How do I test a car battery properly?
Short answer: Charge it or check state of charge first, then use a conductance or load tester set to the battery's type and rated cold cranking amps; a voltage reading alone does not tell you battery health.
A battery can show 12.6 volts at rest and still fail under load, because voltage reflects charge level, not capacity. A good test starts with state of charge. Roughly, a fully charged flooded battery rests around 12.6 volts, and lower readings mean it is partially discharged. Remove surface charge by turning on the headlights briefly or letting it rest before reading.
Next, use a conductance tester or a load tester. Enter the battery type, such as flooded, AGM or EFB, and the rated cold cranking amps from the label. The tester compares measured performance with the rating and reports good, charge and retest, or replace.
If the tester says charge and retest, charge fully and test again; testing a discharged battery can condemn a good one. Many testers also check cranking voltage and charging system output, which helps you find out why the battery went flat. Wear eye protection and avoid sparks near the battery.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.