Battery Cell Voltage Readings !?!??

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tkc100
tkc100 Solar Expert Posts: 67 ✭✭✭✭
in General Solar Power Topics #1
I am hoping someone can help me understand some battery voltage reading I recorded today while testing a battery.
Or perhaps more importantly I am hoping I can explain this in a manner that makes sense.
It was a typical 6 cell 12 volt lead acid battery.
I wrapped a couple inch piece of solder around the positive lead of my Digital Multi-meter so that the solider protruded about an inch past the tip of the probe.
I connected the negative probe to the negative post on the battery.
Starting with the cell nearest the negative post I started taking voltage readings by dipping the tip of the solider in the each cell’s electrolyte.
The readings I got were as follows:
Cell 1 = 0 volts, Cell 2= 2, Cell 3=4, Cell 4=6, Cell 5=8, and filially Cell 6 read 10 volts.
Then I switched the entire setup:
I put the solder on the negative probe and attached the positive probe to the positive post.
Then I began taking my reading at the cell nearest the positive post.
The readings I got were as follows:
Cell 1 = 2 volts, Cell 2= 4, Cell 3=6, Cell 4=8, Cell 5=10, and filially Cell 6 read 12 volts.
There is probably a simple explanation but at the moment it completely eludes me.
Why aren’t the voltage readings the same no matter which way I start off measuring??
Quite honestly I thought I had a bad cell after taking the first set of measurements and was left scratching my head after taking the second set.
Enlighten me please.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #2
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    Re: Battery Cell Voltage Readings !?!??

    You're not measuring from one battery electrode to another battery electrode. You're using your solder as one of the electrodes, and that solder is acting as a negative electrode. Therefore, in case one, you measure 0 volts between the two negative electrodes, while in the second instance, measuring from the battery + terminal, you read 2 volts between it and your solder negative electrode.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Re: Battery Cell Voltage Readings !?!??

    I am assuming that the lead in the solder behaved like a negative plate...

    When you connected to the negative post, plate potential = solder potential = 0 Volts.

    Positive terminal is the "other plate" and had a 2 volt potential vs the solder / negative plate.

    I am not sure I would do this very often... I would worry about contaminants on the solder (and possibly the tin/other materials in the solder) "poisoning" the cell (probably not a big issue--but a possibility).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • tkc100
    tkc100 Solar Expert Posts: 67 ✭✭✭✭
    #4
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    Re: Battery Cell Voltage Readings !?!??

    Why is the solder acting as a negative electrode no matter which of the multi-meter's probes it is on.
    I am really trying to wrap my head around this but not succeeding yet.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: Battery Cell Voltage Readings !?!??

    With a charged lead acid battery, one of the plates is (more or less) pure lead, and the other is lead oxide.

    When you place the solder (~40-50% lead or so) in the electrolyte... You will have virtually zero volts difference between the lead solder vs the Lead Plate and ~2 volts difference between the solder and the Lead Oxide plate.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
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    Re: Battery Cell Voltage Readings !?!??

    i would not put solder into a battery as the flux may contaminate the electrolyte.

    as far as the readings go you did not place the solder to the correct point to read the voltage of the first cell, or 2nd, etc. extreme care must be observed in doing individual cell voltage readings as this is not normally done on 6v and 12v batteries.
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  • tkc100
    tkc100 Solar Expert Posts: 67 ✭✭✭✭
    #7
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    Re: Battery Cell Voltage Readings !?!??

    Thank you all for your input.
    However at the expense of seeming dense I still not seeing it.
    Sometimes it is better to work from a picture for those of us who who present a challenge.
    Please refer to the attached file.

    First I put a piece of 50/50 non fluxed solder on the end of my probe because I did not want to dip the probe into the electrolyte. It makes no difference whether the probe is dipped directly into the electrolyte or my make shift solder tip is used the voltage reading will be the same.

    Correct me when I go astray. I'm not saying this as fact just my understanding.
    I order for a cell to function electrons flow out of the negative post through any load and back to the positive post.
    In order to get a voltage reading electrons must flow out of the negative post through my meter and back to the positive post.
    When the multi-meter's negative lead is attached to the negative post and I probe the closest cell with (lead or steel) there is no potential difference. The probe does not function as a cathode.
    When I move the probe over to the next cell the electrical connection is completed thought the cathode in the first cell. Hence I am actually reading the voltage potential of the first cell.
    The only way I can account for the difference in reading when start from the other end is that the probe can function as an anode but not a cathode.
    Does that make sense?
    Have I ventured too far down the rabbit hole?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
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    Re: Battery Cell Voltage Readings !?!??

    it must make contact with the + inside the cell as just dipping it into the electrolyte does not work for this. when you went to cell #2 it had to pass through the + of the 1st cell and - of the 2nd cell to reach the 2nd cell's electrolyte. this allowed you to read 1 cell when you thought it was 2 cells. do something wrong and you could weld pieces of solder into the cell as solder has a low melting point and could send small shards of solder to float loosely in the electrolyte to later short out portions of the cell.

    i strongly advise you not to do this unless you are willing to risk the battery and possibly more
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: Battery Cell Voltage Readings !?!??

    I have to point out that this is a completely futile operation. You are not reading the Voltage of the cell this way. You are in essence creating a "different" battery where one plate is comprised of those within the cell itself and the other is the piece of solder on the end of the other meter probe. It will produce random and meaningless readings. If it accidentally touches a plate within the battery it will read either zero or +/- 'X' Volts against the opposing plate. If it touches two plates at once it can short internally. As Niel pointed out, introducing foreign matter to battery cells is a bad idea. You should never stick any metallic probe into a cell.

    Regrettably most batteries today do not have accessible inter-cell connectors on top. There is no practical way to get a per cell Voltage reading. Under the circumstances, taking an SG reading is a better idea.
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  • tkc100
    tkc100 Solar Expert Posts: 67 ✭✭✭✭
    #10
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    Re: Battery Cell Voltage Readings !?!??

    OK, I got the picture....don't be sticking things in your...batteries.
    Actually the genesis of this twisted train of thought was the fact that my hydrometer was broken. I've got one of those nice little refractometer but somehow water got into where it does not belong.

    Then I got to wondering and well you can see where that got me.
    I do now have a better understanding of how a lead acid battery works, and of course not to stick thing in it.

    If I had to sum this little line of though up I would say:
    The metals in my probe (lead or whatever) can only function as a anode (-) and can not function as a cathode (+)
    I believe that would account for the voltage reading I got.
    Secondly, any reading I got are meaningless.
    And finally don't be sticking things in your batteries.
    Thanks all!!!!!!!
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    #11
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    Re: Battery Cell Voltage Readings !?!??

    ''And finally don't be sticking things in your batteries.
    Thanks all!!!!!!!''


    It's soooo nice when someone takes/understands the advice and doesn't cry about ''how it works, I've always done it this way, you guys are all bullies blah blah blah!''


    tck100, my hat's off to you. There';s so much to learn from so many people here.

    Ralph
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #12
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    Re: Battery Cell Voltage Readings !?!??
    Ralph Day wrote: »
    ''And finally don't be sticking things in your batteries.
    Thanks all!!!!!!!''


    It's soooo nice when someone takes/understands the advice and doesn't cry about ''how it works, I've always done it this way, you guys are all bullies blah blah blah!''


    tck100, my hat's off to you. There's so much to learn from so many people here.

    Ralph
    Totally agree! Well said Ralph. Well done tck100. Very refreshing :)
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #13
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    Re: Battery Cell Voltage Readings !?!??

    Wait till your bit of solder hits an opposite plate in the cell. that will get exciting real fast.
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