3 battery bank; how far apart?

[japawil]

I currently have a 2 battery house bank (group 31) on the port side of the engine and a starter battery on the starboard side. I’m looking to add another battery to the house bank.
There is basically no room to expand for another battery on the port side, but there is room for another battery on the starboard side.
What would be the consequences of having a 3rd battery say a 6/7ft cable length away from the other 2 as part of the same bank?

[Checkswrecks]

Assuming you are like most in not regularly running high C charge or discharge on the house bank, there'd be no problem by connecting with heavy enough cable.

[john61ct]

Are we talking paralleling 12V units?

Not ideal, imbalance issues will reduce lifespan.

But not a critical issue, use nice fat wiring

[billknny]

Originally Posted by japawil

I currently have a 2 battery house bank (group 31) on the port side of the engine and a starter battery on the starboard side. I’m looking to add another battery to the house bank.
There is basically no room to expand for another battery on the port side, but there is room for another battery on the starboard side.
What would be the consequences of having a 3rd battery say a 6/7ft cable length away from the other 2 as part of the same bank?

Making a parallel battery bank isn't quite as simple as it looks at first glance, especially if charging or discharging at high currents. The small, but non-zero resistance of batteries and cables can cause significant problems with keeping charges equal across the bank.

You'll find some reasons and data and solutions here:
https://fetchinketch.net/boat_though...y-connections/

[Frankly]

I have 6- 6V GC2, 4 midship and 2 in the fwd bilge, normally all 3 sets in parallel. Connecting wiring #4 AWG maybe 30 ft round trip (+ fused on both ends). Fwd bank is tapped directly to run bow thruster (330 Amps), anchor windlass (75 Amps). Start 3GM from the main bank. Been running that way for 14 years and if it has created any significant problems I haven't noticed.


Frankly

[Stu Jackson]

I have 3 Grp 31s, two forward about 6 feet from the third. I used proper sized wiring between them and used the run-through wiring (+ / -) as recommended by the right stuff guys. Been working fine for 22 years. Not an issue if wired properly.

[Cheechako]

It will work fine. Forget all the theories. That's all about micro effects to the user. Just use big enough cable.

[japawil]

Thanks for the replies.
Billkinny, that’s what I had in the back of my mind when I asked the question. I guess the easiest/best thing would be to put a couple terminal posts in the engine compartment and feed the batteries equidistance from there to give me the best bet at maintaining equalization.
The house bank is currently at the farthest run of the cables, so I wouldn’t need to buy any more cable than I would have to connect to the opposite side of the compartment anyway!

[Clivevon]

I found this article useful on this subject:
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

about connecting batteries so they are balanced. Might be of help to you?

[Clivevon]

Or as a simpler alternative, as there isnt room for a third battery there, is there room instead to put 2 larger house batteries on the port side to get some extra capacity?

The problem with unbalanced battery connections meaning batteries possibly failing at different times is that I have always understood it is best to replace all house batteries together so the older ones dont pull down the new one? Balancing the battery banks helps to extend overall battery bank life in that situation.

[john61ct]

less of an issue with parallel than serial

fine points for ideal longevity vs good enough adapting to what needs must