Odd battery voltages on battery bank

cdabillycdabilly Registered Users, Users Awaiting Email Confirmation Posts: 12 ✭✭
Want to thank everyone in advance for their help.

I had an account before and the community helped me a ton when we were going to put a solar system on our RV.   Now a few years later we are getting ready to go off grid with our off grid cabin, and while things are heading the right direction I do have some questions.   Can't figure out what my old login is so I created a new one.

I thought we were going to get into our cabin last year and so I purchased a used bank of batteries.   We didn't end up getting the cabin when we planned and these sat in storage for a year.

Just pulled them out (2v cells) and the cells measure at 2.0v on my ohm meter.   According to charts I am finding it shows they are about 65% discharged.

I want to put a charge on them as it will be another few months before I can put them in use.

I connected 12 of them in series for a 24v battery bank.  When I connect 12 in series and throw the ohm meter on it shows me as like 25.7v which would be over 100% full for a 24v battery bank.

I am really confused on how I can have mostly dead individual cells show as mostly full when I connect them up as a bank.

Want to make sure I have everything right before I put the charger on them so would appreciate any help you can offer.

They are AGM batteries.

Thanks in advance!

Comments

  • mike95490mike95490 Solar Expert Posts: 9,091 ✭✭✭✭✭
    Sitting idle for a year has likely damaged them a bit.

    I would get a charger onto them soon, and be SURE it is set to the recommended voltage for those batteries, with AGM, a volt or 2 can make the difference between blowing the vents or not.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • mcgivormcgivor Solar Expert Posts: 3,602 ✭✭✭✭✭✭
    edited October 2018 #3
    The resolution of the meter may be rounding out to the next decimal point, for example 2.0V could be 2.0495, not enough to rregister as 2,1V, conversely 2.051V may read 2.1V. When all cells are connected in series the apparent error in resolution will be reduced, however  the accuracy of the meter will be, for the most part, directly related to the cost of the meter itself. Case in point, I have two identical Fluke 179 meters which are not cheap, connected to a cell of 3.43X volts, the last digit X, may be +/- 5 points different or 5mV, if the meter used only diplays tenths of a volt as opposed  hundredths and thousandths, there deviations may explain why the apparent reading is high. 

    Either way batteries/cells which have been in storage for an extended time should be changed according to manufacturers recommendations, don't be tempted to use a charger designed for automotive batteries with AGM, as pointed out above a couple of volts could be critical, especially on the higher end, which automotive chargers  are generally designed for. Almost forgot, if the meter used has minimal resolution, get one with higher resolution as it will be useful when dealing with individual cells.
    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.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    In addition to getting a higher resolution meter (or changing scale on the one you have), , and using a charger capable of proper voltages for these batteries, you may want to consider charging smaller series strings (eg 6v or 12v). In each smaller group you should put batteries together such that they're charged with other cells with similar resting voltage and SOC.

    With proper measurement there may be some at lower state of charge than others (temperature variance effects etc in storage), and ideally it's best to have all cells at the same SOC before putting in series.

    Some(most/all?) AGMs have quite low self-discharge, especially if stored cool/cold, so may not need all that much charging to top up.
    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
  • cdabillycdabilly Registered Users, Users Awaiting Email Confirmation Posts: 12 ✭✭
    Good feedback on getting a better meter, on my way to getting one now.

    Will be interesting to see how the new meter reads the bank.

    I will finish wiring up my 24v charger / inverter (schneider 4024) tonight or tomorrow and will hopefully be doing some charging sooner then later.

  • cdabillycdabilly Registered Users, Users Awaiting Email Confirmation Posts: 12 ✭✭
    Okay got a nice Fluke meter today and it gave me different numbers, different enough that now I wonder which meter is wrong....I feel like I need a third meter to test lol.

    My original meter was a $25 or so Southwire meter from Lowes that I have had for a few years.   It shows each individual cell that I tested at 2.0 volts.

    I just picked up a Fluke meter and it shows the cells ranging from about 2.14 - 2.17.   Pretty different results, so that is weird.

    I am still not understanding my original issue however so I appologize in advance for being daft.

    According to the Mfg the cells should definitely be charged if they are below 2.1.   Depending on what meter you look at I am below that or kissing it.   Either way it is obvious my cells are discharged.

    However when I connect them in series and take a measurement from the bank as a whole, the bank measures about 25.7.   Using this chart I found (found a bunch like it, just linked to one I found with google images here) 25.7 is definitely a 100% charge for a 24v battery bank.

    So how can it be possible that my cells are greatly discharged but my bank is at 100% charge.   That makes zero sense to me and I am hoping someone can help explain that part of this.



     
  • mcgivormcgivor Solar Expert Posts: 3,602 ✭✭✭✭✭✭
    Don't assume anything with regards voltage, a battery which has lost 99.9% of its capacity can have a perfect appearing voltage reading  but once a load is connected the voltage collapses. The truth will be revealed when put on charge, they should accept current and the voltage should climb slowly, climbing rapidly would  indicate either they are near fully charged or have lost capacity, since there is no way of measuring the SG, the only way to get an accurate idea of capacity, would be to fully charge, then perform a controled discharge to a given voltage. The specifications for the load test should be available from the manufacturer.
    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.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • mike95490mike95490 Solar Expert Posts: 9,091 ✭✭✭✭✭
    Just charge them, and then see what the voltages are after they sit idle for 12 hours 
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • PhotowhitPhotowhit Solar Expert Posts: 5,415 ✭✭✭✭✭
    I don't understand the reason, but AGM tend to have a higher resting voltage. Perhaps @Marc Kurth could give us some idea or reasoning. I've seen AGM battery voltage reference charts that use 13 volts as a 100% charge. I would look to the manufacturers for information.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • BB.BB. Super Moderators, Administrators Posts: 31,159 admin
    Assuming this Wiki is correct:

    https://en.wikipedia.org/wiki/Lead–acid_battery#Absorbed_glass_mat_(AGM)
    When a lead-acid battery loses water, its acid concentration increases increasing the corrosion rate of the plates significantly. AGM cells already have a high acid content in an attempt to lower the water loss rate and increase standby voltage, and this brings about shorter life compared to a lead-antimony flooded battery. If the open circuit voltage of AGM cells is significantly higher than 2.093 volts, or 12.56 V for a 12 V battery, then it has a higher acid content than a flooded cell; while this is normal for an AGM battery, it is not desirable for long life.
    More or less, the resting voltage of a lead acid battery is an indication of the Specific Gravity of the electrolyte (higher acid concentration, higher terminal voltage). (there are many other contributors to terminal voltages... Other materials used in plates to reduce water usage, increase strength, etc.; battery temperature, state of charge)

    Some other general information about Lead Acid Cells.
    BB. said:
    Re: SG versus voltage

    From an earlier battery discussion thread:

    Well mixed electrolyte is important--so if you notice all cells not coming back to "full charge" level of s.g., then there is a good chance that "mixing" (equalization) will bring the s.g. back up to your logged readings (keep these as your "good batteries" / "fully equalized" numbers.

    The amount of variation you are seeing is not that great (less than 0.030 between all cells). And having "high" s.g. is not always a good thing either.

    ran across this page on why different types of batteries have different starting s.g. fills... Is pretty interesting:
    Specific Gravity vs Applications
    1.285 Heavily cycled batteries such as for forklifts (traction).
    1.260 Automotive (SLI)
    1.250 UPS – Standby with high momentary discharge current requirement.
    1.215 Geral applications such as power utility and telephone.

    As mentioned earlier, the specific gravity (spgr.) of a fully charged industrial battery, or traction battery, is generally 1.285, depending on the manufacturer and type. Some manufacturers use specific gravities as high as 1.320 in an attempt to gain additional Ah capacity, but at the cost of a shorter cycle life.

    ...

    Higher Gravity = vs Lower Gravity =
    More capacity / Less capacity
    Shorter life / Longer life
    Higher momentary discharge rates / Lower momentary discharge rates
    Less adaptable to "floating: operation / More adaptable to "floating" operation
    More standing loss / Less standing loss
    Also on that page is the formula between cell resting voltage and specific gravity:
    Specific gravity = single-cell open-circuit voltage - 0.845 (example: 2.13v – 0.845 = 1.285)
    Or
    Single-cell open circuit voltage = specific gravity + 0.845.

    -Bill


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,384 ✭✭✭✭
    edited October 2018 #11
    As Mike said: charge them fully, let them rest and then use the voltage as your new reference voltage (after adjusting for temperature).  With flooded batteries, also equalize and check fill level (both effect the resting voltage).

    IMO, all charging related voltages should be referenced to this resting voltage.  Never use generic voltages from the Internet for anything.

    I am available for custom hardware/firmware development

  • cdabillycdabilly Registered Users, Users Awaiting Email Confirmation Posts: 12 ✭✭
    Really good feedback, thanks!   I think that clarifies quite a bit for me.

  • cdabillycdabilly Registered Users, Users Awaiting Email Confirmation Posts: 12 ✭✭
    will let you all know how it goes.
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