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Battery capacitor, why?

Discussion in 'General Electronics Discussion' started by JPU, Dec 6, 2018 at 6:23 PM.

  1. JPU

    JPU

    277
    1
    May 19, 2012
    Hi All

    I am hoping someone can help me with this one.

    I have a battery (29.2v DC) and its made up of Lithium cells. Inside the battery there is a capacitor across the positive and negative wires which then lead directly out of the battery. The capacitor is a 35V 220uf.

    Please can some one tell me what is the purpose of the capacitor in the battery?
    Also, would the presence of the capacitor positively effect the outcome of an EMC test on the device the battery powers. ( A brush less motor and controller)

    Thanks

    JPU
     
  2. kellys_eye

    kellys_eye

    4,289
    1,143
    Jun 25, 2010
    If the battery is made up of lithium cells (i.e. multiples of) then there will be some form of 'balancing' circuitry built in and the capacitor will be associated with smoothing/decoupling the supply to that circuitry.

    It should not affect any EMC tests.
     
    FrankB likes this.
  3. JPU

    JPU

    277
    1
    May 19, 2012

    Hi

    Yes there is a BMS in the battery.

    Please can you help me further? Would the presence of the capacitor would help in an EMC test?
     
  4. JPU

    JPU

    277
    1
    May 19, 2012
    I am sorry, I can see that you have said it would not affect the test, but would the presence of the capacitor help if there were to be any problems. I'm just about to send a set for EMC testing. We have never placed the capacitor in the battery since its original design and the equipment has worked perfectly for us in testing. However a last minute check over before sending the kit for EMC testing and Ive just realised the cap had not been inserted through my tests. Now I don't know if I should add it just in case, Belt and braces so to speak, or just leave it out in case it effects the test in a negative way?

    Thanks
     
    Last edited by a moderator: Dec 6, 2018 at 7:15 PM
  5. kellys_eye

    kellys_eye

    4,289
    1,143
    Jun 25, 2010
    The capacitor is of a value that wouldn't (imho) make any difference to EMC tests, fitted or not.
     
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