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Old 01-09-2011, 18:03   #1
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Simple Battery Question


I'm looking for a bit of advice on my battery set up. I have a Bristol 35 with 2 dual purpose marine batteries with a simple 1, 2, and both switch. I have been starting on both, running the engine on both to charge both and then while under sail or over night switching to one of the batteries to save one for starting.

One of my batteries seems to not be holding a charge as well and I will replace. Should I stick with 2 deep cycles or go for one starting and one deep cycle?

Am I managing these batteries correctly and if I switched to one starting, how should I change what I'm doing?


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Old 01-09-2011, 18:13   #2
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Re: Simple Battery Question

Never use your start battery as a house battery.

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Old 01-09-2011, 18:38   #3
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Re: Simple Battery Question

I'd agree with the sentiment to not use the starter battery for long discharge. It will not hold up as well.

You don't have to use both batteries to start, in most instances. I'd tend to leave the starter battery for starting and the deep cycle battery for house usage.

So, I'd recommend one starter battery, one Deep Cycle battery. Even though they can be used for either roles, they are optimized for the specific role and will give better performance and life if used coorespondingly.
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Old 01-09-2011, 18:43   #4
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Re: Simple Battery Question

With your kind of setup, I prefer to go with two identical batteries. However, the downside is that you never want to mix an old battery with a new one in this type of system. Therefore, I recommend you replace both batteries with new deep cycles.

This system is simplicity at its finest. You just need to remember to keep working the switch.
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Old 01-09-2011, 18:46   #5
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Re: Simple Battery Question

my boat uses a single 8d battery for starting. is also usable for house. my house are 4 golfcart batteries, 6v. both banks are deep cycle.
if you are on anchor, do not use your starting battery for house. if you use deep cycle batteries for each, is not a problem i have ever seen.
at anchor you can run out of juice. is harder to do that on a dock.
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Old 01-09-2011, 19:18   #6
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Re: Simple Battery Question

In the future you might consider purchasing a battery combiner. This way you do not have to remember to disconnect or reconnect the start battery. You can also use the start battery as a house battery. The combiner will disconnect the house load on it before it gets too low to start the engine and reconnect it when the DC system voltage is high enough to charge it.

You would want two deep cycles. I agree with Bash on getting two deep cycles now.

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Old 01-09-2011, 19:54   #7
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Re: Simple Battery Question

1) Two deep cycle batteries are better than one starting + one deep cycle

2) Blue Sea Systems makes a nifty kit that includes a battery switch and a charging relay: Add a Battery - Blue Sea Systems .
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Old 01-09-2011, 20:32   #8
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Re: Simple Battery Question

When you switch to 'both' to start, you may be in for a surprize. If one of your batteries is dead, the good battery will dump it's juice into the bad battery leaving you with two dead batteries and no start. Start with the battery switch on 'one' or 'two' but not on both except as a desperation move. Switch to 'both' after the engine is running and preferably with the engine only at idle. I'm sure you have a make before break switch but it you don't, switching at idle, with no load on the alternator, should keep you from blowing the diodes.
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Old 01-09-2011, 22:31   #9
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Re: Simple Battery Question

The 'dual purpose marine' batteries you have now are not true deep cycle batteries. They are a hybrid that can be discharged somewhat more than a starting battery but less than a true deep cycle. You may have been inadvertantly abusing them by treating them like deep cycles and that is why one of them is going now. Couldn't say for sure without knowing how old they are, how much maintenance you do to them and at what frequency, their recharging protocol, how low you go before recharging and how many cycles they have been thru. Don't bother to figure all that out, not germain to what to do going foreward.

I would keep one of your existing batteries as a starter and get one or two deep cylces for house loads. The marine battery can handle a deeper than normal discharge occasionally handling house loads.

Then the question becomes how do you want to charge everything. Adding a high capacity alternator with smart regulator for the house batteries, retaining the stock alternator for the starting battery and have two independant electrical systems would be my pick. It's simple, it charges somewhat faster and has redundancy. But it also presuposes that you have room for the extra alternator and the bucks for everything.

Various combining and isolating electronics would be my second choice if there was a constraint on the system previously described, but I haven't really ever researched those alternatives.
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Old 02-09-2011, 18:49   #10
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Thanks everyone for the comments!

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