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Following on from a previous question.
Again, thank you. Unfortunately it's a single alternator on a BMC 1.5 engine. Is it possible to increase the size of an alternator, add a second or somehow increase the efficiency? I believe it's currently a 70amp?

Asked by: Carl Young  | 12.23pm, Tuesday 3 July

WW says:

There are several options open to you as you state:-
Adding another alternator obviously requires a double pulley on the crankshaft and brackets. Sterling make a unit which will combine the outputs of two alternators, but the usual arrangement is to use one for the start battery, and the higher output one for the cabin battery. This also does away with the need for any split charge relay or diodes.
Fitting a bigger alternator is a fairly easy option, just make sure it will fit and the wiring is adequate.
Increasing the output of the current alternator is impossible, but by using an external controller (there are several types available) it can be made to charge the batteries more efficiently. A normal automotive alternator will reduce the charge current too quickly, but bypassing the internal regulator will increase the charge rate.

Rupert Smedley  | 2.39PM, Tuesday 3 July

Must be said that Calcutt Boats (the principle mariniser of BL1.5 and 1.8 engines on the inland waterways) rarely fitted a larger alternator than 70A (though did fit twin ones). You are lucky- often the standard alternator was 35A, later becoming 55A as the engines became (slightly) more refined!
As Rupert says, a normal alternator for cars, etc. has a built in regulator that lowers the charge rate (it isn't designed for heavily discharged batteries). Fitting an external regulator (like the Kestrel, or a Sterling alternator controller) massively increases the effective charging rate and current.
For a very simple-to-fit unit, the Sterling digital Alternator to Battery charger is very good- it fools the alterator into giving a full current (at lower voltage) and then it steps up the voltage, to give the charging characteristics of a 230V mains battery chargers. It only needs to be fitted between the alternator and batteries, and doesn't need any internal wiring to the alternator, which is good!
If you want to fit a second, or larger alternator, I would suggest that you contact the specialists, Calcutt Boats (www.calcuttboats.com). They will be able to provide appropriate brackets, pulleys and mounts (and even the alternator) that you need, and can advise on what size alternator can be fitted (bearing in mind power constrainsts and pulley bearing loads).

Mark Langley  | 6.16PM, Tuesday 3 July

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