Different Battery Capacities in Parallel

1 Posts
Oct 29, 2008 08:36 pm
Different Battery Capacities in Parallel

Hi, newbie here, I have a boat with 3 4D 12V batteries in parallel and I would like to increase the battery bank capacity.  Because of space constraints, I can only increase 2 of the three batteries to the 8D size.  Will having 2 8D's and one 4D have a larger usable capacity?
1 Posts
Oct 30, 2008 09:24 am
Re: Different Battery Capacities in Parallel

It is not advisable to use different battery capacities. you must use the same capacities if you want to increase battery bank capacity. add 4D also if space constrain take out existing 4D and replace it with 8D.
Nov 4, 2008 09:57 pm
Re: Different Battery Capacities in Parallel

Correct me if I am wrong but, aren't 8D batteries "big rig" cranking batteries? If so, are they being used for cranking an engine or lighting and entertainment such?
If I am not mistaken about 8D, they only have about 100 amp hours at the 20 hr. rate. Although the height of a Trojan T-105 "golf cart" battery would be greater, you might be able to get 8 of them on the same foot print. Wired in series parallel you could have 800 amp hours at 12 volts nominal instead of only 200.
Of course this is just a thought. You have the actual measurements, so...
Nov 4, 2008 10:23 pm
Re: Different Battery Capacities in Parallel

If you could give us the measurements L x W x H of your battery compartment and what the primary use of battery power is we could help you to get the most out of that space. Not that you can't do this on your own but, we may know of options that you may not be aware of.
Here are some places that may help you to decide a few things.
578 Posts
Nov 7, 2008 01:25 pm
Re: Different Battery Capacities in Parallel

hi thomas and all,

here is a link to an 8d battery used in RE systems for those that are able to lift it Wink


245ah @12v  they are pretty big and heavy with terminals on the same narrow side.  universal makes a version as well in sealed agm, and i am sure that other companies do as well.

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