Most no-start calls are batteries, and most failed batteries gave warning that a voltmeter could not see. Understanding the difference between the two measurements makes a cheap tester genuinely useful.
Voltage tells you charge, not health
A multimeter across the terminals gives state of charge, roughly:
- 12.7 V or above, rested: fully charged.
- 12.4 V: around 75 per cent.
- 12.2 V: around 50 per cent, and being left here regularly is what kills lead-acid batteries.
- Below 12.0 V: deeply discharged.
The measurement must be taken rested — at least a few hours after driving, ideally overnight — because a surface charge from the alternator inflates the reading substantially. A battery read straight after a drive can show 12.8 V and be at half charge.
What voltage cannot tell you is whether the battery can still deliver current. A battery with sulphated or shed plates can hold a perfectly respectable resting voltage and collapse the instant the starter draws 300 amps.
Cold cranking amps: capability, not charge
CCA is the current a battery can deliver for 30 seconds at minus 18 degrees while holding a usable voltage. It is the standard measure of a starting battery’s capability, and it is printed on the label along with the standard used, usually EN.
A conductance tester injects a small AC signal, measures internal resistance and estimates the CCA the battery can still deliver. Comparing that against the label figure gives you state of health.
The interpretation is simple:
- Measured CCA at or above the rating: healthy.
- Around 70 to 80 per cent of rating: ageing, will struggle on the first genuinely cold morning.
- Below roughly 65 per cent: replace it before it strands you.
This is the number that predicts failure. Voltage tells you what happened yesterday; measured CCA tells you what will happen in January.
Testing correctly
Results are only meaningful if the method is right:
- Charge the battery first if it is below about 12.4 V. Conductance testers give pessimistic readings on a discharged battery and will condemn a good one.
- Test at the battery posts, not at the jump points under the bonnet, unless the tester specifically supports them.
- Enter the correct rating and standard. Entering an EN figure as SAE, or guessing, makes the state-of-health percentage meaningless.
- Note the temperature. Cold reduces available current substantially; most testers compensate, cheap ones less well.
The tests a good tester also runs
Beyond the battery itself, most units will check:
- Cranking voltage — how far the voltage dips while the starter turns. A healthy system holds above roughly 9.6 V. A deep dip with a healthy battery points at the starter or its cabling.
- Charging voltage — typically 13.5 to 14.7 V at the terminals with the engine running. Consistently low means the alternator or its wiring; consistently high means a regulator fault, which cooks batteries.
- Ripple — AC leaking through from failing alternator diodes, which destroys batteries quietly and causes strange electronic faults.
That charging test is why a battery tester is worth owning even when the battery is fine: a battery that dies repeatedly is usually being killed by the charging system or by a parasitic drain.
Start-stop cars need the right battery and the right step after fitting
Cars with stop-start use AGM or EFB batteries, which tolerate the constant shallow cycling. Fitting a conventional battery to such a car results in early failure.
Just as importantly, many of these cars have a battery monitoring system that must be told a new battery has been fitted. Without that registration the charging strategy stays tuned to an old, tired battery, and the new one is undercharged from day one. That step needs a diagnostic tool — and is a good reason for a battery tester and an OBD scanner to live in the same box.
What to buy
A conductance tester with clear CCA and state-of-health output, cranking and charging tests, and support for AGM and EFB. Avoid units that report only a colour or a smiley face; the whole value is in the number.
