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DC motor start-up surge is tripping the battery protection circuit - help!

  1. Jun 21, 2009 #1
    Hi all,

    I've got a problem which is so daft it hurts.

    I'm using lithium batteries, and I'm well aware of the dangers, so despite being fine with working on an un-protected cell, I changed to protected ones, and then issues appeared. The current is drawn from a pair of parallel 18650's, good for about 6A continuous.

    So, my circuit is incredibly simple - a DC motor, a switch and a battery. But because of the battery protection circuit, and the start-up current surge of the motor under the load, it simply shuts the battery power off. It does this shut-down in 12ms, which I've worked out after buying an expensive digital 'scope, and it stays off until you take away the "short circuit".

    Sometimes it fires up, sometimes not. If I add a DVM to the circuit to measure the current, it also works perfectly. I think this is due to the resistance of the leads. If I add a power resistor it simply cooks that, and doesn't turn the motor. I've tried a 1mF cap to smooth it, but that gives another issue, in that if you manage to press the switch "just so" it dies and stays dead, as the cap keeps the battery protection on!

    What I need is some (possibly mythical) part that will sit in the circuit and be a small resistor (an ohm) for a fraction of a second, then become a bit of wire (milli-ohms) until the switch is let go. Oh, and just to make it even harder, there is only a 10mm diameter by 35mm long space to fit anything inside the case!

    I know this can be done, I know the current draw is safe, I know that it sometimes helps if I tweak the lengths of the leads a bit (though I don't understand quite how or why, it might be inductance or it might be resistance) - so please, if anyone knows what I should be looking at adding, please let me know!

    I can post as much detail as you like, if you need any more. I've trawled Google, asked friends who might know, tried endless fiddling. It's killing me!

  2. jcsd
  3. Jun 21, 2009 #2


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  4. Jun 21, 2009 #3
    I've had a read up on them, it's rather interesting, they use a crystal of an exotic material that drops it's resistance with increasing temperature. That temperature is caused by the current through the device.

    Very clever. Thanks for the suggestion, I'll get some ordered in the morning.
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