Bad cell?

Been off grid for 5 years, the system basics include PV modules: 24 Kyocera KU265-6MCA (6.36kW); Batteries: 16 Rolls S-480 Flooded Lead Acid Battery Capacity: 750Ah; Generator: 15kW Generac EcoGen, the input and storage is 48v.  The dual inverters and charge controllers are Outback with a Mate3 interface.

I equalize the batteries every three months (sometimes sooner) and check levels every month. Yesterday when checking levels before equalizing, I noticed one cell (the 43 rd I checked) did not appear to vent off any fluid over the past 30 days.  All others where okay, though a few might have been lower than I would have expected, but this could have just been they were not quite as full from when I added distiller water 30 days ago.  After equalizing, I noticed that the fluid level in the 43 cell was appeared unchanged. 

A closer inspection inside the cell revealed the electrolyte to be brownish, like there was rust in the cell.  The plates also look to has some corrosive build up that is rust in color.

Still, the system overall is performing with in anticipated parameters.  So I do not want to replace the entire battery bank.  Any suggestion?


  • BB.BB. Super Moderators, Administrators Posts: 32,088 admin
    Welcome to the forum David,

    Do you have a hydrometer of some sort to measure the specific gravity of the electrolyte?

    And have you measured the voltage across each battery (6volt?) during charging/loading? A battery with an open cell would be higher voltage than the rest... And a battery with a shorted cell would be lower voltage than the others.

    Ideally, all 16 batteries should be about the same voltage. Any battery that reads substantually "different" voltage (or even S.G. differences) needs to be looked at closely.

    And do yo have a DC Current Clamp DMM (digital multimeter)?: (cheaper, "good enough" for our needs) (mid-priced meter--Also really nice higher current readings and other functions)

    You have two parallel strings of batteries, so you can use the DC current clamp meter to see if the batteries are sharing charging & discharging currents or not.

    Note: There are AC only Current Clamp Meters (and they have AC+DC volt meter functions, be careful to make sure you get the AC+DC current clamp type). AC only CC DMMs are very nice too... But current clamp does not do DC measurements (which we really need).

    I am not sure about the color in the water/plates.... Usually not a good thing, but if battery is working well--Keep using it.

    Rolls batteries can benefit from extended Equalization Charging current. (Using 12 volt bank numbers), typically EQ voltage is around 15.0 to 15.5 volts... For the Rolls, EQ charging >16.x volts is common when trying to get a bank "balanced" (all cells at same S.G., and higher SG overall--closer to new fill). The EQ Current should be around 2.5% to 5% of battery bank capacity (i.e., 2.5% of 100 AH battery is 2.5 Amps EQ Current).

    This can take several days--And watch battery bank temperatures. With extended EQ charging, battery banks can easily overheat when doing hours of extended/elevated voltage EQ (i.e., stop EQ, and restart the next day after bank cools).

    You have been filing/using "distilled or demineralized" Water (no hard water, no organics, etc.)?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • davidlewisoffgriddavidlewisoffgrid Registered Users Posts: 3
    Hi Bill,
    Thank you for the replay and sharing your expertise. 
    I have only ever added distilled water to the batteries. The potential discoloring issue may stem from incident from 3.5 years ago.  In my absence the MATE3 failed to shut off the generator after it automatically turned on.  It ran for nearly 18 hours or so.  Some plates were exposed at that time. After refilling and equalizing once a month for 3 months and seeing the specific gravity readings within one standard deviation of the mean I reverted back to equalizing every 3 months. Since then the system has really operated within anticipated parameters, but I no longer depend on the MATE3 to shut off the generator and put the system on manual if I am not going to be around for more than 3 or 4 hours.
    I do have a hydrometer.  The cell in question reads low (1.090) relative to the average of the 1.212 of the 48 cells.  This nearly three standard deviations (0.044) from the mean. The range is 1.235 to 1.090.  The next lowest reading is 1.112, which is two standard deviations below the mean. There are also two cells that are three standard deviations above the mean. 
    I do have a Fluke multimeter for reading the DC voltages, but not a DC current clamp meter. I guess I should get one.
    If I understand you recommendation correctly, my next step would be to check each battery's voltage while charging.  What would I consider a significant variation?
    I assume given the significant variation of the specific gravity readings, I will find considerable variability of the voltages. 
    I also interpret your replay to suggest one potential remedy would be to do an "extended Equalization Charge".  If this is accurate, i will need to do some research regarding the specification you outline.  If I have any other questions, may I reach to you?
    I have to travel and will not be able to get back to voltage readings etc until 9/9/2020.  Thanks again for your time and expertise.
  • BB.BB. Super Moderators, Administrators Posts: 32,088 admin
    Exposing plates to air is usually a very bad thing... And, guessing, what you are seeing are the results.

    Your batteries (when new) should have a specific gravity of around 1.275 (range of 1.260 to 1.280) units... And the variation between high and low cells should be no more than ~0.030 units. (page 15, table 3 is SG vs state of charge)

    1.235 sg is ~75% state of charge
    1.112 sg is ~0% state of charge
    1.090 sg is ~0% state of charge

    If your battery is being charged with the correct voltage set point (58.8 volts @ 77F/25C--See page 13 for charging voltage vs temperature), and sufficient current (I think Rolls recommends 10% rate of charge minimum).

    You want to hold 58.8 volts for upwards of 6 hours--To ensure the battery is charged. Check SG of each cell (temperature corrected/logged to log book). And then try EQ at 62.4 to 63.6 volts (usually around 2.5% to 5% rate of charge). Monitor the cells and when they stop rising (check every 30-60 minutes), then that is the "new full SG for each cell". And monitor battery temperatures and if too hot. Stop EQ when SG's stop rising.
    4. Place batteries on charge. Please see Table 2(a) & 2(b) Flooded Charging Parameters for required charging settings. Do not let the cell temperature exceed 52°C (125°F). If the temperature becomes excessive or the cells begin to gas vigorously, reduce the rate of charge. Continue charging until all cells reach the specific gravity of the filling acid. All cell specific gravities should be even (1.260-1.280).

    You may wish to measure the voltage/SG of each cell and group the "good batteries" (highest SG) into one string. And Charge/EQ that string first.

    And the second group of "weak batteries", you could use a 12 volt (or 6 volt) charger and try corrective charging/EQ on each (pair) of batteries and see if you can bring any of them up to a reasonable SG (and avoid "cooking" your "good" batteries).

    My fear is that your bank is toast... I don't know if it was the "boiling" dry from genset charging and/or your charging voltage/loads/solar array/etc. (or lots of deep cycling, letting batteries "sit" with low states of charge which causes sulfation--A permanent loss of charge capacity).

    Before you replace the batteries, I would review how the system operate today. What is your daily WH or AH usage, what is your charging current and bank voltage (i.e., holding 58.8 volts for 2-6 hours). Make sure your system is configured and operating correctly before you commit to a set of exp$ive new batteries.

    You can also look at the charging termination current. You want to see the current fall below ~1% state of charge. If you see the ending current settle at 2% or higher rate of charge, the battery bank is end of life.

    You probably want the batteries to match voltages within 0.05 volts (for 6 volt batteries). Just an FYI, roughly a change of 0.010 in SG is a change of ~0.01 volts in the cell voltage (i.e., 3 cells * 0.030 low SG = 0.09 volts "low" on a 6 volt FLA battery compared to a battery with 3x cells and "proper SG").

    Best way to continue the conversation, is to keep posting here in "your thread". You will get a good range of answers from others too (I am certainly not perfect). And it will be helpful for the next person who has similar questions.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VicVic Solar Expert Posts: 3,117 ✭✭✭✭
    edited September 2020 #5
    Hi David,

    Thanks for the info on the SGs.   However,  it is not clear if those readings represent batteries that are what you believe are fully-charged.

    Please run the batteries through a full charge,   let them stay on Float,   and measure the SG of each cell,   record those,   and  please report those readings.   Waiting  thirty,  or so minutes after going to Float,  befor taking the SGs,  may allow them to stabilize,  a bit.

    What are your Absorb,  Float,  and EQ voltage settings.   How is Absorb terminated,   and what is the duration of Absorb,  and EQ?  And,  approximately how deeply is the battery discharged in each cycle?

    And,  assume that the Kyo PVs are arranged in strings of three PVs,  is this correct?

    Exposing plates,  is very hard on FLA batteries.

    Thanks,   Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • SurfpathSurfpath Solar Expert Posts: 439 ✭✭✭
    edited September 2020 #6
    The cell in question reads low (1.090) relative to the average of the 1.212 of the 48 cells.  This nearly three standard deviations (0.044) from the mean. The range is 1.235 to 1.090”

    Hi it may help to clarify at What point in the charging cycle were these SGs taken, at the end of the day? First thing in the morning? 

    Still, the system overall is performing with in anticipated parameters.  So I do not want to replace the entire battery bank.”

    When one of the cells on one of my batteries started to go bad on my last battery bank of L-16’s the overall bank voltage looked fine all day up till about 12.30am then it would collapse rapidly to around 48.5 volts.
    the other batteries were still healthy so I replaced the bad one with 2 golf cart batteries and got another 22 months from the bank. But if you have exposed plates on multiple batteries perhaps this is not just a single battery issue?
    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.
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