Gary Warner wrote:
The electrical setup I've created calls for two
12 volt batteries. They are on an A/B/ALL
switch with a battery-combiner (BC) between them.

It would be simpler, cheaper, and more foolproof to simply have each
battery on a key switch. The A/B/Both/Off switches create all sorts of
problems from dead batteries to fried alternators.


Normally I plan to run with the switch in
just the "A" or "B" position. So when the engine
is running the BC will combine the batteries and
both will be charged. When the engine stops and
we are drawing current for light & such, only one
battery will be supplying power. If that one got
drained we could switch to the other to restart
the boat.

Exactly!


QUESTIONS:

TYPE OF BATTERY: I know that Gel or Absorbent Glass Matt
(AGM) are supposed to be better. But for this application
where these is not much vibration, no need to mount the
batteries in any position except "normal" and where the
battery may not get 100% proper storage (ie no trickle
charger all winter) I'm thinking the extra cost is not
worth it. I'm thinking just use regular flooded batteries.

Agree. I don't see enough advantage in your usage to get AGMs, unless
the boatteries are in an inaccessible place and will be difficult to
service.

Also, you don't mention it but are you putting in a hard wired charger?
Definitely get & use a 3-stage smart charger no matter how you rig it.



SIZE will be group 24. I believe the space that I have
for the batteries (under the passenger seat) will take
two Group 24 batteries.

Might be too small. If you're using 2 dual purpose instead a starting &
a deep cycle, then you're getting less cranking amps and less amp-hours
capacity.

http://www.cs.ucf.edu/~matt/leadacid/guide.html


CCA/MCA: I really don't know what size Cold Cranking Amps
or Marine Cranking Amps is necessary/good to turn over the
engine I have. I'm guessing that anything in around about 550
is fine. If I can get more for not much more $$ than that's what
I'll do.

Agreed. This is one case where more is better.


RESERVE CAPACITY: I really don't have much draw on these batteries
so pretty much whatever the RC rating is will be fine for my use.

TYPE: I'm thinking that Dual-Purpose is the way to go. It is possible
that I'd draw a batter down when occasionally sleeping on the boat
overnight and Starting batteries don't like that very much.

neither do dual purpose, for that matter deep cycle batteries get rather
upset if you discharge them past 50%


TWO OF THE SAME: Since they will be charging in tandem (together)
it's best to have two of the same. Also, I'll sometimes have the switch
on "A" and sometimes on "B" so both get approximately the same
useage pattern.

If you were oversizing the batteries so that there wasn't risk of
damaging or hard sulfating them starting the engine, I'd say fine. But
since you're limited in size I'd suggest getting a starting battery and
a deep cycle and marking the switches as such. You might also consider
getting two dual purpose batteries and then finding a place for two golf
cart batteries somewhere else for house loads.



BUYING:
+ Check the date manufactured on the battery and try to get ones that are
less than 3 months old.
+ Try to get them with a longer free-replacement period as this is a good
sign of quality
+ Watch out because replacement are sometimes pro-rated and based on the
LIST price, not retail.

Good. I bought two Exide Gr-31 batteries that were approx 6 months on
the slef and they are not really satisfactory. If you can buy dry
batteries and have the vendor fill & charge them, that'd be better too.
Only a battery store will do this though.

Fair Skies
Doug King