Battery testing — state of charge versus capacity

State of charge: how full is it

Open circuit voltage, measured after the surface charge is gone, tells you how full the battery is right now. It says nothing about how much the battery can deliver.

Surface charge from recent charging reads artificially high. Remove it by turning on headlights for a minute or two, then let it settle before reading.

Capacity: how much can it deliver

A load test pulls roughly half the CCA rating for 15 seconds and watches whether voltage holds up. A conductance tester estimates the same thing electronically in seconds and works on a partly discharged battery.

Either way this is the test that matters, and it must be done on a battery that is already reasonably charged — a flat battery fails a capacity test whether it is good or not.

Banks share the blame

Heavy trucks and apparatus run multiple batteries in parallel. One weak cell drags the whole bank down and will kill the new batteries connected alongside it.

Test them individually, disconnected from each other, and replace as a matched set. Mixing ages and types in a bank guarantees a repeat visit.

How to test it

1. Remove the surface chargeHeadlights on for a minute or two, then off, then wait a few minutes before reading.
2. Read open circuit voltageCompare to the state of charge table below.
3. Charge first if neededBelow about 75%, charge before testing capacity or you will condemn a good battery.
4. Load or conductance testHalf of CCA for 15 seconds, watching that voltage stays above about 9.6 V at room temperature.
5. Test bank members separatelyDisconnect them from each other first, or a good battery will mask a bad one.

Numbers to work to

Common mistakes

• Testing capacity on a discharged battery and condemning it.
• Reading voltage straight off the charger and calling it fully charged.
• Testing a parallel bank as one unit, so a good battery hides a failing one.
• Replacing one battery in an aged bank and expecting it to last.