6v bank vs 12v bank

[ranger58sb]

Originally Posted by S/V Illusion

The whole “if one fails” argument seems pretty lame. How many people have only one bank of 2 -6V batteries? Or one 12V battery? In either case, “if one fails”, who’s bad landing is that?

FWIW, I think it's a legit point for examination, but then I also think it's situational.

For boats with room for only 2x 6Vs or 1x 12V battery, you probably make do as best you can, hope for the best, and keep your towing insurance up to date.

OTOH, for boats with more space, more systems, more batteries in a given bank, and maybe away from services/stores for longer periods... I don't think it hurts to keep that "what if one fails?" point in mind. Then maybe or maybe not act on it...

Given our own situation, two dual-purpose engine/house banks, I wouldn't go with less than 4x 6Vs or 2x 12Vs.

(Of course it also happens a remaining 2x 6Vs or 1x 12V battery wont start one of our engines anyway... but I do have a parallel switch for both banks and that could possibly solve that problem in a pinch.)

-Chris

[mikeo16]

FWIW, I presently have 3 12-volt group 31 batteries with 2 being used for house and one as a reserve. When my house bank is replaced I plan on switching to 2 6-volt and keeping the reserve with the single group 31 12-volt battery. By going to 6-volt batteries for the house, the house capacity will be a little larger at 230 amps versus 210 amps using 2 12-volt group 31 batteries.

[EmeraldCoastSailor]

Never heard of a six volt battery producing over 400AH.

[Dsanduril]

Wow, such a simple question from the OP and no one wants to actually answer. With the same ultimate bank size, which is better, two 6V batteries in parallel or two 12V batteries in parallel? Sure, size and cost and cycle capability matter in selecting the batteries, but if those things are equal, what is a better installation?

FWIW I have a 12V parallel bank. One of those batteries died in the middle of a passage across the Indian Ocean. I did not observe (different than there were none) any signs of impending failure prior to its occurrence (if I had I’d have replaced the battery before setting out). I was able to remove the paralleling jumpers and continue on my merry way. Had that been two 6V batteries in series I’d have been without juice in the middle of a passage. So, I, at least appreciated the redundancy.

If I had 4x6V batteries then I’d also have been OK, but for the specifically asked question of two batteries I’d prefer parallel.

[Gtstricky]

There is also the voltage drop issue. 24v can utilize smaller gauge wire and make longer runs (like to a windless) more feasible. However a downside is that some electronics only come in 12v requiring a method to lower the voltage.

[EmeraldCoastSailor]

Maybe I am missing something but it doesn’t seem reasonable to compare 2 6v batts in series to 2 12v batts in parallel. I get comparing 4 6v batts in series and parallel to 2 12v batts in parallel. I switched to the 4 GC batt approach 6 years ago and will repeat when I need to replace.

[ramblinrod]

Originally Posted by EmeraldCoastSailor

Maybe I am missing something but it doesn’t seem reasonable to compare 2 6v batts in series to 2 12v batts in parallel. I get comparing 4 6v batts in series and parallel to 2 12v batts in parallel. I switched to the 4 GC batt approach 6 years ago and will repeat when I need to replace.

The comparison makes perfect sense.

2 x 6 Vdc batteries in series may provide ~225 Ahr @ 12 Vdc nominal.

2 x 12 Vdc batteries in parallel may provide ~ 225 Ahr @ 12 Vdc nominal.

Apples to apples.

I do not recommend the 2 x 6 Vdc bank option to my clients due to the lack of redundancy/ usability in the event of a battery failure.

[john61ct]

Fine, so long as true deep cycling batteries are purchased, not crap from Wally world.

If only true deep cycling in 6V FLA were available in smaller sizes like ~100Ah at a reasonable price.

Personally I can't imagine getting by with only a couple hundred Ah but people do.

Also, I prefer to get redundancy from a Reserve bank you can activate simply by flipping a switch.

Having to physically reconfigure the bank in an emergency would make me nervous but I guess each unit could have its own isolation switch at the parallel connections.

[Michael Cobbe]

Originally Posted by CaptMDG

What are the pros and cons of having either 1) two six volt batteries in series producing 450 AH, or 2) two twelve volt batteries (225 AH each) in parallel producing 450 AH. I am interested in the practical physics of these configurations. On paper they are the same, or are they?
I am not interested in costs, sizes, or weights of each bank, just in the practicality of each.

6v volt batteries wired in series last for 5 years, 12 volt ones last less than that.

[john61ct]

oversimplification but in general, true proportionally

[ranger58sb]

Originally Posted by Michael Cobbe

6v volt batteries wired in series last for 5 years, 12 volt ones last less than that.

Might be true if comparing deep cycle 6Vs to dual-purpose 12Vs. Might be true if comparing batteries of average quality. Don't think that's dogma, though.

Lifeline offers 12V deep cycle batteries (e.g., GPL-31XT); wonder how they'd compare.

I'm getting 10-12 years from Odyssey 12V AGMs.

-Chris

[S/V Illusion]

Originally Posted by ramblinrod

I do not recommend the 2 x 6 Vdc bank option to my clients due to the lack of redundancy/ usability in the event of a battery failure.

If the intent is to provide true redundancy maintaining sufficient capacity, it makes no sense to rely on one 12 v battery. Better advice would be two separate 6 v banks which can also be combined.

[john61ct]

Originally Posted by ranger42c

Might be true if comparing deep cycle 6Vs to dual-purpose 12Vs. Might be true if comparing batteries of average quality.

No still holds most of the time, apples to apples.

> Lifeline offers 12V deep cycle batteries (e.g., GPL-31XT); wonder how they'd compare.



> I'm getting 10-12 years from Odyssey 12V AGMs

Yes if you need AGM, they are more available in good quality 12V format than FLA.

Not as good value though, if you don't.

[Pete7]

Originally Posted by john61ct

Personally I can't imagine getting by with only a couple hundred Ah but people do.

Well what about those who don't want or need a 50ft mono or 45ft cat and the running costs involved? Since a battery is only a store of energy, if you use it you have to replace it at some point. A couple of hundred AH for a house bank is fairly typical for a 25-35ft yacht btw.

The reality on a smaller cruising yacht is that weight is so important and it's dead easy to fill the boat with stuff bit by bit and not realise. Does an extra pair of 6v FLA matter? perhaps not on their own, but then what about the genny, dinghy, outboard, bicycle, fridge, solar panels, cruising chute and spare sails etc. Weight mounts up and has to be stored somewhere.

I know from personal experience (our total is 230AH) a small yacht can run quite successfully on a couple of hundred AH. Match the demand with appropriate sized solar or wind generation with occasional engine runs or shore power and you have a winning formula. The genny then becomes less important, if needed at all saving weight and cost.

The difference might be being able to go cruising on a smaller yacht minimising on extras or trying to save for ever for a larger yacht. We all have a choice, what will you choose?

Originally Posted by john61ct

Also, I prefer to get redundancy from a Reserve bank you can activate simply by flipping a switch.

Easy done with a couple of battery switches to combine house and engine start unless your thinking twin house banks, meh

[ranger58sb]

Originally Posted by Michael Cobbe

6v volt batteries wired in series last for 5 years, 12 volt ones last less than that.

Originally Posted by ranger42c

Might be true if comparing deep cycle 6Vs to dual-purpose 12Vs. Might be true if comparing batteries of average quality. Don't think that's dogma, though.

Lifeline offers 12V deep cycle batteries (e.g., GPL-31XT); wonder how they'd compare.

Originally Posted by john61ct

No still holds most of the time, apples to apples.

> I'm getting 10-12 years from Odyssey 12V AGMs

Yes if you need AGM, they are more available in good quality 12V format than FLA.

Not as good value though, if you don't.

Have you seen a longevity comparison between Lifeline's GPL-4CT 6V AGM and their GPL-31XT 12V AGMs? (Or a similar comparison between 6V FLAs and deep cycle -- i.e., not dual-purpose -- 12Vs? (Rolls, I guess?)). The info on Lifeline's website suggests an equal number of lifetime cycles...

For me, "value" includes not having to yutz around trying to get access to service our AGMs, and reduced off-gassing... so its not a strict $$$ evaluation.

-Chris