Parallel strings in battery bank question.

SaggysSaggys Solar Expert Posts: 98 ✭✭
Well it looks like parallel battery strings have finally caught up to me. I have 3 parallel strings of 385 amp hour L16's configured 2S/3P, The center string is not load sharing. Almost all of my discharge/charge current flows through the outer strings, less than 10% flows thru the center. I disassembled the center bus and cleaned all connections, everything seemed fine, The battery bank is bolted together with same length copper bus bars and the positive bus is pulled from one end, the negative the other end.

The strange thing is the voltage across the battery tied to the positive bus is about 7.5 volts and across the battery tied to the negative is much lower like 6.5 volts. When I remove the series bus they both balance out nicely? The SG in all cells is 1.290 and above, as high as 1.300 on my Hydra volt tester. Verified all connections with an ohmeter for this string and all checked good and also cleaned and inspected with same results.

When checking strings with a clamp on ammeter while under load, the battery connected to the positive bus would drop in voltage (while only drawing minimal current) while the battery connected to the negative bus would keep a constant voltage? Is this a open cell? Next trip up I will load test each individual battery and see if each one can supply a load.

The batteries are 5 years old with weekend use and are only seeing about a 6% charge rate. The system is in need of an upgrade to 24 volts and MPPT charge control. I know its time for new batteries and system uprade, just curios if anyone has seen a battery behave this way?

Happy Easter everyone.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    And the issue with parallel strings of batteries... What to do when they do not properly share loads/charging current.

    Sounds like you have all the right tools (AC+DC Current Clamp DMM).

    You have 6 "identical" batteries... And you are looking for one or two (typically) batteries that are different.

    Check specific gravity for each cell... They should all be within 0.015 SG units from min to max (no more than 0.030 sg units). If over 0.015-0.030+ sg units different--Generally you should EQ the bank (or each string one at a time).

    The idea with EQ is to "over charge" the full batteries, and use that "extra current" forced through the "full cells" to bring up any low cells (and mix electrolyte, etc.)... Generally, for a 12 volt bank, you would charge at 15.0 to 16.0 volts--Looking for 5% rate of charge (or >2.5% rate of charge)... You do not want more current as even 5% rate of charge (200 AH string, 10 amps EQ current) can overheat the batteries after a few hours of EQ (watch battery temperature).

    The 7.5 vs 6.5 volt difference in battery voltage is a big sign... Generally that would seen in SG between cells being quite "different".

    If this voltage difference is measure when charging, and the SG is similar between all cells--You can use your volt meter to check each connect (battery + to next battery - terminal)--You are looking for bad electrical connections/failed wiring/bus bars (cracked/corroded/ertc.)... The + to - voltage drop across a bus bar should be pretty near zero volts--If you see 0.1 volts or higher, then I would suspect a bad connection (you can sometimes even feel "poor connections" getting warm/hot--Be careful not to burn yourself as a weak connection can get near red hot).

    I would focus on the 7.5/6.5 volt batteries... You should see around 0.050 to 0.100 volt difference between any two batteries (in theory)...

    When charging, a "high resistance" battery will read higher voltage, a low resistance battery lower voltage. "High resistance" might be an open cell (or simply charged to 100%) and the low resistance/voltage battery may have a shorted cell (or simply need recharging)...

    Similar when testing under load... A low battery voltage may be a shorted cell/low state of charge. A high voltage battery may be an open cell, or simply a battery able to supply its current (fully charged).

    And then, if you find a "bad battery", you are left with the question to replace 1 battery, or all batteries. Or simply disconnect the one string and "limp by" on the other two strings.

    It is not unusual for a small percentage of cells/batteries to fail, and the rest to be able to continue on for another few years. Replacing one failed battery--Then get another year or so--Then replace whole bank--Not a bad choice. If a second battery fails in another 4 months--Perhaps not a great result... Nobody knows what will happen.

    Hot batteries/hot climates--Batteries age faster (tropics). Cold/sub freezing climates, batteries age much slower. Every 10C rise in battery temperature/age 2x faster... Every 10C drop in temperature, a 2x longer aging life (assuming 25C/75F is standard temperature).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SaggysSaggys Solar Expert Posts: 98 ✭✭
    BB. said:



    You have 6 "identical" batteries... And you are looking for one or two (typically) batteries that are different.

    Check specific gravity for each cell... They should all be within 0.015 SG units from min to max (no more than 0.030 sg units). If over 0.015-0.030+ sg units different--Generally you should EQ the bank (or each string one at a time).

    The idea with EQ is to "over charge" the full batteries, and use that "extra current" forced through the "full cells" to bring up any low cells (and mix electrolyte, etc.)... Generally, for a 12 volt bank, you would charge at 15.0 to 16.0 volts--Looking for 5% rate of charge (or >2.5% rate of charge)... You do not want more current as even 5% rate of charge (200 AH string, 10 amps EQ current) can overheat the batteries after a few hours of EQ (watch battery temperature).

    The 7.5 vs 6.5 volt difference in battery voltage is a big sign... Generally that would seen in SG between cells being quite "different".

    If this voltage difference is measure when charging, and the SG is similar between all cells--You can use your volt meter to check each connect (battery + to next battery - terminal)--You are looking for bad electrical connections/failed wiring/bus bars (cracked/corroded/ertc.)... The + to - voltage drop across a bus bar should be pretty near zero volts--If you see 0.1 volts or higher, then I would suspect a bad connection (you can sometimes even feel "poor connections" getting warm/hot--Be careful not to burn yourself as a weak connection can get near red hot).

    I would focus on the 7.5/6.5 volt batteries... You should see around 0.050 to 0.100 volt difference between any two batteries (in theory)...

    When charging, a "high resistance" battery will read higher voltage, a low resistance battery lower voltage. "High resistance" might be an open cell (or simply charged to 100%) and the low resistance/voltage battery may have a shorted cell (or simply need recharging)...

    Similar when testing under load... A low battery voltage may be a shorted cell/low state of charge. A high voltage battery may be an open cell, or simply a battery able to supply its current (fully charged).

    And then, if you find a "bad battery", you are left with the question to replace 1 battery, or all batteries. Or simply disconnect the one string and "limp by" on the other two strings.

    It is not unusual for a small percentage of cells/batteries to fail, and the rest to be able to continue on for another few years. Replacing one failed battery--Then get another year or so--Then replace whole bank--Not a bad choice. If a second battery fails in another 4 months--Perhaps not a great result... Nobody knows what will happen.

    Hot batteries/hot climates--Batteries age faster (tropics). Cold/sub freezing climates, batteries age much slower. Every 10C rise in battery temperature/age 2x faster... Every 10C drop in temperature, a 2x longer aging life (assuming 25C/75F is standard temperature).

    -Bill
    Did not mean to Quote BB 's full reply; mainly wanted to know if anyone has had this happen where I see 12 amps of current flow thru the other 2 parallel strings and like .1 or .2 amps thru the center string? SG's measure 1.295 and above, voltage rises on the more positive battery under charge and decreases on discharge? More negative battery stays at a fairly constant voltage. Is this the result of a sulphated battery or is it a physical open in the battery?
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    You can always swap one of the batteries in the center string batteries to another string--And see if the "issue" follows the battery or not.

    If you are seeing 12/0.1/12 amps during charging the batteries in the other 12a strings are around 7.0 volts each (14.0 volts charging)--Then I suspect that the 7.5 volt battery (or its bus bar connections) is the one with problems. It is "high resistance"--Possibly an open cell or sulphated (kind of doubt sulphated--That usually comes with a lower SG reading). Could be "cracked" plate--Or did you ever have low electrolyte levels and expose the plates?

    7.5 volts charging is 15.0 volts charging (@ 12 volt bus) or even EQ charging voltage; and 6.5 volts is 13.0 volts (@ 12 volt bus) and is not even float charging.

    When the bank is being discharged, what do you see for current flow per string and battery bus/each battery voltage? Does the center string have low current, does the 7.5 volt battery have lower voltage than the other batteries?

    Swap that battery with another battery from a 12a string and see what happens.

    By the way, what are the battery voltages (each, and bus voltage) when charging? 14.0 volts ( (7.5+6.5)/2 = 14.0 volts)--Is this during "bulk" charging or absorb charging (i.e., 14.0 set point)?

    12 amps into a 385 AmpHour string is pretty low charging current--Only 3% rate of charge... (battery and bus voltage at that charging current per string?).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SaggysSaggys Solar Expert Posts: 98 ✭✭
    When discharging very low current, same as charging and higher voltage battery is lower voltage than the other on in the series string. The negative battery in the string almost acts as though it is isolated. I have cleaned and verified all conections on this string with an ohmmeter.The voltage numbers are at near full charge and are temp compensated for both inverter/charger and charge controller. I have absorbtion set at 14.7v and float at 13.4v on a PWM charge controller.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Sounds like the "7.5 volt" battery (during charging, next to positive bus--And to confirm--voltage drops a lot during discharging, with low current flow too) is bad.

    I don't think there is anything that you can do to "recover" it. (possible failed internal bus/plate connections?)

    Choices are running on 2x strings (as you are essentially now). Replacing with new (or used if you can find one) battery (or paralleling a pair of 6 volt GC batteries for a bit more "cheap" bank life)--Or start your plans to redesign your system to 24 volts.

    If batteries were (on average) cool/cold--Probably 5-7+ year life (Minnesota in unheated space?)... If the bank was "hot" on average (tropical)--Then closer to 3-5 year life expected.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SaggysSaggys Solar Expert Posts: 98 ✭✭
    You are correct, battery next to positive bus does act the way you described. Cabin is in northern WI., and batteries are in an unheated space. I am going to limp along and start on 24 volt system. Thanks for your input.
  • SaggysSaggys Solar Expert Posts: 98 ✭✭
    Quick follow up, tested both batteries this morning with a load and the battery tied to the positive bus is bad. With load on battery a small amount of current flow was present and battery voltage collapsed.  Other battery in string behaved as expected.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    edited April 13 #9
    Sounds like you have a handle on the issue... Next $tep.   :s

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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