Many vessels have more than one alternator on the one engine and this has its advantages. The idea is that each alternator charges their own bank of batteries.

2 alternators - 2 battery banks

This is simple enough, the only consideration is that the exciter current for each alternator must be fed from the battery to which it supplies, so its best to have the ignition switch feed for the original alternator, power a relay that then connects the exciter current to the other alternator from its associated battery.

2 alternators 1 battery bank

There are many vessels with 2 alternators wired in parallel, feeding the same charge splitter, and there are many people claiming it's fine as it seems to work. It is not. It may work, but it is not in the slightest bit efficient, it stresses out both alternators, and it provides no redundancy should one fail.

The problem is simply that each alternator will 'bounce' electrically off the other, this yo yo effect is due to the inevitable differences in cut off voltage, characteristics of the regulators between alternators, even 100mV difference is enough. A typical bouncing situation is:

1. A heavy load causes the voltage to drop
2. Both alternators A and B spot this and increase their output voltage,
3. Alternator A senses the voltage rise before B, A reduces its output voltage
4. B will increase its voltage, and is now doing all the work
5. Only when B is maxed will the voltage drop enough to activate A
6. Go back to point 3, ad infinitum.

This process basically keeps repeating, so one alternator is maxed out each time before the other takes over, they're not exactly working in harmony.

A simple solution to this is provided by a company called Balmar, this provides 2 field coil outputs, and controls the battery voltage for multistage charging.

Twin Engine Alternator configuration

Vessels with 2 engines with an alternator on each are often in situations where one engine seems to end up powering much more than the other, so for these a there is a device called a Centre Fielder that controls both alternators from a common location, so they can do the same amount of work. There is a company called Balmar who are industry leaders in this sector.

A centre fielder ensures that the main domestics are still charged should one engine fail. A typical arrangement will see the centre fielder power a diode and split charge to domestics, engine start 1 and engine start 3. All batteries are charged regardless of which engine is running.