probe to the negative (-) terminal. For the full walkthrough, see my guide on how to test a car battery with a multimeter.
2. Lowest cranking voltage
Press the MIN/MAX button on your meter, then have a helper start the engine while the probes stay on the battery. The MIN value shows the lowest voltage during cranking. If your meter has no MIN/MAX, see the best automotive multimeters.
3. Running voltage
Start the engine and let it idle with the headlights and A/C off. Measure at the battery terminals again. If it stays low, follow my guide on how to test an alternator with a multimeter.
Car Battery Voltage Chart (12V, Engine Off)
| Resting voltage | Approximate charge | What it means |
|---|---|---|
| 12.6 V or higher | 100% | Fully charged |
| 12.4 V | 75% | Good |
| 12.2 V | 50% | Needs charging |
| 12.0 V | 25% | Low, charge soon |
| Below 11.9 V | Close to 0% | Discharged, may be damaged |
Values are typical for a standard flooded battery at room temperature. AGM batteries usually rest a little higher.
Charging and Cranking Voltage Chart
| Test | Good | Problem |
|---|---|---|
| Cranking (lowest reading) | 9.6 V or higher | Below 9.6 V means the battery is weak under load |
| Running (engine idling) | 13.7 V to 14.7 V | Below 13.7 V: undercharging. Above 14.7 V: possible regulator fault |
Battery or Alternator? How to Tell
- Good resting voltage, poor running voltage: the alternator or its wiring is the likely problem.
- Good resting voltage, but voltage collapses while cranking: the battery is probably near the end of its life.
- Low resting voltage that goes flat again overnight: check for a parasitic draw. See how to measure amps with a multimeter.
- Low battery and low running voltage: fix the charging system first, or a new battery will go flat too.
Frequently Asked Questions
What is a good car battery voltage?
A healthy 12V car battery reads about 12.6 V or higher with the engine off. With the engine running, you should see 13.7 V to 14.7 V.
What should my battery voltage be while cranking?
It should stay above about 9.6 V. If it drops below that, the battery is weak under load, even if the resting voltage looked fine.
How do I know if it is the battery or the alternator?
A good resting voltage with a poor running voltage points to the alternator. A battery that tests fine at rest but collapses while cranking is usually the battery itself.
Why is my reading higher than 13 V with the engine off?
That is a surface charge from recent driving or charging. Turn on the headlights for a minute, turn them off, wait 15 minutes, and measure again.
Does cold weather change the readings?
Yes. Batteries deliver less power in the cold, so cranking voltage dips lower. Treat borderline results in winter with caution.
Can this tool replace a load test?
No. It gives a good estimate from voltage readings, but a professional load test is the final check on a weak battery. Most auto parts stores will do one for free.