What are the signs of a failing alternator? How do I differentiate between a battery problem and an alternator problem?

Every car has a red “alternator” or car battery symbol in the instrument panel that illuminates when the ignition is switched on.

Once the engine starts, the light should go off, indicating a properly functioning alternator.

The alternator is a belt-driven “electricity generator” that charges up the 12-volt battery when the engine is running.

Among the parts that make up the alternator is a “voltage-regulator” that quite accurately maintains the charging voltage supplied to the car’s rechargeable battery, regardless of engine speed.

When the battery does not receive charge, the warning light on your instrument cluster – usually a red symbol of a battery with the “+” and “-” signs – will light up.

A slipping or broken belt driving the alternator will provoke the warning.

There are other factors that can cause alternator performance to deteriorate over time. Often, an under-charging alternator may not signal alternator failure on your dashboard.

Not many cars have an on-board voltmeter that shows if the alternator is charging sufficiently with an output of between 13.5 and 14 volts.

Even if your car does not have a voltage display, most workshops can quite easily check if the alternator is in a good state. This is done by measuring the voltage at the battery when the engine is idling while the lights, air-conditioner and other accessories are in use.

The battery voltage under such conditions should always be maintained at a minimum 13.2 volts.

A significantly lower reading, or a drop to below 13 volts, indicates that the alternator is not performing optimally.

This is as far as the alternator is concerned, but the health of the car’s battery is another matter.

The best way to check a battery’s health is to measure the voltage between the battery terminals when the engine is running and then again after the engine has been switched off for some time, preferably overnight. This lets you know if the battery is still capable of maintaining charge.

Often, a weak battery will show a voltage of 12 volts or just below that before a cold-start. This is when you should consider a replacement, so you will not need to call for recovery when you suddenly find a dead battery that will not crank the engine.

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