Why does the first cell on my batteries banks always have the highest voltage ?

fca1
fca1 Solar Expert Posts: 124 ✭✭
in Off Grid Solar & Battery Systems #1
Hi in all my banks I noticed that the first cell connected to positive cable has always the higher voltage of group...
Is this normal ? It's only me ? *;)
Any one have the noticed the same behavior ?
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #2
    Re: Why does the first cell on my batteries banks always have the highest voltage ?

    How much higher voltage ? Maybe they have become unbalanced somewhat.

    Swap it with another one in the string and see what happens to the others.

    boB
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #3
    Re: Why does the first cell on my batteries banks always have the highest voltage ?

    If you have both + and - wired to the first cell, that can happen. Usually best to tie + to the nearest cell and - to the one at the opposite corner.

    Also check cable connections.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: Why does the first cell on my batteries banks always have the highest voltage ?

    Here is a good explaination on how to correctly wire batteries so they share current properly:

    http://www.smartgauge.co.uk/batt_con.html

    Also, it is possible that you have the best ventilation/cooling on the first battery/cell in your setup. Cooler batteries will have higher resting voltages (by a little bit).

    What is your bank? a series of 2 volt cells in series--Series/parallel 6/12 volt batteries?

    In general, you want to ensure that all cells/batteries are pretty close to the same in voltage--And differences can be an indication of failures somewhere (bad cell, bad connection, bad wire, poor wiring setup, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • fca1
    fca1 Solar Expert Posts: 124 ✭✭
    #5
    Re: Why does the first cell on my batteries banks always have the highest voltage ?

    Hi this is something that i notice some time ago and it happens with both my banks with different cells different cables..

    yes i have + connected to first cell and - o the las cell (12 2v in series to make 24v)..


    already swaped ther order, but the first (the one with + connected) always have more voltage when charging....

    it have some logic because the first batt receives the charge and even if small my ohms law i have losses between batts (just thinking loud; waiting for echo) :)

    in one bank they are connected with copper bars so no small gauge cable....
    i-2qRBJJr-M.jpg

    i-rd33fdT-M.jpg

    regards
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Why does the first cell on my batteries banks always have the highest voltage ?

    OK, since during charging, it would seem to indicate a "colder" battery than the rest.

    These appear to be sealed batteries--so you cannot check SG (?).

    You are measuring voltage on the battery post itself--And not on one of the cable lugs/bus bars?

    And what is the voltage (high/low) you are measuring?

    In general, battery position should not matter at all in a single series string--other than heating/ventilation or differences in electrical connections (a small or "weak" connection getting hot).

    If all the connections are cool and batteries are all about the same temperature--Then there is no "engineering" reason for the differences.

    However--no matter how many times I type that, we have folks here rotate cells/battery positions in strings once or twice a year because they see the same thing (differences in battery voltage or SG levels).

    I won't say it cannot happen, but I cannot give you any reasons beyond my guesses above.

    Very nice looking battery bank.

    -Bill :confused:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #7
    Re: Why does the first cell on my batteries banks always have the highest voltage ?
    BB. wrote: »
    You are measuring voltage on the battery post itself--And not on one of the cable lugs/bus bars?

    And what is the voltage (high/low) you are measuring?

    -Bill :confused:

    I think that Bill has your answer here. Your cell-to-cell connectors are very flat and resting against the plateau of the battery post, so that they have the potential of making a very low resistance connection (low voltage drop.) Depending on the design of the + and - cable ends, they may be higher resistance, causing a higher voltage drop between the lug and the battery post.
    Then, depending on how the wire to the second parallel string is connected, that wire and the lug connected to it, may be carrying less current and therefore have a lower voltage drop between the post and the lug.
    Finally, based on the relationship of the various wiring resistances, you may actually have a higher battery post voltage on the first cell. But if this is the case, you should see the same effect on the first cell connected to the - side of the charging cable too. Unless you have a bad connection on the + cable and not on the - cable.

    When measuring cell voltages under charge, the two concerns you have to keep in mind are:
    1. Are the currents equal at each string? and
    2. Am I measuring the voltage of the battery itself, not something connected to it through a (small but significant) resistance.
    SMA SB 3000, old BP panels.
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