Disconnect Circuit While Charging Battery

Discussion in 'The Projects Forum' started by jpanhalt, Feb 22, 2015.

  1. jpanhalt

    Thread Starter AAC Fanatic!

    Jan 18, 2008
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    Many battery powered devices do not allow them to be operated while the battery is being charged. I know how to do that with a switched charging jack like this:
    upload_2015-2-22_12-38-32.png

    1) Is there a more common, sophisticated, expensive, or solid-state way to do that?
    2) I am going to be using an On/Off controller (e.g., MAX16054) and a boost converter (NCP1402 at 5V). The battery pack will consist of three, AAA NiMH cells in series. Is it really necessary to have a disconnect during charging? Assuming the charge voltage doesn't exceed 5V, how would turning the system on, say accidentally, during charging be a problem?

    Thanks, John
     
  2. #12

    Expert

    Nov 30, 2010
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    If the charging voltage never exceeds the Vmax for the chips and capacitors, I don't see a problem.
    You might cause glitches by using unfiltered AC to charge in pulses, but a DC flow would be well behaved.
    Just go through the circuit pretending you have to design it for the charging voltage. That should draw your attention to any problem areas.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Depends on the charging algorithm. Some NiMh chargers look for a slight drop in battery voltage to terminate charging. Plugging in a load could confuse the charge controller.
     
  4. #12

    Expert

    Nov 30, 2010
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    I could have sworn I read that as NiCad. :confused:
    Then somebody else catches it.
    That's what make this site work so well.
     
  5. jpanhalt

    Thread Starter AAC Fanatic!

    Jan 18, 2008
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    The part of my question about turning on accidentally was an "abundance of caution" sort of thing. I couldn't think of any problems. My charger is decent and overvoltage won't be a problem. As you mention, Mike, turning the device on might cause a false end-of-charge detection, but I also monitor the mAH and would probably detect an early end. This device will not be used in a model airplane, so if the battery is not fully charged, that will be only a minor inconvenience.

    My real question is whether there is a better or more common way to disconnect the circuit during charging. Since some of my equipment has a negative center pin (namely my JR transmitters), I will probably also be including reversed polarity protection.

    As for NiMH vs. NiCd, since the self-discharge rate of NiMH cells has been so reduced, I have switched to them for everything except high discharge service.

    John
     
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