Simple and unregulated undervoltage protection for lithium cells

Discussion in 'The Projects Forum' started by danrulz98, Sep 17, 2012.

  1. danrulz98

    Thread Starter New Member

    Sep 17, 2012
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    Hello,
    This is probably going to be a really simple question, but I've been trying to do some research on a solution and I haven't really found anything that's really simple and elegant.

    I have a cordless drill that uses 9.6 volt NiCad batteries with only 1.3 Ah of capacity.

    What I would like to do is use some high performance lithium cells to replace the NiCad cells.

    Three lithium cells gives me 11.2 volts, which should be fine. The batteries I'm looking at are some high amperage RC toy batteries.

    Now, what these batteries seem to lack (other than information!) is undervoltage protection. I don't actually know if they do or not, but in product demonstrations of these batteries, they seemed to go down to almost 8 volts before the tests were stopped by the people conducting them.

    So for my application, I would like something just as simple as possible for undervoltage protection. I have a fancy charger that will keep it from going over its voltage rating and control charging and all that, so all I need is something to kill the power when it goes below 9 volts, and let the power come back on when there's over 10 volts or so (it doesn't actually matter, so long as the circuit comes back on when the battery is charged).
    I already figured I would have to have a separate charge connector, so that's not a concern.

    Now, I figured this could be done with a FET of some kind, perhaps a FET that takes more than 9 volts to close the circuit. If there is such a FET, that should work just on its own. Perhaps a FET and a zener diode.

    The solutions I've found so far have been ICs that do a whole lot more than undervoltage, or ICs that wouldn't be able to take the amperage or voltage I'm hoping to get (whatever the battery can put out - this is a cordless drill after all).

    Anyway, any help is appreciated. I'm hoping this is something simple and I just didn't look hard enough.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Last edited: Sep 18, 2012
  3. Audioguru

    New Member

    Dec 20, 2007
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    In addition to under-voltage shutoff, you need a circuit to disconnect the motor it it becomes stalled and uses a very high current.
    A Ni-Cad battery simply gets hot but a Lithium battery explodes or catches on fire and you do not want to be near Lithium that is burning. The fire is very difficult to stop. Water on the fire makes it burn hotter.

    You also need a charger that is made for a 3-cell Lithium battery.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Li-Ion cells have a higher internal impedance the Ni based cells (Ni-Cd or Ni-MH). May not be good for peak current.

    Li-Ions require very exact charging and must not be over discharged. Li-Ion packs have built in protection for over/under voltage as well as over current that opens a FET and cuts the pack off. No way to do it "simple" and still be safe.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    He is not using a Li-Ion battery for a laptop that has built-in protection circuits. Instead he is using a very high current Li-Po battery for a high speed (high current) model car and the battery has no protection circuits because they are in the charger and in the car.

    The battery is 2.2Ah and is rated for a discharge of 50C so the maximum allowed current is 110A!
    The battery is Chinese so maybe you cannot believe it but I betcha it burns very well, or will a stalled motor or speed controller burn out first?

    The wiring in the drill might be incandescent and also burning!

    Or does a cordless electric drill have a circuit breaker?
     
  6. sceadwian

    New Member

    Jun 1, 2009
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    Buy a proper Lithium powertool set, they've come down quiet dramatically in recent years.

    Any attempt to hack an RC power pack into a power tool without proper thermal monitoring (not just under voltage) will lead to possible explosion or fire.

    Many smaller modern devices like cell phones simply use polyfuses for thermal management, and the IC's that do charging are simply fed from a decent LDO with a current feedback resistor. Nothing to it, I routinely charge individual cells by hand using only a constant current source and a voltage meter.

    But a power tool requires a bit more monitoring due to the high discharge rate and the desire as well to have a high charge rate, with multiple cells in series.

    Personally I believe the warning's of how exact a Lithium cells charging requirements are seriously overblown, a single cell is stupid simple to charge and care for, it's a PACK that's a problem because of balancing, which is critical especially with high discharge currents and balancing charging currents.

    Ni-Mh and Ni-Cads would tolerate float low current charges for pack balancing where the same float currents will destroy a Lithium pack, sophisticated charge/discharge managements IC's are required especially as the number of cells increase.
     
  7. danrulz98

    Thread Starter New Member

    Sep 17, 2012
    15
    0
    Thanks for the input -
    For charging, I have one of Turngy's universal battery chargers. It handles charging voltage, amperage, balance, all of it. Hence, I'm not concerned about charging things.

    Since I'm pretty sure a peak load of 110 amps would fry just about anything, limiting the current to 20 or 25 amps would be good, which should also keep the temperature down? (I would think so at least).

    As to buying a new tool set... yeah, I looked into them. $200 for the modern version of my tools with lithium batteries. A bit less affordable than the $18 battery and all the fun of making it work. (Makita also wants $100 for a replacement nicad battery, and I know the el-cheapo ones don't last). I probably will get new tools, but its up on the eventually list. If anything, I want to learn more about Lithium batteries and getting them into projects.
     
  8. sceadwian

    New Member

    Jun 1, 2009
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    Yeah.. Lets not mention voiding your home owners or renters insurance policy..
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Might find something here:

    http://www.batteryjunction.com/pcb.html
     
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