Lithium Ion battery charger

Discussion in 'The Projects Forum' started by Experimentonomen, Apr 17, 2012.

  1. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
    331
    46
    I'm working on building a charger for a 22.2V pack i made with six 18650 cells from a laptop battery back wired in series.

    Im considering active balancing by using the shared transformer method, see fig 7 in this pdf: http://users.utcluj.ro/~atn/papers/ATN_2_2010_1.pdf

    I'm thinking a 30 turn primary and six 5 turn secondaries as that would turn the full pack voltage into the correct voltages for each cell.

    The part i don't quite understand in that pdf is that the secondaries are wired the wrong way, the diode is facing the negative pole rather than the positive pole.

    Is this a drawing error or does that reverse current act like the passive shunt balaner by using a reverse current over the cell to divert the charger current to the other cells ?

    And i imagine the box that sez "control" is more or less just a 50% duty cycle clock that gets an enable signal from the charger.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    IMHO, it's not an error. The diode diverts current around the cell rather than thru it. But I don't really understand why this arrangement wouldn't drain the cells if no charge pulses are applied. You should write the author.
     
  3. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
    331
    46
    Ah i never realized that, with no switching signal on the mosfet, the diodes and secondaries actually impose a dead short onto the cells.

    I should just be able to flip the diodes and doing the charge current limiting primary side and sensing the total pack voltage for the constant voltage regulator. It should provide a more balanced SOC than just a plain old series charger.

    Since all six secondaries have the same number of turns(like a six strand litz wire) would produce exactly the same voltage, there should be no need to monitor the voltage of each cell.

    Its just a matter of limiting the primary current so that each of the six secondaries only supply up to 1.1A of current to the respective cell.

    This current limiter could be a current shunt in the switching inverter feeding back a current signal to the error amp that controls duty cycle and thus controlling the voltage of the six outputs, limiting the charge current.

    Or simply a current transformer on each output, feeding back to the PWM controller, now i dont have six identical small ferrite cores though.

    I'll have to build a prototype and test this out.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    In Fig. 7 it looks that way, but that's not really a schematic. There may be more to the story, one of those "details left to the reader" sort of things.
     
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