Testing a battery the right way takes two steps. First figure out how charged it is. Then test whether it can actually deliver current. Skip the first step and the second one doesn't mean much.
Here's how it works for a regular 12 V lead-acid battery.
Get rid of the surface charge first
Right after a drive or a charge, a battery reads higher than it really is. That's surface charge. Turn the headlights on for about a minute, let it rest a bit, then measure. Otherwise you'll think the battery is more charged than it is.
Check the state of charge
Measure the open circuit voltage, with no load, at around 70°F:
- About 12.6 V: fully charged
- About 12.4 V: around 75%
- About 12.2 V: around 50%
- About 12.0 V: around 25%
Load test
A load test only means something if the battery is at least about 75% charged. A good battery that's half dead can fail a load test, so charge it first.
On a fully charged battery at about 70°F, load it at half its CCA rating for 15 seconds. It should hold at least 9.6 V. A 600 CCA battery gets a 300 A load. If it drops below 9.6 V, it can't deliver its rated current and needs to be replaced.
Conductance testers
A conductance tester doesn't put a big load on the battery. It sends a small signal through it and figures out its condition from the response.
It still needs a decent state of charge to give you a pass or fail. If it says "charge and retest," that's what you do. At 12.1 V, for example, the battery is too low to judge.
Lithium batteries are different
A 12 V lithium (LiFePO4) battery has its own electronics and voltage limits. Charge it only with a lithium-approved charger or mode. A regular lead-acid charging profile or a carbon pile load test can damage it or give you a bad reading.
Cheat sheet
- Remove the surface charge before measuring
- 12.6 V full, 12.4 V about 75%, 12.2 V about 50%, 12.0 V about 25%
- Charge to at least about 75% before a load test
- Load test: half CCA for 15 seconds, hold 9.6 V or better
- Conductance testers use a small signal, not a big load
- Lithium batteries need a lithium charger, no carbon pile
Common questions
What voltage is a fully charged car battery?
About 12.6 V at rest, around 70°F, once the surface charge is gone.
What should a battery hold during a load test?
At least 9.6 V at half its CCA rating for 15 seconds, at about 70°F.
Why does my tester say charge and retest?
The battery is too low to judge. Charge it fully, remove the surface charge, and test again.
Getting ready for your ASE tests?
ASE Pro has 2,100+ practice questions with clear explanations, so you know why every answer is right.
