Equalizing 3 strings of FLA's?

ligwydligwyd Registered Users Posts: 155 ✭✭
Obviously best bet is to equalize (when necessary) each of three strings separately. As it stands now, batteries are in standby.
Once a mth I give a top up charge and 1 hour (XW default) EQ charge to mix up electrolyte.
 
Question is:
If one string is falling short will an EQ charge bring all batteries up to par (based on S.G. measurement of course) or will it just further the gap in strings? (if a gap does in fact exist) - gap meaning discrepancy in voltage between strings

Thanks guys.
John

Comments

  • mcgivormcgivor Solar Expert Posts: 3,274 ✭✭✭✭✭
    When the strings are connected in parallel the voltage of each string would be identical, whilst charging or dischargeing the differential in current of each string would reveal imbalance. In parallel circuits the current will flow equally through each leg provided the resistance of each is equal, if one leg has a higher resistance the current will flow at a greater proportion through the other leg (s). So it is possible that one string  in a parrallel arrangement of 3 will receive less current whilst the overall voltage satisfies the charging source.

    The result would be that the weaker  (higher resistance) string  would become undercharged over time thus doing less work at the expense of its lower resistance healthy strings. Should the SG readings become lower in one string it would be prudent to remove it and charge seperately until SG readings match the others. Often a single cell becomes weaker in a particular string  despite the voltage appearing normal in open circuit, this is a more difficult situation to remmidy particularly in monoblock batteries where single cell access is not possible.

    The method of parallel connection plays an important part in the ability of achieving a ballanced bank, having a means of checking with a clamp on ammeter for example, will answer vastly more questions than voltage itself,  because even a battery with zero capacity can display a voltage which appears normal.


    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
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  • EstragonEstragon Registered Users Posts: 4,451 ✭✭✭✭✭
    EQing the whole bank should eventually get individual cells as close as possible, but in doing so you're overcharging the high SG cells.  Limiting the overcharge to only the low cells is ideal, but often impossible or impractical.

    As Mcgivor notes, seeing different voltages between strings suggests to me there may be an issue with wiring which may warrant investigation.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • ligwydligwyd Registered Users Posts: 155 ✭✭
    All wiring is done with 4/0 tin coated copper battery cable and professionally crimped on tin coated copper quick cable battery terminal lugs. All 3 positives parallel cables the same length and all 3 negative parallel cables the same length going to heavy duty tin coated copper bus bars with each string fused. All connections had smooth surfaces and an ever so thin film of NO OX ID paste and all torqued to spec with stainless fasteners and re-torqued. Then lightly brushed on vaseline to to coat the battery posts before the battery terminal boots/ covers were put in place.

    I guess this is why 2 volt cells are the absolute best? Since you can give each battery individualized maint. if necessary?
    Then again, in having only one string, is it rare that any individual battery would need individualized maint. or is that not necessary given that there would be only one string?

    Heres a pic of my battery bank and box.




  • EstragonEstragon Registered Users Posts: 4,451 ✭✭✭✭✭
    Nice box!

    How much of a difference is there in voltage between strings?

    Have you checked voltage of individual batteries making up the strings?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • ligwydligwyd Registered Users Posts: 155 ✭✭
    There is no real concern present. Just being diligent and thinking ahead.
    When the bank is receiving max charge rate or when a heavy load is applied mid point voltages are all close. I believe sometimes there can be a .05V difference in Voltage from one string to another.
    Under max charge or heavy load, what would be the most voltages should differ before there was cause for concern?
    Also to consider, if mid point voltage discrepancy is .05 you'd times that by 2 since the mid point is measuring  24V. The whole 48V string could then be .1 of a volt difference. Just a thought.
    Don't have a clamp on Amp Meter yet. Not sure which one to buy. Will have to shop around a  bit.

  • EstragonEstragon Registered Users Posts: 4,451 ✭✭✭✭✭
    Seems to me 0.05v is likely getting into territory where many multimeters might have trouble getting repeatable, accurate readings.  To me, ~1/10th volt is close enough for our purposes.

    I got my DC clamp meter from Amazon, ~$50, which is good enough for what I need.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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