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Adding a 4th battery #11586236 05/06/16 02:41 AM
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Bobby Milam Offline OP
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Just bought a new boat (new to me) and it came with new batteries. 2 deep cycle for the TM and 1 starting battery. I have a good battery (2 months old) left over from my old boat (deep cycle) and am thinking of adding it to the new boat since I have the room. I'm not sure which is the best way to go. Run it in parallel with the starting battery, connect it with a switch or just add it by itself and run my electronics off of it?

Right now I just have a 2 bank on board charger which I have connected to my 24v TM system. If I can run them in parallel, then I'll only need a 3 bank system?

I probably don't NEED the battery but having been stranded with a dead battery before and the fact that I already own a good one, I figure more is better.

Re: Adding a 4th battery [Re: Bobby Milam] #11586318 05/06/16 03:22 AM
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Flippin-Out Offline
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OK, here's the technical correct thing to do, which is not what some do, and you'll probably hear from them too.

Never hardwire anything in parallel. If one of the two batteries has a problem, it WILL negatively impact the other battery. There are many cases of people with 2 main batteries but still stranded because both are too low to start the outboard. As one battery dies, it drags down the other battery. Result: TWO useless batteries.

Get a battery switch. I can point you to a good one available on an auction site for $30. The two batteries get wired to this, and typically you select #1. IF, some day, #1 goes south, you move the switch to #2, and boom, it's like you never had a battery issue. Finish your day, then address replacing battery #1.

Charging: Best answer, get another 2 bank charger. Next choice: get a 3 bank, and connect bank #3 to the common connection on the switch. Then you can charge battery #1 or battery #2 by changing the battery switch. This choice works fine especially if you normally don't need to use battery #2. (That battery would just need periodic top-off charge to keep it ready as your back-up.) The switch is an advantage that you can also turn off all electrical if you need to.

Re: Adding a 4th battery [Re: Bobby Milam] #11586359 05/06/16 03:53 AM
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Bobby Milam Offline OP
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Thanks Flippin-out, I kind of figured that might be the advise that I was given. Sounds like the way I'll go. I think I'll be better off with a second 2 ban charger than losing money on selling the 2 bank to buy a 3 bank. I just have to get a little creative to fit that 2nd charger in there.

Re: Adding a 4th battery [Re: Bobby Milam] #11586367 05/06/16 03:59 AM
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Flippin-Out Offline
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Here's another advantage of a dual 2-bank charger setup. Let's say you are on a multi-day trip, and the TM charger drops a bank. You can borrow the #2 bank of that second charger to get both TM batteries charged. (The back-up battery should be OK if you haven't had to use it.) A 10A/10A two bank charger is all you should need for the starting batteries as the outboard helps with that too.

Last edited by Flippin-Out; 05/06/16 04:01 AM.
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