Two Banks used as one.

[JohnB]

[quote=Charlie]Hello All:

"I am about to purchase a new (to us) boat. It has 4-6volt batteries and a starting batterey. The boat has alot of electronics on it: radar, GPS, three different 12v refer/freezers, and a 110v washer dryer that can --I've been told -- run off the inverter. I've been told that their are 400 ah of power. "

Doesn't 4 six volt batteries @ 100 ah each equate to 200 ah at 12 volts ?? This would give a max effective ah of 100 using a 50% discharge. Not enough to run the above.

JohnB

[btrayfors]

They're likely to be golf cart batteries, rated at 200-225AH each at the 20-hour rate. Thus two 6-volt golfcart batteries in series = 225AH @ 12 volts and if you series/parallel the other two batteries you have 450AH @ 12 volts.

[hellosailor]

Charlie, if you look at the online & printed materials from battery makers, and from most of the folks in the 12v "power" businesses, you'll see a consensus that the best way to hook up batteries in MOST situations, is to make 12V banks by adding cells in series--not in parallel. Once you go to a series+parallel or plain parallel setup, you add possibilities for incorrect charging and other problems that simply can't exist with a series-only setup.
If you read up on that, and you understand and appreciate the potential problems, only then think about these series+parallel setups, no matter what fancy charging or monitoring gear you have.
The simplest most robust way to go is series-only banks, and if you need more than one, you switch between banks. That also gives you some redundancy and less urgency if one cell fails and only takes out one bank. The most cost-effective way to build battery banks is by adding 6V cells to make 12V (Sam's Club or Trojan "traction" batteries for golf carts are popular) or if you want more power, go to an industrial battery supplier where they sell single 2.2V cells for fork lift batteries. Wire up six of those, and you'll have one whopping huge 12V bank--with each cell still small enough to lift (well, more or less) and replaceable if it fails.
As for the series+parallel bank you've already got...well...I'd say probably leave it be, treat it as one bank and use it most heavily--so you can replace it with something better, sooner.<G>

[Charlie]

Thanks Hello Sailor:

Between your words of wisdom and a few other peoples I am going to just wait and see. I'll use the boat see how it works out and study it some more then decide. At this point I think it would be good for me to get either a second or a bigger alternator (only rated at 75 amps) and see how my actual Ah consumption actually is.

Bill youare correct they are golf cart batteries. It is 440 Ah. What do you think of Hellosailors idea with forklift batteries. One thing that comes to mind is trying to replace them in a third world country may be nye impossible. Probably have to carry an extra cell or two.

[GordMay]

Charlie (&hellosailor):
1. Series wiring a set of Fork Lift cells is not a DIY job for most of us. There’s a lot of bus-work to get right.
but
2. There are probably more fork lifts than sailors in third world countries. Locating single cells may not be an insurmountable problem.
and
3. A 400 A/H bank c/w 75A Charger represents a fairly substantial elect. capacity. As long as you continue learning while you do; waiting to see may be a good first choice. You’re getting lots of good information here, and there’s much more available on-line.

[Charlie]

Thanks Gord:

I think I need to wait till I'm a little further up the learing curve before I make major ($$$) changes. I'm thinking of replacing the engine instead of increasing the battery power. Engine has 5k hours on a yanmar. If the former owner kept up the engine like the elec its probably a good idea to repower. Never Monday says he can get me a good price on the engine.

[hellosailor]

Charlie-
"forklift batteries. One thing that comes to mind is trying to replace them in a third world country may be nye impossible."
As Gord notes, I was going to ask if you think 3rd world boondocks are more likely to have fork lift batteries" Or golf cart batteries? But then again, former imperialist powers and odd dictators may indeed have spread golf carts further than fork lifts. Hmmm....Odd little planet.<G>
A 400AH bank is actually well matched to a 75AH alternator. The safest max charging rate for a 400AH wet cell bank is 400/5=80AH, and that's the max. So assuming your regulator will allow a full charge, and your alternator can take the heat, that's about all you need. 90-100AH probably would be ideal since that could run the boat (electronics, reefer, etc.) while charging, but 75AH isn't so bad. 100AH alternators were fairly rare in cars until maybe 10 years ago when they changed from "police/taxi service" to being normal for all the new gizmos.
If nothing else, the existing alternator will make a useful spare when/if you do upgrade.

[Pblais]

[quote]A 400AH bank is actually well matched to a 75AH alternator. /QUOTE]

This is pretty much what I have used for the past 3 years. You can go a few days before you need to worry if you conserve power and just let the fridge do what it needs. I added a battery monitor and it really is the ticket. You learn what you actually use and what conservation really works and you learn about charging and how well you do on the plus and minus side of the bank account. That alone builds the habits to do what needs to be done when you have to.

Things work best when you really have an instinct of what you use, what is left, what you want, and what you can have. It would be pretty easy to trash a bank in a few months of bad habits and misconceptions. The monitor translates all the rules of thumb to what you really do. They suddenly work. Avoiding really deep discharges just is going to save you a whole lot of money.