Engine Battery Drain - Is it the Prop Shaft Alternator?

Slavko Despotovic

Hi Alan,

Amel Super Maramu 2000 owners manual on page 16 have this:

12 V circuit: One only battery feeds the starters of both main engine and power generator engine. It is situated towards the front of the boat.
It is charged from two 12 V alternators one on each of these engines.

I do not if this is the same for SM. But you can easily check. Run engine and measure voltage on starter battery. Turn off the engine.  Then run generator and measure voltage. It should be higher than when both engines are turned off. 

SM 2000
#279 Bonne Anse in Portoroz

Courtney Gorman

Hi woody the only thing that charges the starter battery is the engine alternator so if you don’t start it regularly it dies sorry 😞 

On Jan 16, 2021, at 12:04 PM, Alan "Woody" Wood <woody@...> wrote:

Well this was an interesting one.. if you're into this sort of thing!

I had replaced a defunct engine room blower a few days previously, exchanging the Jabsco 35440-0010 with a 35770-0094 continuous duty blower.. and it completely drained the battery.
I was aware it was on the 12v starter battery system (even though its a 24v blower) but presumed if it wasn't operational then it wouldn't draw any power.
Evidently I was wrong :/ 

Clive Chapman

You could try disconnecting the engine start battery and see if the voltage drop continues. That would establish if the continuing volts drop is actually due to a dead battery or because there’s an unexpected load remaining connected to the battery (alternator exciter coils maybe? Alternator regulator failed?).

I don’t know the Amel electrical systems so am basing this on a reasonable knowledge of car electrics. 

Alan "Woody" Wood

We're currently in a marina, on shore power with the battery charger on.. and the starter battery (2 year old Victron 110ah) has suddenly died - showing less than 2v! and reducing 0.01v every 5 minutes).

We had our Valeo prop shaft alternator bench tested recently by a local electrician and put back.  When he replaced it, the switch for the generator had been reversed.. ie 'on' was now 'off' and vice versa.  I suspected the diode had been reversed or damaged in the re-istallation so disconnected the prop alternator hoping this would stop the drain - it didn't - so maybe it wasn't the problem.

The Mastervolt Chargemaster 24/100 has had intermittent display problems lately although still keeping the house batteries at 100% throughout.. could that be related?

I am at a loss at what investigate next - any suggestions? 


SM 1997