Alternator/Battery Bank Sizing

[pmyatt]

We are an all-electric boat, through choice. Our present electrical system comprises a Balmar 70 Amp alternator (with smart charger) charging a Domestic Bank of 4 X 105 Ah Delphi Freedoms, a Starter battery of 1 X 105 Ah Delphi Freedom, and a 90 Ah Red Flash AGM Windlass battery, charged by way of a Xantrex EchoCharge Unit off the Domestic Bank. The engine is a Yanmar 3GM30F. Additionally we have a Fischer Panda 4000iAC generator to run the air conditioner, the electric (induction) hob, and microwave combo oven, and the water-maker. To reduce battery charging frequency, we want to increase the size of the domestic bank but, to do so, requires that the alternator size also be increased. The present alternator belt is 3/8" and will not really be suitable for a larger capacity alternator. Has anyone any thoughts/recommendations on sources of alternative wide, pulleys for the alternator belt and what size alternator should we be aiming for if we up the domestic bank to 8 X 105 Ah batteries - choosing the 105 Ah instead of going for 210 Ah as can't lift the latter.

 

[William_H]

Alternator size

I think it is a bit of a myth about the alternator size as related to battery bank size.
An alternator will run out of power ie deliver no more than its rating, when it is asked by flat batteries or smart charge controller to provide more current than it can do. Alternators generally fall in voltage so usually this gives a sharp reduction in charge voltage despite what the smart charge controller asks. The charge controller can only provide max field current so max power assuming alternator is turning at max revs.
So apart from possible overheating of the alternator and possible reduction in service life due to higher field current in brushes and higher mechanical loads on bearings an under rated alternator will simply not charge so much as you might want.
If on the other hand you fit the larger battery bank and find that your alternator current is apparently maxing out when you do you engine run for battery charging then you might benefit by a larger alternator and so more current.
If you find that your 70A alternator is mostly only giving 50 amps or less then a bigger alternator will give no more. Remember battery charge current is what the batteries will take not what the alternator will give.
Regarding belt size. Again if the existing belt is slipping due top alternator load then yes you need to upgrade the belts. But if in normal service this doesn't happen don't worry. Just keep spare belts.
good luck olewill