State of charge and state of health are not the same
Resting voltage tells you the state of charge. Around 12.6 volts is a fully charged lead acid battery, roughly 12.4 is about three quarters, and below about 12.0 it is substantially flat. That is useful and it says nothing about whether the battery can still deliver current.
State of health is about capacity and internal resistance. As a battery ages, plates sulphate and internal resistance rises, so it can sit at a healthy 12.6 volts at rest and collapse the moment the starter turns. This is why a car that reads fine on a voltmeter still fails to start on a cold morning.
A proper tester measures both, and the health figure is the one that predicts failure. Testing a battery that has just been driven, or one that is flat, gives a health reading that is wrong in both directions, so charge it and let it settle before drawing conclusions.
| Reading | Means | Does not tell you |
|---|---|---|
| Resting voltage | How charged it is now | Whether it can crank the engine |
| Measured CCA against rated | How much capability remains | Whether it is charged |
| Cranking voltage | How far it drops under real load | The cause of a slow crank |
| Charging system test | Whether the alternator is holding voltage | Battery condition |
CCA, and why you must enter the right number
Cold cranking amps is the current a battery can deliver at a low temperature for a set time while holding a minimum voltage. A tester works by measuring the battery and comparing what it can now deliver against the rating you tell it the battery was sold with.
That comparison is the whole test, so the number you enter decides the result. Enter a rating far above the true one and a healthy battery is condemned; enter one far below and a failing battery passes. The rating is printed on the battery label, and it is worth taking a moment to read it rather than accepting the tester default.
The rating standard matters too. CCA, EN, DIN, IEC and SAE are measured differently and give different numbers for the same battery, so a tester set to the wrong standard is comparing against the wrong scale. Most testers ask; set it to whatever the label states.
Surface charge, and testing at the right moment
A battery that has just been charged or driven carries a surface charge, an elevated voltage sitting on the plates that has not settled into the chemistry. It reads high and flatters every measurement taken from it.
Removing it is simple: switch on the headlights for about thirty seconds, or leave the vehicle standing for a few hours. Then take the reading. Skipping this step is the most common reason a test says a battery is fine and it fails a week later.
Test at the battery posts rather than at a clamp where possible, because a corroded or loose clamp adds resistance that the tester reads as battery condition. If a reading looks poor, clean the terminals and repeat before condemning the battery, since terminal corrosion imitates a failing battery closely.
Battery type, and testing the charging system too
Modern vehicles use several lead acid variants and they do not behave alike. A conventional flooded battery, an EFB and an AGM all have different internal resistance and different voltage characteristics, so a tester that offers a battery type setting needs it set correctly or the health figure is measured against the wrong expectation.
The other half of the job is the charging system. Most of these testers will measure charging voltage at idle and under load, and check for ripple from a failing diode in the alternator rectifier. A battery that keeps going flat is often a charging fault rather than a bad battery, and testing only the battery finds the symptom.
A carbon pile load tester is the older approach: it draws a genuine heavy current and watches the voltage sag. It is unambiguous and it stresses a marginal battery, which is why some workshops keep one. Conductance testers are quicker, safer and work on a battery that is not fully charged, which is why they have largely replaced them.
