Cross-tie parallel strings of batteries - yes or no?

BrentBrent Posts: 64Solar Expert ✭✭✭✭
Can anyone tell me what the pros/cons are of a cross-tied battery bank? I have two strings of 8 - 415AH batteries which are cross-tied. I have measured the current in the cross-tie cables during heavy charging and discharging and have only measured 200-300 milliamps so it would appear that high ampacity cables are not required.

What would happen if a cell shorts in one of the batteries with another battery across it as would be the case in a cross-tie configuration. If this is a potential problem would a fuse in each cross-tie be of use?

Thanks,

Brent

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Cross-tie parallel strings of batteries - yes or no?

    No.

    Definitely do not do this.

    It creates even more pathways for the current to flow through and the current will always pick the one with the least resistance, which can be in one string and out another.

    When you're putting batteries together your trying to build one larger battery. The 'first coupling' is always done to increase Voltage. The 'second coupling' is done to increase current.

    Always create strings of the proper Voltage first, then parallel the strings to increase Amp hour capacity.
  • BB.BB. Posts: 27,575Super Moderators admin
    Re: Cross-tie parallel strings of batteries - yes or no?

    I would say "NO" to cross tying a battery bank.

    If all is working well, then cross ties do nothing (as you have confirmed with your current measurements).

    The only time cross tying would "help" is when there is an open cell or electrical connection--And that comes with lots of issues:

    If the cross tie "works", then the bypassed failed cell means that other series cell(s) will be in danger of being over discharged (i.e., if you have two parallel strings, that would mean a 50% discharge of the "whole" string would be a 100% discharge of the cell carrying the bypassed current).

    And, the whole issue of fusing/breakers... Normally, with paralleled battery strings, you only need one breaker/fuse per string for over current protection. With cross tied batteries, in theory each cross tie would need fusing/breakers too (nobody is going to do that).

    Lastly, it makes checking the battery bank for faults (open/shorted cells, bad cabling, etc.) that much more difficult. You have to measure cross tie current to make sure it is zero, and if it is not, then you have to figure out why it is not zero (probably disconnecting the cross tie for debugging). Makes voltage measurements across cells/battery meaningless--If you measure 12.x volts across a cross tied battery, which battery is supplying that voltage (are other batteries low/failed?).

    I cannot think of a reason for cross tying that would give any real advantages--And lots of real disadvantages.

    And, like Marc/Cariboocoot finds when he makes a black/white answer, I will be awaiting somebody telling me I am wrong and there is/are real advantages for doing this (just none that I can think of). :p

    -Bill "been there, done that" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Cross-tie parallel strings of batteries - yes or no?

    Bill;

    I recall someone on the forum tried this once. Got fireworks. :p
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Cross-tie parallel strings of batteries - yes or no?
    BB. wrote: »
    I will be awaiting somebody telling me I am wrong and there is/are real advantages for doing this (just none that I can think of). :p

    Sorry to disappoint you, but I think you are correct. But Trojan battery must think differently because they have a line of 2 volt batteries that have three parallel cells per battery.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Posts: 27,575Super Moderators admin
    Re: Cross-tie parallel strings of batteries - yes or no?

    I think Trojan did that because is was mechanically simple... And other vendors do make battery stacks that parallel cells too.

    Still don't think it is a good idea. ;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Cross-tie parallel strings of batteries - yes or no?

    It's a bit different making the parallel connections within the battery itself: no worries about wire lengths and quality of contact. More or less it's just extending the size of the plates.

    BTW, those Trojan re-wired 6's aren't desirable for another reason: three cells to check instead of one, so the advantage of having a 'single' larger capacity 2V unit is somewhat diminished.
  • BrentBrent Posts: 64Solar Expert ✭✭✭✭
    Re: Cross-tie parallel strings of batteries - yes or no?

    Thanks for the responses. Something to think about.

    Brent
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