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12V or 24V?

May 16, 2015
42
C&C 37 128 Portland
Dear battery heads:
With the imminent installation of solar on my C&C 37, I'm trying to decipher the actual voltage of the house bank. The PO put together a #2 bank consisting of:
  • 2 6V AGMs wired in series to make 12V
  • 1 newer AGM 12V (added later under q-berth)
  • + Xantrex Trucharge 20
6V combo wired in series.
Screen Shot 2021-01-28 at 11.47.26 AM.png

The 6V combo and 12V are connected. Below is a surveyor's shot of the single 12V in the q-berth--an incomplete picture but perhaps showing enough to explain what's going on--to someone who understands it.

12V wired in parallel with the 6V combo. Correct?
1611865311537.png

I'll of course look at this connection more carefully when I'm back on the boat in a week, but your comments are welcome.

A larger controller is needed for SunPower panels to charge a 12V system, driving the question: is this a 12V or 24V house bank?

Pretty green to this stuff, so be gentle.
 
Last edited:
Jul 7, 2004
7,904
Hunter 30T Cheney, KS
I get " You do not have permission to view this page or perform this action. " when I try to take a peek at the attachment.
 
Jul 7, 2004
7,904
Hunter 30T Cheney, KS
That works. It's not possible for me to see how the 12v batt is connected to the others though.
Did they use red cable for positive and negative connections? Why would they do that?
 
Jun 2, 2011
339
Hunter H33 Port Credit Harbour, ON.
The 6V are in series so you are correct it is 12V. If the small + wire from the 6V bank is the same wire as is connected to the 12V it is connected as 12V. It looks like your system is 12V. The 12V battery has a large + wire. If it goes to the starter the 12V may be a dedicated start battery. The small + may be run through an automatic charge combiner. Not really enough in the pictures to offer anything else. I don't believe that pleasure craft use 24V. Work boats with big diesel engines - probably. Good luck.
 
May 17, 2004
3,420
Beneteau Oceanis 37 LE Havre de Grace
Hard to tell from the second picture, but if the 12V is in parallel then what you have is a 12V bank, not 24. The 12V battery would need to be in series with the series-6V bank to add up to 24V. It does kind of look like the red wire from the 6V bank is connected to the positive post of the 12V battery, which would be parallel. Series would be positive to negative, like you see the two 6V batteries connected.

Having a newer 12V battery connected to two older 6V batteries in parallel isn’t really ideal. You might find that one or more of those batteries are depleted as a result. The batteries in a bank really should be better matched than that for maximum longevity. If you find that the batteries still have enough capacity for your use then I guess keep with it for now, but consider revising the next time you need to buy new batteries.
 
May 16, 2015
42
C&C 37 128 Portland
That works.
It works well. Learning that wiring in series to a third battery causes issues. I have none, so all is good;) I'm going with the 12V assumption.

Yep, that red cable throws me off, too. I'll confirm it's a parallel connection & mark neg cable accordingly.
 
May 16, 2015
42
C&C 37 128 Portland
If it goes to the starter the 12V may be a dedicated start battery. The small + may be run through an automatic charge combiner.
No and yes. #1 bank is a separate dedicated start battery and not in this supply. And yes, the small wire comes from the Xantrex.

A separate question is the importance of inserting "over-current protection" (fuses) in battery cables. Our surveyor recommended this, but some say not necessary.
 
Jan 11, 2014
7,722
Sabre 362 113 Fair Haven, NY
It is highly unlikely that you have a 24v system. Most boat electronics are 12v. Some boats do use 24v for high loads like windlass, bow thrusters and so forth.

If you don't have one yet, you should purchase a decent quality Digital Voltmeter. Don't buy a cheap Harbor Freight. Fluke meters are the top of the line, there are others that close. Klein and Ampprobe are also decent quality. Next learn to use the meter, it will be invaluable and will answer your question.

To really understand what you have you'll have to trace the wires and diagram them. Even in these 2 photos there is much to be suspect of. Where does the lighter gauge cable go? Where are the switches and how are they wired? Without knowing this we are just guessing.

Batteries wired together in series or parallel should be matched, same brand, same size, same manufacturing run.

The Xantrex 20 only puts out 20 amps which is under sized for your battery bank. You have about 300 amp hours of power between the 2 6v batteries and the 12v battery. Ideally, the batteries should be charged at 10% of the total capacity, which would be about 30 amps. From a quick look at the manual it looks like this is an older charger and not a "smart charger" smart chargers will extend your battery's life.

The small cable looks a little small for a paralleling cable. It should be the same size as the larger cables on the 2 6v batteries.

There is lots to think about when adding solar. Getting your money's worth out of solar takes more than a big panel and large controller. See the article below for more information about solar panel construction and planning.

This is all probably more than you wanted to know. A good book to start with is Charlie Wings Boatowner's Illustrated Electrical Handbook (Amazon Link).

Keep us posted.

 
Jan 11, 2014
7,722
Sabre 362 113 Fair Haven, NY
A separate question is the importance of inserting "over-current protection" (fuses) in battery cables. Our surveyor recommended this, but some say not necessary.
Yes add over current protection, the easiest way to use Marine Battery Terminal Fuses. They should be sized for the cable.

 
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Jan 11, 2014
7,722
Sabre 362 113 Fair Haven, NY
If it is any help, this is all a mystery until suddenly it isn't and you wonder why it seemed so mysterious. ;) Read, research, ask questions.

Visit the Musings With MaineSail forum here on SBO

and visit

 
Feb 11, 2017
121
former Tartan 30 New London, CT area
A fuse is NOT required in the lead to the starter. A fuse IS required in the lead to the house breaker panel. It should be within 6' of the battery (as I recall) and sized to protect the cable to the panel.
 
Jan 11, 2014
7,722
Sabre 362 113 Fair Haven, NY
A fuse is NOT required in the lead to the starter. A fuse IS required in the lead to the house breaker panel. It should be within 6' of the battery (as I recall) and sized to protect the cable to the panel.
ABYC requires a fuse within 7 inches of the battery. No fuse is required on a dedicated start battery, but it is a really good idea to put one there.