There are two port engine batteries, two starboard engine batteries, and two generator batteries. The port batteries also supply all the 12v systems including bilge pumps. The existing batteries were one starting batter and one deep cycle. Mismatched cranking amps and amp hour ratings. I replaced those with two deep cycle 800 cranking amps and 200 amp hours batteries.
The reconfigure plan is to move the 12v supply to the generator battery bank and beef it up with 3 high amp hour deep cycle batteries.
StarsMan, I am new to the powerboat realm and know nothing about twins but I wonder at why you have two batteries for each engine and genset? Is this common with this boat?
I would assume a single good starting battery for each engine and the Genset would work well. Then a separate house bank for all your electrical needs.
I am guessing the double batteries on both engines and genset are basically insurance. The manual only references one battery per engine and in the original factory configuration the 12v system was split between those two batteries with port battery running port 12v stuff and starboard running starboard 12v stuff and both bilge pumps coming from the port battery.
Anyway, in answer to original post, yes a single battery per engine would be adequate for starting. Same with gen. Using two is like wearing a belt AND suspenders I guess. My uncle has the same two battery set up for his single engine trawl boat.
Our boat -- and most (maybe all) built by the various companies in the same manufacturing group -- are usually configured with two "large" battery banks, plus a genset starter battery if a genset is installed.
Each of the two large banks does triple duty: starts an engine, runs about half of the DC house, and then also services one of the bridge functions (electrics, on one, the electronics suite on the other).
There's also a parallel switch, so either large bank can augment the other to start an engine should that be necessary. And then there's the genset/charger option, if it comes to that.
I wouldn't have thought to do it this way for a cruising boat, given potential for days at anchor... but IMO it has turned out to be a relatively simple, relatively efficient design/implementation. I suspect it was probably influenced buy the manufacturer's expectation that a major percentage of these boats would be used more for marina-hopping than for long periods of cruising and at anchor. Still, it's turned out to be reasonably manageable.
Ours started life with at least two "dual purpose" Group 27s (or maybe 29s) on each large bank, and I think the genset battery was a G27, too. (I'm a little unclear on thae two larger banks because the original owner either replaced one battery in -- or maybe added a battery to -- each of those banks, so when we got it, each bank was kind of mis-matched.) I replaced all those with 3x G31s for the large banks and a single G34 for the genset starter.
That has turned out to be quite sufficient... although my next step, whenever I have to replace our (now 10-year-old) starboard bank -- the one that runs all our electronics while I'm trolling on the port engine -- is probably to replace the G31s with 4x 6V golf cart batteries. That would still be in spec with engine starting requirements... fit in approx. the same physical space... and at the same time would increase Ah on that bank from a nominal 300 to a nominal 440 or so.
-Chris
