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Old 19-06-2021, 07:39   #1
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Battery bank wiring 6V Series Parallel

Hello, my new to me sailboat has six 6V deep cycle batteries wired in series and parallel for one 12V bank. The series wiring is straight forward with each pair. The parallel wiring is achieved by the positive of each pair joining at a bus as shown in attachment. My understanding is that this is acceptable but not as balanced or efficient as it could be. I have re-wired it on paper as shown in the attachment. This appears to me cleaner and more balanced and efficient. Is this a correct line of thinking and worth the effort to change? Thank you.
Attached Files
File Type: pdf Existing setup.pdf (43.2 KB, 204 views)
File Type: pdf Proposed setup.pdf (45.6 KB, 173 views)
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Old 19-06-2021, 07:49   #2
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Re: Battery bank wiring 6V Series Parallel

Leave it.

Existing wiring allows you to quickly remove a section in case of emergency.

Rewiring with 1 switch means you lose control of isolating a set.

Is of same efficiency but with less control if rewired.
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Old 19-06-2021, 08:16   #3
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Re: Battery bank wiring 6V Series Parallel

The PO has handed you an ideal arrangement (IMNSHO). Assuming there are some fuses downstream of that positive bus find something else to work on.


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Old 19-06-2021, 08:29   #4
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Re: Battery bank wiring 6V Series Parallel

A vote for the existing topology, ie three battery connections to a common bus bar, followed by a switch. I’d replace the three switches with MRBF fuses. Since 1965 when I started sailing, only once have I needed or wanted to isolated one set of batteries is a paralleled bank.
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Old 19-06-2021, 09:33   #5
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Re: Battery bank wiring 6V Series Parallel

We have nearly the exact setup on our boat as your present scheme, and have had it that way since 2002 when I redid the DC system. After a massive battery(175lb and full of sulfuric acid) had an internal short and almost caught fire, I wanted a way to isolate and remove a bad battery....by myself and without shutting down the entire boat. We now use 6-Trojan T-105s.The only difference/change from your existing is that I added another on/off switch between the (+) Bus and the 175 amp fuse that leads to the house load. If you are concerned about balancing, make all your (+) cables the same length, all your (-) cables the same length, and the cables to/from your switches the same length(equal length = equal resistance). FWIW, we also use a separate , stand-alone sealed (car) battery for eng(main and/or genset) starting. That battery is kept charged by a Balmar Duo Charge off the house bank busbar, and by the genset alternator. The main engine alternator feeds the house bank busbar, as do our solar panels & our windgen; our 3-bank battery charger. feeds each 2-battery "battery" directly. Our system has worked perfectly for us since 2002, and our T-105s last 6-7 years typically.....and if one fails I can isolate it and safety carry it up/down the companionway steps...by myself!
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Old 20-06-2021, 07:30   #6
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Re: Battery bank wiring 6V Series Parallel

Notwithstanding what others have said here, I've always been told by many more knowledgable than me that your proposed connections is the better way to wire. It will result in more even distribution of the energy with more balanced charging/discharging of the batteries.


I understand the possible benefit of being able to disconnect one pair of batteries, but that can be easily done with your proposed wiring as well.


Might want to check with the battery manufacturer to see what they recommend, or a QUALIFIED marine electrician.
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Old 20-06-2021, 12:15   #7
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Re: Battery bank wiring 6V Series Parallel

If all of the cables are the same AWG and length, then the existing setup is satisfactory and each battery will be charged and discharged equally. The downside is that in achieving equal length cable lengths, you may well end up with excess cable that you have to deal with.
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Old 20-06-2021, 12:57   #8
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Re: Battery bank wiring 6V Series Parallel

The two configurations, eliminating fuses, switches, etc., have exactly the same number of wires and terminations. If all the wires for the same purpose, ie all the wires used to connect the positive or the negatives bus bars, are the same length, the batteries will be equally "balanced" in either configuration.

In the "existing" configuration, the failure of any single connection only effects one pair of batteries. In the “proposed" configuration, a failed connection can affect any or all of the pairs. So, IMHO, the current topology is a balanced as you can get and less subject to single point failure.
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