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Capacitor in series voltage imbalance

  1. Jul 27, 2013 #1
    dear experts

    i connected several capacitors in series .
    their initial voltages are different. some were at 2.5v and some at 1v.

    Connected them in series, the volt differences remained.

    i am not sure why the voltage from to higher potential capacitor does not get.spread.across all tye rest.

    all of them are same brand same capacity and model.

    one thing to note about these caps is they appear to be sleeve shrinked using 2 caps.
    one such sleeve shrinked cap is rated at 2.7v supercap.

    is tye imbalance causer by such configuration?

    thanks.

    regards
    ramone
     
  2. jcsd
  3. Jul 27, 2013 #2

    Baluncore

    User Avatar
    Science Advisor

    I need a better description of your test circuit. After connecting them in series did you close the circuit through a load ?

    When Super-caps or electrolytics are used in series they should have a power diode wired across each. That will prevent some of them becoming significantly reverse biassed when close to discharge. Variations in leakage current can discharge one faster than the others. The one with the highest leakage will become reverse biassed first. Reverse bias can detonate tantalum capacitors, wear eye protection while testing.

    If identical capacitors in series start out with different voltages and are charged or discharged through a load they will all have the same voltage change. That is because they must all have the same current flow and so the same change in charge. Remember that C = Q / V and Q = I * T, so dv = I * dt / C.

    Voltage will be shared between series capacitors in an inverse relationship to their capacitance. If there are +/-10% variations in capacitance then a series charged group can vary 20% in voltage.

    Electrolytic capacitors have a chemical memory and can behave like a battery sometimes. Once discharged they can recover some voltage later.
     
  4. Jul 28, 2013 #3
    Dear Baluncore

    Yes, I closed the circuit. The pack of 6 batteries started charging. But the volt differences remained.
    So, each cap voltage was rising steadily. My concern is the fact that the caps with initial higher voltage does not seem to spread its charge to the caps with lower voltages, as should be the case.

    If the volt difference is little, its quite normal. But the difference was higher than expected.

    Yes, there seems to be quite a bit of leakage.

    Thanks, now I know why the volt diff remains since they caps each get same charge.

    I am using china-made supercaps that seems like Electrolytic caps.

    Ok, i will monitor with caution.

    Thanks very much for your advice!

    Regards
    Ramone
     
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