Why a multimeter isn't enough
A multimeter shows voltage, which is a rough guide to how charged the battery is. It says nothing about how much current the battery can still deliver, and current is what turns the starter.
An ageing battery can sit at a perfectly normal voltage and still collapse the moment you crank on a cold morning. That gap between "looks charged" and "can actually start the car" is what a battery tester measures.
How a battery tester works
A handheld tester clips onto the terminals, sends a small signal through the battery and measures its internal resistance (or conductance). As a lead-acid battery wears, its internal resistance rises and the current it can deliver falls.
You enter the rating from the battery label, and the tester compares what it measured with what the battery should deliver. The whole test takes seconds and doesn't drain the battery.
Reading the results
| Reading | What it means |
|---|---|
| Voltage | How charged the battery is right now |
| Measured CCA | The cold-cranking current it can still deliver, versus the label rating |
| Internal resistance | Rises as the battery wears |
| State of health | A percentage estimate of remaining life |
| Verdict | Typically Good, Recharge, or Replace |
CCA stands for cold-cranking amps: the current a battery can deliver at −18 °C for 30 seconds. It is the number that matters on a cold start. A healthy battery measures close to its label; a tired one measures well below it.
Rough voltage guide (resting)
Measured after the car has been off for a few hours, for a typical 12 V flooded battery. AGM batteries sit a little higher. Treat these as approximate.
| Resting voltage | Approximate charge |
|---|---|
| about 12.6 V or more | Fully charged |
| about 12.4 V | Around three-quarters |
| about 12.2 V | Around half |
| below about 12.0 V | Discharged — charge before testing health |
Cranking and charging tests
A battery is only one part of starting and charging, and good testers check the other two:
- Cranking test. The tester watches the voltage while the engine starts. A deep, sustained dip points to a weak battery or a starter drawing too much current. A common rule of thumb is that a 12 V system shouldn't fall below about 9.6 V while cranking.
- Charging test. With the engine running, the tester checks that the alternator is raising the voltage above resting level. Many older cars run roughly 13.5–14.8 V; cars with smart charging vary this deliberately, so compare with the car's expected behaviour rather than a single number.
Put together, the three tests tell you whether to replace the battery, look at the starter, or chase a charging fault. That avoids the classic mistake of fitting a new battery to a car whose alternator killed the old one.
Getting an accurate test
- Let it rest. Right after driving or charging, a surface charge makes a battery look better than it is.
- Clean the terminals. Corrosion and loose clamps give low, misleading readings.
- Enter the label exactly. Rating standard (CCA, EN, DIN and so on), value and battery type all change the verdict.
- Mind the temperature. A very cold battery tests weaker than it is at room temperature.
What a battery tester won't tell you
- Parasitic drain. If a healthy battery keeps going flat overnight, something is drawing current with the car off. That needs a current measurement, usually with a clamp meter or multimeter.
- Lithium batteries. Handheld conductance testers are designed for lead-acid batteries; check the tester supports your battery type.
Questions people actually ask
How often should I test my car battery?
Does a battery tester drain the battery?
Can it test the alternator?
Why does the tester say "recharge and retest"?
The tester this page is about.
KONNWEI KW600: 100–2000 CCA, flooded, AGM and GEL, plus cranking and charging tests. €55, shipped free, 2-year warranty.
See it in the shop → Which tool do I need?