Battery voltage drop question

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  • mfoobar
    mfoobar Registered Users Posts: 13 ✭
    #32
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    Re: Battery voltage drop question

    I just did a longer test on the battery with my battery monitor connected to it. The battery started around 12.55v as I wanted to start with it a bit discharged just to see if the voltage drop I was seeing was just due to a high surface charge. As you can see from the graph there is definitely the Coup de Fouet on there with the small bounce in voltage at the start of being under load. This test was done using a 4.4A headlight bulb as a load and ran for about 112 minutes. I still think the voltage drop is too great, but as you can see at the end, the battery does recover to the same point as the start. It has consumed about 8.4Ah in that time. This is a single 12v 110Ah 2.5 year old SLA battery. What is worrying is the speed at which the voltage is dropping when under load. I will try and give it a good charge back up and try again and run the test for longer.

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    -Matt
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #33
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    Re: Battery voltage drop question
    mfoobar wrote: »
    What is worrying is the speed at which the voltage is dropping when under load. I will try and give it a good charge back up and try again and run the test for longer.

    For a 110AH battery delivering ~4 amps (C30 rate) that voltage drop does look pretty bad. Either there is a wiring problem and you are not measuring the voltage right at the battery terminals or the internal resistance of the battery is way too high. (The smaller voltage drop between voltage under load at start and end, 2 hours later, does not seem to be as much of a concern.)
    If it is internal resistance of the battery, then it may be the result of bad sulphation or other damage. A true capacity test to something like 50% SOC should tell you more.
    SMA SB 3000, old BP panels.
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