Charge LiPo while in use?

Discussion in 'General Electronics Chat' started by philipm, May 3, 2013.

  1. philipm

    Thread Starter Member

    Jun 27, 2012
    I've been using a MCP73811 to charge a single LiPo cell with great success. It is connected with a MOSFET so that power to the main circuit is cut whenever charging is in progress. I'd like to simplify the circuit by removing this protection.

    My question is - what would happen if I tried to charge a LiPo while it is in use?

    I'm guessing it's not an issue. The MCP73811, supplying 4.2V and constant current (450mA) will win in an analog OR with the battery voltage (3.0-4.2V, depending on charge). So the charger would supply the first 450mA and then the battery would add any more mA draw.

    Anything to watch out for?
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    You are guessing wrong. LI batteries are dangerous (ask Boeing) and have to be charged carefully. The charger ICs have very accurate detectors looking at cell voltage and charge current. If the battery is under load (in use) the current going into (and out of) the battery is constantly varying so the detector can't work.

    There is a reason the designers disconnect the battery to charge them.
  3. #12


    Nov 30, 2010
    Cars don't work that way, but that's because cars use lead acid batteries. They are much more forgiving and rarely burst into flames when irritated.
  4. philipm

    Thread Starter Member

    Jun 27, 2012
    Flames you say? Suddenly, I'm liking the MOSFET protection again. ;)