Quick answer
Let the car sit overnight with everything off, set a multimeter to DC volts and put the red probe on the positive post and the black probe on the negative: about 12.6 V means fully charged, 12.4 V is about 75%, and anything below that is discharged and should be charged before you judge the battery. Voltage only tells you the state of charge, not the battery’s health, so if a fully charged battery still struggles, have it load tested or checked with an electronic battery tester. Testing straight after driving or charging gives a falsely high reading.
Is this a DIY job?
- Difficulty: Easy
- Time: 5 minutes for a voltage check; overnight wait for an accurate reading
- Tools: digital multimeter, safety glasses, gloves; optionally a hand-held electronic battery tester
- Skills: basic use of a multimeter
Anyone can do a voltage check. A proper health test needs a load tester or an electronic battery tester. Many auto electricians, battery outlets and roadside services will run one for you. If the battery is swollen, cracked, leaking or smells of rotten eggs, don’t test it: leave it alone and get help.
Three ways to test a battery
| Test | What it tells you | Who can do it |
|---|---|---|
| Resting voltage (multimeter) | State of charge only | Anyone with a multimeter |
| Electronic tester (conductance or impedance) | Estimated cold cranking amps (CCA) and a pass/replace result | Anyone with a tester; most workshops |
| Load test (carbon pile or load tester) | Whether the battery holds its voltage under a heavy current draw | Usually a workshop |
None of these measures a battery’s remaining capacity on its own. A battery can pass a CCA test and still fail soon after because its capacity has faded. That’s why a battery with a history of going flat is worth a full check even if one test looks fine.
Step by step
Resting voltage test (multimeter)
- Let the battery rest. Park the car overnight with the engine off and everything switched off. Driving or charging leaves a “surface charge” that makes the voltage read high for several hours.
- Look before you touch. Safety glasses on. Check the case for swelling, cracks, leaks or heavy corrosion. If you find any of these, stop.
- Set the meter. Black lead in COM, red lead in V/Ω, dial on DC volts (20 V range on a manual meter). (see How do I use a multimeter on my car?)
- Measure. Red probe on the positive (+) post, black probe on the negative (−) post. Touch the lead posts or clamp bolts, not the plastic covers.
- Compare with the table below. At 12.6 V the battery is fully charged. At 12.4 V or less, charge it fully with a battery charger, let it rest again, and retest before you decide anything. (see How do I charge a car battery with a battery charger?)
Cranking check (multimeter and a helper)
- Keep clear of moving parts. Hands, hair, clothing and leads away from the belt and fan.
- Watch the voltage while a helper starts the engine. Keep the probes on the posts (alligator clips make this easier). For comparison, the formal battery load test passes a battery that holds at least 9.6 V under a heavy load for 15 seconds (at 10°C or warmer). If the voltage falls well below that while cranking, or the engine cranks slowly, the battery is weak or there’s a connection or starter problem, and a proper load test or electronic battery test will confirm which. (see Is it my starter motor or my battery?)
Charging check (engine running)
- With the engine idling and accessories off, measure across the battery again. Most conventional charging systems read roughly 13.5 to 14.8 V. A reading at or near the resting voltage means the battery isn’t being charged, so a “dead battery” may really be a charging fault. Smart charging systems can read lower at times. (see How do I know if my alternator needs replacing?)
Electronic battery tester
- Charge first if needed. Electronic testers give unreliable results on a battery that’s well below full charge. Battery testing guidance suggests at least 70% state of charge for a conductance test.
- Connect the tester’s clamps red to positive, black to negative, directly on the battery posts.
- Enter the battery details. Select the battery type (flooded, EFB, AGM) and the CCA rating and standard printed on the battery label. Entering the wrong figure gives a wrong result.
- Read the result. Most testers give a measured CCA, a state-of-charge figure and a verdict such as “good”, “charge and retest” or “replace”. If it says “charge and retest”, charge it fully, let it rest and test again before buying a battery.
Load test
- Leave this one to a workshop unless you own a load tester. The standard carbon pile test draws half the battery’s rated CCA for 15 seconds at 10°C or warmer. A battery that stays above 9.6 V passes. Load testers get hot and can spark, and the battery must be at least 70% charged first.
If the battery fails
- Check the cause before you replace it. A battery that keeps going flat might be fine and the real fault might be a drain or a charging problem. (see Why does my car battery keep going flat?)
- Replace with the correct type. Stop-start cars need an AGM or EFB battery, and some cars need the new battery registered to the car’s computer. If you do replace it yourself, disconnect the negative terminal first and reconnect it last. (see How do I replace a car battery?)
Specs table
Resting voltage and state of charge (standard 12V lead-acid battery, rested)
| Voltage | State of charge | What to do |
|---|---|---|
| 12.6 V or more | About 100% | Fully charged |
| 12.4 V | About 75% | Charge it; RACV treats anything under 12.4 V as discharged |
| 12.2 V | About 50% | Charge it, then retest |
| 12.0 V | About 25% | Flat: charge it, then retest |
| 11.9 V or less | About 0% | Dead flat: charge and retest; may need replacing |
The US battery industry’s standard table (24-hour rest, 26°C) gives almost the same figures. AGM batteries read slightly higher when full, and cold lowers the reading. A battery tested straight after driving or charging can read well above these figures because of surface charge, which settles after a few hours’ rest. Check the battery maker’s figure for your battery.
Other test figures
| Test | Figure | Source note |
|---|---|---|
| Running (charging) voltage | About 13.5 to 14.8 V | Conventional charging; smart systems vary |
| Load test (carbon pile) | Half rated CCA for 15 s; pass if above 9.6 V | Battery at 70% charge or more, 10°C or warmer |
| Cold cranking amps (CCA) rating | Amps the battery delivers for 30 s at −18°C while staying at or above 7.2 V | SAE J537 definition |
| Electronic tester | Battery at 70% charge or more | Readings drop as charge falls |
| Replacement guide | Replace when CCA drops to 50% of rating | Heat-failed batteries; CCA doesn’t show capacity loss |
What you’ll need
- digital multimeter
- battery tester (electronic conductance tester for 12V batteries)
- smart battery charger (to charge before retesting)
- battery terminal brush (clean posts give accurate readings)
- safety glasses and nitrile gloves
When to call an auto electrician
- The battery is swollen, cracked, leaking, very hot or smells of rotten eggs.
- The battery tests good but keeps going flat (likely a drain or charging fault).
- The charging voltage is outside the normal range, or the battery warning light is on.
- Your car needs battery registration or coding after a replacement.
- The car is a hybrid or EV. Its 12V battery can be tested, but the high-voltage battery and its wiring are professional-only.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: What voltage should a car battery read? A: About 12.6 V when fully charged and rested overnight with the engine off. With the engine running, most cars read roughly 13.5 to 14.8 V because the alternator is charging.
Q: Can a battery show 12.6 V and still be bad? A: Yes. Voltage only shows the state of charge. A worn battery can show full voltage with no load, then collapse when you try to start the car. A load test or electronic tester shows its real condition.
Q: How long should I wait after driving to test my battery? A: Overnight is best. Lead-acid batteries need at least four hours of rest, and battery makers suggest 24 hours, for an accurate open-circuit reading.
Q: How long does a car battery last? A: RACV gives about three to five years for a standard lead-acid battery and five to seven for AGM, though how you drive and the climate make a big difference. (see What’s the difference between flooded, EFB, AGM and lithium car batteries?)
Q: What does the coloured “magic eye” on top of my battery mean? A: It’s a hydrometer that shows the condition of one cell only, so it’s a rough guide. A multimeter or battery tester gives a more reliable picture.
Sources
- RACV, “Car battery guide: how they work, when to replace them, and how to jump-start” – multimeter on 20 V DC after the car has been off overnight; voltage vs state-of-charge guide; “anything under 12.4 V is considered discharged”; lifespan of three to five years (lead-acid) and five to seven years (AGM). https://www.racv.com.au/royalauto/transport/cars/car-battery-guide-how-to-know-if-it-needs-replacing.html (checked October 2026)
- NRMA, “Tips from an NRMA patrol: car battery maintenance” – magic eye shows one of six cells only; newer cars can fail with little warning. https://www.mynrma.com.au/open-road/advice-and-how-to/car-maintenance/tips-from-a-patrol-batteries (checked October 2026)
- NRMA, “Common car battery myths” – CCA definition (30 seconds at −17.8°C, 12V battery). https://www.mynrma.com.au/open-road/advice-and-how-to/car-maintenance/car-battery-myths (checked October 2026)
- Battery University, “BU-903: How to measure state of charge” – rest at least four hours (24 hours recommended); surface charge; BCI open-circuit voltage table at 26°C; AGM reads slightly higher; temperature effect. https://batteryuniversity.com/article/how-to-measure-state-of-charge (checked October 2026)
- Battery University, “BU-902a: How to measure CCA” – SAE J537 (−18°C, 30 s, 7.2 V); carbon pile test at half CCA for 15 s above 9.6 V at 10°C or higher; 70% state of charge for load and conductance tests. https://batteryuniversity.com/article/bu-902a-how-to-measure-cca (checked October 2026)
- Battery University, “BU-905a: Testing starter batteries in vehicles” – CCA testers can’t detect low capacity; replace when CCA drops to 50%. https://batteryuniversity.com/article/bu-905a-testing-starter-batteries-in-vehicles (checked October 2026)
- JS Alternators, “How to test your alternator: complete guide” – running voltage about 13.5–14.8 V on conventional systems; smart charging reads differently. https://js-alternators.com/blogs/blog/how-to-test-alternator (checked October 2026)
- RACV, “Mobile car battery replacement” – some vehicles have battery management systems that require new batteries to be registered. https://www.racv.com.au/cars-transport/vehicle-maintenance/car-batteries.html (checked October 2026)
- Banner Batteries, “Battery sensor – application and function” – don’t fit a conventional battery to a start-stop vehicle that uses AGM or EFB. https://www.bannerbatterien.com/en/Battery-knowledge/20-Battery-sensor-%E2%80%93-application-and-function (checked October 2026)