2 Chargers, 1 Battery Bank - Different Voltage?

KenBrownKenBrown Registered Users Posts: 4
Hi, I've been looking around trying to get this question answered - but have been unsuccessful so far.  My issue is that I have 12x12v 250ah Deka batteries wired for a 48v system, 4s3p.  I am having a hard time keeping the 12 batteries with matching voltage, my parallel strings stay right in line with each other, but not each 12v battery.  I'm currently using 2 flexmax 80 charge controllers to bring me back up to full charge each day.

My idea, is to wire the battery bank 1s12p and connect that to one of my two charge controllers and let that one charge it at 12v for however long it takes to equalize each 12v battery, and leave the other controller charging at 48v.  I've read that you don't want to apply different loads and 'tap' the batteries to make them go out of sync, but that's not what I'm trying to do.

Is there anything I'm missing as to why this wouldn't bring all 12 batteries into alignment with each other?

Thanks in advance for any/all feedback.

Comments

  • KenBrownKenBrown Registered Users Posts: 4
    Or, even just wiring the batteries themselves 1s12p and leaving everything else alone and just let them 'self adjust' with time?
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Your basic problem is that parallel batteries are a very poor design choice for a system that cycles frequently.  Putting 12 batteries in parallel will only make your problems worse.   When you charge parallel batteries it is difficult to get the charging current to divide evenly among the parallel paths.  With your existing configuration, the batteries should be combined with a heavy bus bar.  Each string of batteries should have its own fuse or circuit breaker. 

    Your best approach to equalize the batteries is to charge each 12 volt battery one at a time.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,485 admin
    Do you have a good quality hydrometer? Have you measured the SG when the battery bank is "full"?

    Usually, the difference in battery (and cell) voltages are a reflection of different specific gravity levels of each battery/cell. And, the usual answer is to equalize the batteries (controlled over charging the entire battery string so that the current going through the "full cells" is used to charge the less than full cells.

    Generally, for a 12 volt flooded cell deep cycle battery, you are looking at 15.0 to even 16.0 volts (higher voltage for "industrial" type batteries). And around 5% down to 2.5% current during equalization (2.5% of a 100 AH cell would be ~2.5 amps). And watch the battery temperature while equalizing (you may need to stop and let the bank cool, then start again).

    Also, new battery banks do seem to have an issue with individual battery balance vs one that has been cycled 20-100 times or so.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KenBrownKenBrown Registered Users Posts: 4
    They are sealed gel batteries, so without going a little crazy on them drilling holes, not happening for SG check :/  Does it matter that I'm only proposing a temporary 12v parallel and then going back to everything 48v?
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    KenBrown said:
    They are sealed gel batteries, so without going a little crazy on them drilling holes, not happening for SG check :/  Does it matter that I'm only proposing a temporary 12v parallel and then going back to everything 48v?
    It won't hurt to charge them with a 12 volt (nominal) charger... in fact, if you do it correctly it's better than charging them 4 to a string at 48 volts.   The key is to charge them one at a time.  If you put 12 batteries in parallel you will make your problems worse.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,485 admin
    You might need to charge each battery separately.

    GEL and AGM batteries do not do well with higher voltage equalization (especially GEL batteries).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • South AfricaSouth Africa Solar Expert Posts: 295 ✭✭✭
    Here by us, a few guys found these and are using them with great success on 48v banks, where they have midpoint differences.
    See here: http://www.ebay.com/itm/Battery-equalizer-balancer-for-48v-lead-acid-LiFePO4-balanced-charging-/252370442152?hash=item3ac27357a8

    Most of the guys said after a week or so the bank was in near perfect balance.

    Victron has this: https://www.victronenergy.com/batteries/battery-balancer

    And BMV702's are used with great success to warn you about the midpoint mismatches: https://www.victronenergy.com/battery-monitors/bmv-700

    Connections look similar to this: https://www.victronenergy.com/upload/documents/Datasheet-Battery-Balancer-EN.pdf

    Hope this helps.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Here by us, a few guys found these and are using them with great success on 48v banks, where they have midpoint differences.
    Those all work to balance batteries that are in series. 

    In Victron's example of six 12 volt batteries in a 24 volt system, they made a bank of three 12 volt batteries in parallel, and then put that in series with another 12 volt bank composed of three parallel 12 volt batteries.  The balancer can try to keep the two banks at the same voltage, but it can do nothing to equalize the current flows through all the parallel pathways.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • South AfricaSouth Africa Solar Expert Posts: 295 ✭✭✭
    vtMaps, is this not what you would do for 12 x 12v batteries, 48v system?

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    vtMaps, is this not what you would do for 12 x 12v batteries, 48v system?
    The point is that I would never have 12 x 12v batteries in a 48v system.  If I need that AH capacity, I can achieve it with a single string of cells. 

    Designing a system almost always involves compromises.  Parallel batteries are not an optimal design choice.   I would make many other compromises before I would use parallel batteries.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • South AfricaSouth Africa Solar Expert Posts: 295 ✭✭✭
    edited May 2016 #12
    Yes, I agree, same here for the more batteries, the more problems one has.

    But, having said that, the problem is there, and needs a solution, that being my focus.  :D
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