Multiple Smaller Batts VS Fewer Larger batts?

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stereoman405
stereoman405 Solar Expert Posts: 56 ✭✭✭✭
I haven't been able find much info on this. Generally speaking, assuming price per AH is similar, what's the advantages / disadvantages of going (2) 12v batts in parallel vs (2) 6v batts in series for the same AH capacity @ 12v?

I'm assuming there has to be a good reason why smaller voltage cells are so much more popular. Does it have to do with charging characteristics? Or is it as simple as its just easier to move more smaller batts around?

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Multiple Smaller Batts VS Fewer Larger batts?
    I haven't been able find much info on this. Generally speaking, assuming price per AH is similar, what's the advantages / disadvantages of going (2) 12v batts in parallel vs (2) 6v batts in series for the same AH capacity @ 12v?

    Functionally none in that example: same number of batteries, same number of connections. Very minor difference in keeping the cells equalized, but proper wiring technique will eliminate that problem. As cited, the 12V batteries would be 1/2 the Amp/hrs of the 6V in order to make the two banks equivalent.
    I'm assuming there has to be a good reason why smaller voltage cells are so much more popular. Does it have to do with charging characteristics? Or is it as simple as its just easier to move more smaller batts around?

    Bingo! In actuality this too only gets to be a problem as you go up in Amp/hrs. Consider a 1000 Amp/hr, 12V bank:

    6 2V 1000 Amp/hr batteries all in series or 10 12V 100 Amp/hr batteries all in parallel.
    2V cells; 5 'interconnects' and no equalization wiring problems. 6 cells to check SG on.
    12V cells; 18 'interconnects' and a whole lot of keeping leads even to the big bus bars so that every battery draws/charges evenly. 60 cells to check SG on.

    Get it?
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
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    Re: Multiple Smaller Batts VS Fewer Larger batts?

    Well don Marc ... not to mention 10 fuses vs 1
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Multiple Smaller Batts VS Fewer Larger batts?

    Ok, I think I'm getting it. If I want a 400Ah 12v bank I could either have (2) 6v 400AH in series OR (2) 12v 200AH in parallel. The 6v option would net me 6 caps to check SG on, 1 inline fuse, and no sensitivity to matched wire length. The 12v route would give me 12 caps for SG checking, 2 fuses (assuming I'm combining at a bus bar), and a sensitivity to the wire length going to the bus bar. A setup as as small as I'm looking at, probably not a big deal, but I could see it being an issue when you scale it up.

    You keep mentioning keeping the 'cells equalized'. What exactly do you mean by that? Is that only in a parallel scenario? (The wire lengths from batt to batt, or to your bus bar from each batt if you are using a bus bar?) Is equal wire length a concern in one big series loop?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Multiple Smaller Batts VS Fewer Larger batts?

    Cell equalization is two things: keeping the Specific Gravity consistent between cells and returning it to that state.

    In the first part, you need to ensure that the current flow to and from different batteries is the same. This is why you need equal wire lengths with large banks of parallel batteries. Without it, some of the cells will do more of the work and some less. The less used sells will sulphate, meaning the plates get coated with the sulphur from the electrolyte rendering them useless over time.

    The second part is called "equalization charging" and is abbreviated EQ. This refers to applying higher than normal Voltage to a fully charged battery to force the sulphur back off the plates and recombine it with the electrolyte. It is not a perfect, 100% process (nothing is) but it can help if you catch the problem in time. Some battery chargers/charge controllers have built-in or even automatic EQ function. You don't need to do this unless the SG between cells is more than 0.010 out of whack. Excess equalization will damage batteries because of the extra heat generated.

    In series connected batteries it isn't as much of a problem as there is only one pathway for the current to flow and that is through all cells. Minor cell variations aside, this will be even.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Multiple Smaller Batts VS Fewer Larger batts?

    The idea behind equalization is that not all cells in a battery bank perform identically. Some cells may be at 100% capacity and other cells may be at 80% state of charge.

    There are two problems with unbalanced cells... First is that if you deep cycle a battery, the low cells will hit zero volts first, and if taken deep enough, actually reverse voltage and begin to "reverse charge"--pretty much destroying the cell/battery.

    The second is that sulfates begin to harden if the cell is kept below 75% state of charge for days or weeks at a time... If you have your battery at 90% state of charge and one or more cells at 20% lower charge level--those cells may begin to sulfate and ruin the battery AH capacity over time.

    When you equalize a battery bank--You are effectively overcharging most of the series connected cells to get charging current into those few cells that are at 80% state of charge. Yes, you are overcharging the "good cells" (overcharging is hard on cells too) to get enough current into the "low cells" to bring them up to 100% state of charge too.

    Equalization is not needed for a group of parallel connected cells as they all see the same charging voltage (although, "equalization" to cause gas formation is used to "mix" the electrolyte in the battery banks too--if you have "tall batteries", they tend to "stratify" the acid--dense acid at bottom of cell, light mostly water at top of cell--so even parallel connected cells can use equalization to mix too). However, since nobody has a 2 volt battery bank, we always have series cells in any useful bank (12-48 volt is 6-24 cells in series)--so we all have to deal with equalization (couple times a year to once a month or so is typical recommendation. Usually only equalize if cells measure unbalanced voltage of 0.030 volts or specific gravity out by 0.030 or more).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
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    Re: Multiple Smaller Batts VS Fewer Larger batts?
    You keep mentioning keeping the 'cells equalized'. What exactly do you mean by that? Is that only in a parallel scenario? (The wire lengths from batt to batt, or to your bus bar from each batt if you are using a bus bar?) Is equal wire length a concern in one big series loop?

    Equal wire length in one series loop is not a concern .... Equal length is the concern in multiple banks.

    Take a look at this thread http://forum.solar-electric.com/showthread.php?t=7097
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Multiple Smaller Batts VS Fewer Larger batts?

    Ok, I think I got it. I also found this thread that helped a lot.

    http://forum.solar-electric.com/showthread.php?t=5547

    Thanks!