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Old 20-02-2015, 08:36   #1
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Is it OK to combine different size batteries in the same bank?

Hello all, I know you're not supposed to combine flooded with Gel or AGM batteries but is it okay to combine A bank of 4,6v batteries wired in series and parallel with two Group 24 batteries?
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Old 20-02-2015, 08:50   #2
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Re: Is it OK to combine different size batteries in the same bank?

I would not recomend it. Its like chain you are only as good as the weakest link (Battery).

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Old 20-02-2015, 09:22   #3
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Re: Is it OK to combine different size batteries in the same bank?

We have our two group 24 engine start batteries paralleled into our 6 6V house batteries for ~10hrs each day. No problems in 16yrs.

So if you are combining these for charging purposes, or for extra oomph when needed to start something, then there is no problem at all.

If you intend the G24's to be a permanent part of the house bank, then that is only doable if they are also true deep cycle batteries. However, this still is far from optimal.

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Old 20-02-2015, 09:35   #4
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Re: Is it OK to combine different size batteries in the same bank?

Read the countless previous threads on this subject--it has been beaten to death.

Bottom line is that it will work just fine.
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Old 20-02-2015, 09:36   #5
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Re: Is it OK to combine different size batteries in the same bank?

Yes it will work fine. It may not be the ideal setup but if the batteries are in good condition there is no need to discard them just because they are not an exact match.
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Old 20-02-2015, 09:38   #6
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Re: Is it OK to combine different size batteries in the same bank?

I saw a statement from one of the manufacturers a bit ago that said that different sizes were ok as long as they were the same age, category ( gel, agm,...) and and battery type ( deep vs starting). Since different makers use slightly different chemistry even for the same type and category of battery.

Don't remember which company it was.


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Old 20-02-2015, 09:50   #7
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Re: Is it OK to combine different size batteries in the same bank?

OK. SoI guess that's a bad idea.
I was trying to squeeze another hundred 50 amp hours into the system but I'll forget about that and stick with 460 amp hours.
When I wire up the 4, 6vs, is it okay to use busbars to splice two cables together instead of going out and spending $200 on new battery cables?
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Old 20-02-2015, 09:56   #8
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Re: Is it OK to combine different size batteries in the same bank?

FYI, the two group 24's are pretty old but still functioning so that's why I guess it's a bad idea. Although I could buy two brand-new group twenty-fours that would add 150 additional amp hours to my system. The way they're wired now is they are all linked together like a chain. I could have individual positive wires going from the panel to the group 24's and 6vs.
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Old 20-02-2015, 10:04   #9
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Re: Is it OK to combine different size batteries in the same bank?

Think similar is ok. Most controllers assume all batteries in a group are the same. Lead acid, deep cycle, etc.

The trusty deep cycle lead acid battery will put up with a lot of abuse as long as you keep the levels up and keep them charged. Each charge / discharge cycle will consume some of their life.

If you keep a detailed log of water consumption, charge cycles, etc you will quickly see if your proposal is valid or not.

Make sure your connections aren't undersized or have measurable voltage drops.

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Old 20-02-2015, 11:43   #10
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Re: Is it OK to combine different size batteries in the same bank?

It's not the best plan but your boat won't catch fire or anything like that. For even distribution of the load and even charging, they should all be the same. That would be the same size, chemistry, brand and age.


For just "working", others have already reported that it will work. It just works better if they are matched.
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Old 20-02-2015, 12:22   #11
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Re: Is it OK to combine different size batteries in the same bank?

Quote:
Originally Posted by Onemoreproject View Post
...is it okay to use busbars to splice two cables together...
It will work, but each connection produces a voltage drop.

Why not replace the old batteries with another 6v pair identical to you main bank?
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Old 20-02-2015, 12:23   #12
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Re: Is it OK to combine different size batteries in the same bank?

Quote:
Originally Posted by Adelie View Post
I saw a statement from one of the manufacturers a bit ago that said that different sizes were ok as long as they were the same age, category ( gel, agm,...) and and battery type ( deep vs starting). Since different makers use slightly different chemistry even for the same type and category of battery.

Don't remember which company it was.


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Yes, after hearing that information from Interstate (I think it was) I wrote back asking them why. After two more emails they finally said they would ask a technician. The technician responded that there was nothing wrong with mixing different manufacturer, age, category, type, chemistry so long as they were all the same voltage rating.

DUH You are dealing with a kid in an office cubicle who reads the same mis-information everyone does and passes it on.
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Old 20-02-2015, 12:42   #13
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Re: Is it OK to combine different size batteries in the same bank?

Next we can argue about how to connect the batteries to the boat. Which terminals to take the positive and negative from. The same "end", opposite ends, separate leads from each, etc. Get into the .001 ohm resistance difference and how that unbalances the charging, etc.
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Old 20-02-2015, 14:52   #14
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Re: Is it OK to combine different size batteries in the same bank?

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Originally Posted by Terra Nova View Post
It will work, but each connection produces a voltage drop.

Why not replace the old batteries with another 6v pair identical to you main bank?
Because I don't have a room for more 6 V batteries
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Old 21-02-2015, 18:45   #15
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Re: Is it OK to combine different size batteries in the same bank?

NO! all the batteries in the same bank need to be the same size and type. If not the batteries will not recharge properly. Also if you have more than one bank it is best to charge them separately unless you have separate charging circuits.
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