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How to keep a power up spike out of a battery charge controller?

Discussion in 'General Electronics' started by Deleted, Jan 4, 2009.

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  1. Deleted

    Deleted Guest

    I'm having a problem with a circuit that I did not design. The unit uses
    Lithium Ion batteries (laptop battery pack) when AC is not available. The
    problem is, when the unit is turned on with the batteries, there is a large
    initial spike (approximately 8 amps) which the charge controller sees as a
    short circuit and immediately shuts down the battery. My first thought was
    to use a capacitor to send the spike to ground. Then I realized that it was
    the capacitors already in the circuit that are charging up to make it look
    like a short. A resistor was added to attempt to reduce the impact of the
    spike but that failed.

    I then thought of putting a coil in series with the battery. The engineer
    shot the idea full of holes and mumbled something about back emf when the
    circuit was shut down and also complained that the spike could jump over the
    coil. Not having an engineering degree, I shut up but continued to think
    about this problem that should be solved electronically instead of settling
    for cracking open the $100 battery pack and taking battery power directly
    from the batteries to avoid the spike getting into the charging circuit -
    which works fine but is kind of hokey and voids the warranty of the
    batteries.

    In the circuit below I show my thoughts (albeit in a poorly done schematic).
    I figured I could use a coil with a ferrite core and a diode to ground on
    each side. I figured the diodes would satisfy his emf concerns and the
    choice of core would keep the spike from going across the coil. Note there
    is an on/off switch (not shown) between the battery and the rest of the
    drawing.

    Is this an idiot idea? Should I use an iron or any other type of core for
    the coil? Maybe only one diode? Get a job not even remotely connected to
    electronics? Any better recommendations?



    ___________nnnnnnnn___________
    | | |
    + _ _
    BATT ^ ^ Out to rest of circuit
    - | |
    | | |
    ______________________________
     
  2. neon

    neon

    1,325
    0
    Oct 21, 2006
    HERE IS THE IDEA When you turn on a switching regulator like The inrush current can be realy big as much as 10x of what is required to run normaly. What i don't understand is how can you get a spike across a battery that is realy hard to do. if there is a spke is a downward type because of drain on the battery. like your car when you start it deeps into 8v. I think eventualy you will find that the batteries are if not bad at least weak and replacing them will cure this spike. either way there is nothing passive that can solve your dilema. I sugest you do not try adding anything to cure this problem except battery replacement
     
  3. Deleted

    Deleted Guest

    Thanks for your response. I didn't want to go nuts with this design change.
    Adding transistors is like redesigning the charge controller and I can't do
    that.

    Interestingly, my supply guys accidently gave me a brand new battery with a
    larger capacity and it had exactly the same modification to it that we have
    been using - that is, separate the load from the controller and tap power
    directly from the batteries. Unfortunately, said battery is physically too
    large for this particular project. I guess the manufacturer realized they
    had a problem with their controller but chose not to upgrade this model of
    battery for some reason. I don't speak Chinese, so I can't call them with
    any questions...

    Bart
     
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