How to test a car battery
Check the electrolyte in one cell at a time. Put it back in the cell that it came from.
Check the battery’s state of charge with a hydrometer , which measures the strength of the acid in the electrolyte, or battery fluid.
This gives no clue, however, to the battery’s capacity – its ability to sustain a charge well enough to perform its tasks.
Battery capacity depends on the size and number of the plates in each cell. If any plates are damaged, that cell’s capacity is reduced. The electrolyte in a sealed-for-life battery cannot be checked readily.
Battery-condition indicator
A battery-condition indicator calibrated in volts, and with a red-green-red scale.
Some cars are fitted with a battery-condition indicator, which is a form of voltmeter. It may be calibrated in volts, by a sliding coloured scale, or by three bands of red-green-red.
When you switch on the ignition, the indicator shows the battery voltage, just over 12 volts for a 12volt battery or about the red-green division.
A lower reading means that the battery is not fully charged.
If the reading is well down while all the circuits and lights are switched off – the battery is not holding its charge, or is ‘flat’.
When you start the engine, the indicator shows the generator output. It should move slowly to around the 14 volt mark, or midway into the green sector.
It should stay steady at all engine speeds if the car has an alternator, or at speeds higher than idling if there is a dynamo.
If the indicator drops to 12 volts or lower, check the fan belt (See Checking, adjusting and refitting drive belts) or the generator output.
The ammeter
The ammeter shows the amount of current flow to or from the battery.
Some cars still have ammeters fitted on the instrument panel. An ammeter tells you how well the charging system is working, and gives more immediate information than a voltmeter.
The ammeter shows the amount of current going into or out of the battery, or the difference between the two. Thus it tells you at a glance whether the battery is being charged by the generator or discharged by a heavy load.
In practice, if the charging system is in good condition the reading should always be strong.
If the ammeter shows a very low or negative reading, you know immediately that something is wrong, whereas a voltmeter gives less information and is much slower to respond to a problem.
The only disadvantage of an ammeter is that it is connected in series with the battery and the generator. It requires a heavier cable, and if the ammeter circuit develops a fault, there is more danger of damage to an alternator.
Testing a battery with a voltmeter
Remove the high-tension lead from the coil, so the engine will turn over but not start.
You have to put a heavy load on a battery to test its capacity. Some garages use a heavy discharge tester; a similar test, though less conclusive, can be made with a standard voltmeter.
Remove the high-tension lead from the coil so that the engine turns but will not start. Connect the voltmeter across the battery terminals.
Measure the voltage in the battery, then the loss when running the starter.
Note the reading – which should be 12 or 13 volts or possibly more if the battery has just come off charge.
Now have a helper work the starter for 10-15 seconds while you note the reading. If the battery is good, the drop in voltage should not be more than about 2 volts.