Individual Battery Voltages

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jcheil
jcheil Solar Expert Posts: 722 ✭✭✭
in Off Grid Solar & Battery Systems #1
I know that SG is the best way to determine the SOC of a battery.

However, I had some free time, and since small digital panel meters were only $1.49 each, I decided to make a display showing the individual voltages of each battery that I have in my series/strings.

At best, I could use it to visually determine if I had a major failure of a cell/battery somewhere in the string, which otherwise would go undetected. At worst, all the meters look really cool at night :)

So my question is: What is an acceptable voltage variation for each of the batteries in the series?
Mine seem to be within 0.03v of each other although I do have two that are 0.08v different from the others.

I checked the panel meters accuracy against my fluke and they were all within 0.015 of each other.
Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Individual Battery Voltages

    The typical recommendation is to "equalize" a battery bank if the spread in S.G. is 0.015 or 0.030 or greater... That works out to be around 0.015 to 0.030 volts per cell (for 12 volt battery; 6 cells * 0.015 to 0.030 volt => 0.090 to 0.180 volt per 12 volt bank) difference.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #3
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    Re: Individual Battery Voltages

    If you are interested theres a guy on the midnite forum currently working on a battery cell monitor. It outputs serial the voltage and temp of each cell.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: Individual Battery Voltages

    Is this the thread?

    http://midniteforum.com/index.php?topic=1528.0

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #5
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    Re: Individual Battery Voltages

    Yup, that's the one.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #6
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    Re: Individual Battery Voltages
    BB. wrote: »
    The typical recommendation is to "equalize" a battery bank if the spread in S.G. is 0.015 or 0.030 or greater... That works out to be around 0.015 to 0.030 volts per cell (for 12 volt battery; 6 cells * 0.015 to 0.030 volt => 0.090 to 0.180 volt per 12 volt bank) difference.

    -Bill

    Just so I understand you, so on a 6v battery it would be 3 cells * 0.015 to 0.030 volt => 0.045 to 0.090 volts per 6 volt battery.
    Meaning perhaps I am borderline on those two that I have which are 0.08v difference.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #7
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    Re: Individual Battery Voltages
    westbranch wrote: »
    Yup, that's the one.

    That looks really cool, but from what I read it only works with 2v batteries.
    If it worked with 6v batteries I would love to use it!
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #8
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    Re: Individual Battery Voltages

    I think your on a very illusive hunt for any two lead acid batteries to ever have identical anything. There just to many variables to take into account. That's the very reason I got rid of all my shunts and Battery monitors, it'll drive you crazy, like looking for Big Foot. Large differences and trends you can do something about, the small stuff are blips on the screen.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #9
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    Re: Individual Battery Voltages

    And just to add a note of caution... You are measuring three cells in series... Nothing says that all three cells have low (or high) S.G... One could have low SG and two could have nominal or high SG, and you would never "see" the one low cell voltage in the string.

    The old style batteries that had exposed bus bars on the top make checking cells much easier--But made the batteries more difficult to keep clean/sealed... Going with 2 volt cells does have its "charms".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
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    Re: Individual Battery Voltages
    Blackcherry04 wrote: »
    I think your on a very illusive hunt for any two lead acid batteries to ever have identical anything. There just to many variables to take into account. That's the very reason I got rid of all my shunts and Battery monitors, it'll drive you crazy, like looking for Big Foot. Large differences and trends you can do something about, the small stuff are blips on the screen.

    This. In spades.
    Temp differences, wire resistance differences ... trying to hunt down fractional Volts between cells ...
    Spare me from such an obsession.

    If you've got a 5% at rest Voltage difference between batteries, worry.
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #11
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    Re: Individual Battery Voltages
    Cariboocoot wrote: »
    This. In spades.
    Temp differences, wire resistance differences ... trying to hunt down fractional Volts between cells ...
    Spare me from such an obsession.

    If you've got a 5% at rest Voltage difference between batteries, worry.

    I agree, perhaps overkill, but at least with this setup I would be able to see if there was a major cell failure which otherwise might not be noticed for some time.

    But I do thank everyone for their suggestions and comments. They all bring value.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #12
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    Re: Individual Battery Voltages

    You can also simply stick your volt meter probes in adjacent cells to read cell voltage. You want to wash the probes well but it works fine. In Fact, somewhere in my junk I have an old analog batter meter, designed to be dropped in the cells.

    Icarus
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