How it works: the boat alternator

Callum Smedley

Engine power relies on a healthy boat alternator. Callum Smedley shares how to keep it running smoothly

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Depending on the size of the engine, a boat alternator will usually have a maximum output of between 35A and 60A as standard – although it’s possible to fit one with a higher output.

Modern alternators (as opposed to straightforward DC dynamos) generate alternating current (AC) and convert it into direct current (DC) to charge the battery.

The voltage is regulated at a nominal 12v (or 24v) to charge the battery and supply the electrical loads.

When the boat alternator is charging, the output tends to be around 14v, and when the battery is resting it tends to be around 12.5v.

Inside an AC generator, there is a magnet rotating inside coils of wire.

This rotating magnet cuts the lines of flux between the coils, making electricity.

The reason that it makes AC power is because the rotating magnet has a north and south pole, which basically means that it makes positive power for one half a revolution and then negative power for the second half of the revolution.

Hence the term ‘alternating’. Boat batteries are DC, so the second part of the alternator, called a rectifier, changes AC power into DC power.

The battery must never be disconnected from the alternator when the engine is running, or else the alternator’s ‘diodes’ (part of the rectifier) will be destroyed.

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In other words, DO NOT switch off the engine battery switch when the engine is running.

As long as the alternator is kept clean and cool it generally runs without many problems.

Alternators, however, don’t like water, so any leaks, be they fresh or sea water, can damage the alternator if they get inside.

And the alternator can’t be sealed because it is air cooled.

A diagram of an boat alternator

In the diagram shown above, the fan is the green-coloured part that forces air through the alternator, to cool it.

The air vents are at the back of the alternator by the terminal screws.

If the alternator is struggling to charge the battery it could be because the drive belt is slipping.

As the alternator produces more electricity, it gets harder to turn – a bit like pedalling a bike uphill instead of on the flat.

So, if the belt is worn or slack, it will slip. To fix this, the belt should be re-tensioned.

It’s always a good idea to check the belt for any damage, such as wear or cracks, at the same time.

If in doubt, change it. Don’t forget that this belt is often also used to drive a water pump, so it’s very important.

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