6v bank vs 12v bank

[ramblinrod]

Originally Posted by john61ct

Very low expectation, especially if both numbers are SoC there, even a Wally world special can do that

Exactly.

[skipgundlach]

Originally Posted by EmeraldCoastSailor

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

You live a sheltered life

Originally Posted by mitiempo

Based on those numbers you are comparing 2 L-16 6 volt batteries - almost 17" high and 120 lbs each to 8-D 12 volt batteries - nowhere as tall but over 20" long and about the same weight.

If you have room for either of the above why not use 4 Trojan T-105 or similar golf cart batteries in a series/parallel configuration for similar amp hour capacity in a much easier to handle size?

Our Trojan L16H-AC are 435AH each. They are heavy suckers, however, and it takes some serious muscle to change out a battery.

That said, we have two times what the OP was describing; L16 are the same footprint as golf cart batteries but twice as high and twice as heavy in order to achieve the twice as many amps.

Some L16 are as little as 385AH, but the norm is closer to or over 400AH.

You can see installation during our swap into the Trojans here:

Pictures: Flying Pig 2015-2018 Shake-and-Break-Down/Electrical Upgrade/Batteries

[sailcrazy]

For another consideration-getting them in/out of your boat and battery compartment. We had 2 8-D LA cells for a house bank. The night before we were to launch on an 8-week trip around the Philippines, I discovered one was so hot I could have fried an egg on it! Apparently A DEAD SHORT! We disconnected it, and rigged a cats cradle from the boom to hoist it out. Yup-you guessed it-just as we were starting to swing to clear the companion way, it broke....and 175lbs of surfuric acid and lead went falling down. Broke the foot of my helper, made a mess, etc,etc. NEVER again! We now have 6 Trojan T-105s, making 3 12v "batteries". And any one can fail, we still have house power, and I can "easily" remove and replace without issue.

[jalmberg]

Originally Posted by skipgundlach

You live a sheltered life



Our Trojan L16H-AC are 435AH each. They are heavy suckers, however, and it takes some serious muscle to change out a battery.

You ain’t kidding about the weight! I hired some young muscle to lug them around for me because I didn’t trust my back. But weight is what you want in a wet cell Battery.

[MikeInMaine]

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?

Well that was the scenario proposed so the point seems fair to me.

Are you saying that all battery failures are somehow the fault of the owner? What about an internal short-circuit in a cell brought on by manufacturer defect or vibration?

Mike in Maine

[Talbot]

Originally Posted by EmeraldCoastSailor

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

Suggest you need a bit of research.

I have 2x6v batteries 435 amp/hrs The ones I have at the moment are Trojan L16Hhttps://www.tayna.co.uk/industrial-b...rojan/l16h-dt/

My previous batteries were US Battery L16HC - these have been replaced by the L16HC XC2 which is 420 AH.

Anyone considering these excellent batteries (both about the same dimensions, with the trojan slightly taller, be aware that they are a very tall battery, and they are really heavy! - the US battery is 118 lbs (or 54kg).

[skenn_ie]

I agree with Pete7.
I would add that, with a good charging system, 2 x 6V would be the better option. You SHOULD get longer life out of them.

[mdenize]

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.

From my point of view in practical terms the main difference is the ease of keeping all cells balanced for both discharging and charging. Unless you can do this (and it is very hard to do with permanently paralleled FLA battery packs) the series pack will always last longer.

Why? well the charge/discharge current will always choose the path of least resistance, in the series string there is only one circuit so all cells "see" the same current and remain at the same level of charge (plus or minus a bit for cell chemistry and internal resistance). In the parallel circuit the string with the lowest resistance will experience the most current. So what happens is just as others have posted is one string does all the work while the other fails dragging the good string down. Hardly redundancy.

We have a very large nominal 48v 1000AH C100 = 50kVA battery made of 24 x 2v cells in Series,which powers our house. It is nearly 20 years old and I am just off to do some MIG welding which uses 5kW peak. The reason it has lasted so long is that it is very big so no normal load discharges it very much and no peak current worries it much. (eg the MIG uses up to 5kW peak which is about 100A DC however this is less than 10% of its short cct current and the AH drawn in any hour is less than 1% of its capacity which is replaced immediately by our solar. In terms of redundancy our inverter is happy down to 40 odd volts so I have in the past taken a cell (-2v nominal) out of circuit for a month (long story...) with a resultant tiny loss of capacity but only minutes of down time.

I realise that our battery would be too heavy and large for most boats but that is easily fixed by changing battery chemistry which is different subject altogether.

So my vote is one large series string over two smaller parallel strings any day (for ease of charging and long life), if you truly want redundancy get another one the same with a similar inverter charger and load and only parallel them in emergencies.

Happy New Year

[skenn_ie]

AFAIK, charge current is limited by battery voltage far more than battery resistance (O/c charge voltage minus battery internal voltage) divided by (source plus battery resistance). In parallel batteries, unbalance is mostly due to cell self-discharge, so is not really a factor in choosing parallel 12s over series 6s. If one 12V battery increases it's resistance, it will be slightly slower to charge, but the neighbour will run at slightly high voltage so decrease it's admitted current more rapidly thereby bring it back into balance. THe degraded battery will also discharge slightly more slowly which will also help to keep in balance from the voltage perspective.

[mdenize]

Here is a good explanation of the charging of a parallel bank only. It talks about equalising the charging current by equalising the resistance of the cables and what happens if you do not, and why they fail early. However it does not explain the variability in the internal resistance of FLA batteries themselves which does not exist in a perfect world but in practice it does. Good luck with perfectly balancing 4 Paralleled FLA Batteries, I would say it is very difficult.

https://www.impactbattery.com/blog/t...es-in-parallel

[john61ct]

Yes, if you must parallel, best to stick to 2-3 strings.

another good HowTo reference on balancing the intra-bank wiring, this one canonical: http://www.smartgauge.co.uk/batt_con.html

[mdenize]

Originally Posted by john61ct

Yes, if you must parallel, best to stick to 2-3 strings.

another good HowTo reference on balancing the intra-bank wiring, this one canonical: SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

Yes an excellent explanation. The question is have you ever seen a paralleled bank last 20 years like my series battery?

[john61ct]

You need to set an objective criterion for EoL in order to compare apples to apples.

And if you coddle all the longevity factors, you're barely using it, carrying tons of extra "dead lead" around.

Thing is, getting too obsessed about longevity is a pretty weird hobby. I completely agree that

just buying the cheap end of true deep cycling, doing the proper care where it's not inconvenient for your situation, and just replacing as needed

is a perfectly valid POV.

Even drawing down 80% and PSOC recharge every daily cycle and then replacing every six months, hey you do you if that's what works.

I just don't like seeing it happen out of ignorance, use case supports better care the owner just didn't know how to measure what's happening and tweak the gear.

[mdenize]

Originally Posted by john61ct

I just don't like seeing it happen out of ignorance, use case supports better care the owner just didn't know how to measure what's happening and tweak the gear.

True, but that is the rub. In my experience the people who know the least about their battery and its needs are the most likely to think a parallel battery is as easy to maintain as a series connected one.

They are often also the ones who expect it to last a long time and wonder why it has not in ignorance.

I have not met many people who know and care enough about their batteries to maintain a parallel bank in perfect balance.

But in a series bank there is nothing more to know than is it cool, charged and does it have enough electrolyte.

[Dsanduril]

Clearly you haven't been following the other thread here on CF where series connected 6V batteries are showing a 0.3V difference when charging/discharging. These are "identical" batteries and yet one is clearly less capable than the other, and even though the current is the same it has become slowly less charged (not balanced) than its partner. A 0.3V difference at 6V nominal is the difference between 100% SoC in one battery and ~35% SoC in the other battery. The confirmation of the measurements is that one battery in the series string is bubbling and using excessive water at the end of charge (because it is at 100%SoC) while the other is not yet fully charged.

Neither series nor parallel is a panacea. Both require proper design, care, feeding, and monitoring of your batteries and if you don't do that they will die prematurely regardless of how you have them wired.