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Thread: Do I have to unplug the balance connector from the loader before use ? (LiPo)

  1. #1

    Question Do I have to unplug the balance connector from the loader before use ? (LiPo)

    Hello,

    The question is in the title but let me explain the context :

    My next robot will have to recharge itself. To do this, this robot will ship an automatic LiPo loader connected to the LiPo battery.
    If the balance connector can remain connected to the charger, then I could simplify the power management board and save the space taken by the 5 relays that will be used to disconnect the balance wires.

    Thank you !

  2. #2

    Re: Do I have to unplug the balance connector from the loader before use ? (LiPo)

    Whether you need to disconnect the balance connector depends entirely on the charger design. Is any current flowing into the balance port when the charger is off? You will have to measure this. Or, if you design the charger yourself, you could know.

    You don't need five relays (I'm assuming you're using 4S so you have 5 wires.) Ground could always stay connected no matter what. The rest of the wires could be connected with P-channel MOSFETS that turn on when charging (to avoid voltage dropping against the sensor connection,) and that turn into diodes when not charging (so you put them with "cathode" towards the battery.) MOSFETs come in arrays on chips, too, so it could be a single chip on your board.

    Or you could use one relay with four circuits. Something like: https://www.digikey.com/product-deta...2211-ND/647869

  3. #3

    Re: Do I have to unplug the balance connector from the loader before use ? (LiPo)

    Indeed, using P-channel MOSFETS instead of a relay would be better. But is RDS(on) likely to cause charging problems ?

  4. #4

    Re: Do I have to unplug the balance connector from the loader before use ? (LiPo)

    is RDS(on) likely to cause charging problems
    I don't know; is it?

    How much current do you "balance" with? 0.2 amps?
    How much RDS(on) do your MOSFETs have? 100 mOhm?
    This will cause 0.02 Volts of drop between the battery and the charger.

    When using the balance connector only to measure voltage, there is approximately no current flowing through those wires, so RDS(on) will not drop the voltage at all. (This is a form of "Kelvin" connection.)

  5. #5
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    Re: Do I have to unplug the balance connector from the loader before use ? (LiPo)

    Definitely need to know more about the balance charger you are using to say for certain, but...

    Does the balance connector even need to be disconnected when not charging? Probably not.

    Most chargers have two circuits attached to each pin of the balance connector: 1) [sense] a resistive voltage divider to sense the cell voltage and 2) [discharge] a power resistor with a P-FET/PNP-BJT to connect the resistor across that one cell to bypass some of the charging current around that cell and/or discharge the cell slightly. The [sense] voltage divider requires very little current and will not be significantly affected by any series resistance below ~100 Ohms. The [discharge] resistors+P-FET/PNP-BJT will not be significantly affected by any series resistance below ~1 Ohm, and the P-FET/PNP-BJT is usually 'normally off' (pull-up resistors on the P-FET/PNP-BJT and pull-down resistors on the N-FET/NPN-BJT that turns on the P-FET/PNP-BJT).

    Really fancy chargers replace the [discharge] resistors with the primary of a transformer that shifts excess energy back onto the entire pack, but they should be equally unaffected by leaving the balance connector attached while not charging.



    The only thing that probably needs to be disconnected is the positive power lead of the battery from the charger, but even that may not be necessary depending on the charger design.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  6. #6

    Re: Do I have to unplug the balance connector from the loader before use ? (LiPo)

    The question is what the charger does when it's not powered by itself, just sitting there.
    If it goes very-high-impedance, then you can leave it plugged in without danger to the battery.
    For example, a transformer would potentially have pretty low winding resistance.
    Maybe you can measure the current (or just resistance) when it's off?

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