Batteries in series

SystemSystem Posts: 2,511 admin
Is there any advantage to use two 6 volt batteries in series over a single 12 volt battery?

Comments

  • BB.BB. Posts: 28,067Super Moderators, Administrators admin
    Re: Batteries in series

    No deal breakers that I can think of...

    Some things to consider:
    • Extra cable/connections to maintain
    • For the same amount of energy storage, two 6 volt batteries are lighter/easier to move vs one 12 volt.
    • In theory, if a cell goes bad, you need to replace only one battery--in practice, there are many good reasons not to replace one new battery in a bank of older batteries (typically, the newer battery will last, roughly as long as the rest of the older parallel connected batteries. For AGM's may have issues trying to balance/equalize charge between old and new series connected batteries)
    • Compare pricing--get which ever is cheaper.
    If you are building a larger bank of batteries--I personally would prefer to have series connected batteries vs parallel connected (easier to diagnose battery problems with volt meter, less issues with setting up current sharing among batteries in bank).

    One vendor said that series connected batteries last longer than parallel connected batteries. But one of the folks from Northern Arizona Wind&Sun said that from their customers--they do not see any difference in life between series vs parallel connected battery banks.

    For long life, keeping your batteries properly charged, and not over discharging are key.

    A Battery Monitor can go a long way in helping you to manage your battery bank. If you have flooded cell batteries (vs AGM), then a hydrometer and thermometer are also very useful (and "battery caps" can reduce water loss and top-of-battery corrosion problems).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RedfordRedford Posts: 38Solar Expert
    Re: Batteries in series

    Series vs. Parallel comes to mind......

    Batteries wired in parallel tend to try to equalize with each other, constantly charging and discharging each other. Wiring batteries in series do not compete.

    If you look at the sunXtender series though you can get one 6v, 225ah battery for about 276 bucks, so two batteries would be 552

    One 12v, 212ah battery is 483 dollars.

    It seems like it would be a lot easier to move (and maybe ship) two 6v batteries than one large 12v battery.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Batteries in series

    "Wiring batteries in series do not compete."

    maybe not, but it will strain the other cells as all must go through that particular cell too during charging and discharging. to compensate for a weaker cell an overcharge like an eq is needed thus straining the other cells all the more just to bring one cell inline. if it doesn't go inline the charge/discharge curve will be with a permanent loss (lower ah capacity/lifespan). most times this will worsen until the cell fails and the added stress causes premature failures and ah capacity losses for other cells. if those other cells are in other batteries in series then there's a problem in thinking series is not without any problems. 1 battery in a series will influence and affect the others in the series.
    in general, when a cell fails it may appear one of 2 ways. high resistance or shorted. in a series circuit any resistance will limit the battery(s) output capacity and charging ability, but a high resistance failure is more pronounced as there will be little in or out. with a short the cc presses a constant voltage to all of the cells and a shorted cell is like having a battery with one less cell and may still have full ah capacity in those other cells. a shorted cell will cause a constant overcharge stress to the other cells as well as a lower voltage for the battery when charged.
    these conditions rarely all of a sudden just occur and the stresses to the other cells involved in the other batteries in series causes some aging as in reduced capacity/life. this is why in some failures we have said the others probably aren't far behind and can fail at anytime, but this is often unpredictable as to exactly when. there are those exceptional times like in a new battery and a cell just up and dies with the other batteries in series with it having a good chance of being unharmed or influenced if the is caught quickly and replaced.
  • RedfordRedford Posts: 38Solar Expert
    Re: Batteries in series

    How does the construction of a 6v battery differ from a 12v battery? A 12v battery connects six 2.1v cells in series within the battery housing, making A 6v w/ three cells. So if you had one 12v battery and 1 of 6 cells goes bad it would strain the entire battery.

    Say you need 200ah capacity @ 12v.

    You could buy one 12v 200ah battery. If a cell goes bad the entire battery is affected.

    Using 6v batteries in series with 3 parallel strings you could get 70ah, 6v batteries for 210ah of capacity

    If a cell goes bad in one battery, you would still have 2 strings that are fully operational.

    Maybe this would only be true if the series string could be isolated (with charging/discharging diodes) from the other strings, that I would not be sure of.

    Cabling and linking the batteries would be the downfall.
  • mike95490mike95490 Posts: 7,962Solar Expert ✭✭✭✭
    Re: Batteries in series
    string could be isolated (with charging/discharging diodes) from the other strings,

    I have no idea how that could be done, easier to unbolt the cables. Losses in the diodes would be prohibitive.
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  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Batteries in series

    there is no difference other than physically being tied together in a 12v battery and no interconnecting wires. yes, it will affect all of the cells in series whether inside the same battery or not. time is also a factor as i had said. as in a series parallel arangement the 1st series of batteries being of diminshed capacity due to a cell going bad will have less charge/discharge capability and thereby place that extra strain on the series strings that are in parallel with the string that has the bad cell.
    no need to be all that alarmed as most will catch something going on with normal maintenance. this also makes your wiring more crucial than most would think on interbattery connections as they have to be good because bad connections or odd wire resistances due to bad wiring practice could have the same effect as a cell going bad. interbattery wires should preferably be oversized with good connectors to make the bank appear more as 1 battery. symmetry can also help here so avoiding odd wire lengths between some batteries as oposed to others help to keep resistances even.
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