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Battery Charger Considerations?

  1. Jun 19, 2013 #1
    A friend of mine lost his battery charger for his airsoft gun and he naturally comes to me for electrical help. Problem is, I'm freshmen level right now. I looked up how to hack and build a basic battery charger, my question is do the internal resistance/impedance of the batteries matter when hooking everything up? What other major considerations should I include when building a basic battery charger?
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  3. Jun 19, 2013 #2

    Simon Bridge

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    Welcome to PF;
    The internal resistance dissipates energy - which can make the battery get hot, maybe leak or explode.
    It would be pretty unusual to charge a battery with AC so impedance is seldom important - unless you have a lot of ripple of course.
  4. Jun 19, 2013 #3
    Thanks, I've been coming to this forum for a while. Just haven't joined until today. Since that's the case, should I try and match the internal resistance of one battery to the other?

    The way I'm thinking of it right now is if one of the battery's internal resistance is higher than the other's than I think it's intuitive that current won't flow. On the other hand I think that if the battery's internal resistance is lower than the other's than current would flow more than is required and that the flow will damage the battery in some way.
  5. Jun 20, 2013 #4


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    You don't need to know the internal resistance of the batteries.

    You do need to know the voltage and maximum current supplied by the old battery charger. Building one yourself is possible, but unlikely to save you money and you could damage the batteries if you get it wrong.

    Preferably, you or your friend should get a replacement charger from the manufacturer of the gun.
  6. Jun 20, 2013 #5

    Simon Bridge

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    Not really - off the same supply, the batteries would charge at different rates. It's more important to monitor the charging process and keep the current low. There's quite a lot more to the internal structure of a battery than just the internal resistance.

    I've charged batteries off a variable PSU before - the main thing you have going for you is that the batteries are designed to be charged ... you just need to know how the manufacturer expected it to be done.

    You appear to have described opposite results for the same situation.
    I'd be quite concerned about you trying to reverse-engineer a battery charger using only what you know and measurements taken off the batteries.

    Best practice is to find out the specs for the manufacturer's charger and copy them.
  7. Jun 20, 2013 #6
    And if this is a lithium polymer pack, be very careful, as improper charging can cause a serious fire.. just google lipo fire.. we use lipos in the rc aircraft hobby, i use them to power the ecu on my rc jets, and in electric planes they are the primary source of power.. all lipo chargers are c computerized to prevent any issues with charging. I even charge mine in an old army surplus ammo can to contain any fire that might occur. Basically be very careful if this is a lipo pack. Feel free to pm me for a great source for an inexpensive Lipo Charger.

    (Apologies for any typos etc, am posting from my tablet)
  8. Jun 21, 2013 #7
    Right, what I meant is I would think current would flow more into whichever battery has the lower internal resistance. But it seems I don't have to take that into consideration at all...

    Here is a recommended charger by the manufacturer. I see 600 mA is the output current. What should I notice about the voltages?

    It's a NiMH pack, by the way. No lipo fires for me!
  9. Jun 21, 2013 #8
    That they drop a wee bit at "peak" charge. That's my understanding anyways.
  10. Jun 21, 2013 #9
    Yep, NiMH and NiCD will both peak, then drop voltage very slightly, the charger should cut off when that drop occurs.
  11. Jun 22, 2013 #10


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    You might find many chargers that will charge this battery, but you have to consider what would happen if the charger was left on for a week.
    Would it overheat? Would it catch fire?

    In this case, the manufacturer has apparently done enough testing to ensure this charger is safe for the batteries and it won't overheat.

    So, if you believe this, you should be OK if you buy one of these chargers.

    You wouldn't be helping your friend if you tried to construct one when a cheap, recommended replacement was available.
  12. Jun 22, 2013 #11

    Simon Bridge

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    I think commercial battery chargers have a circuit that cuts the current when the battery is charged.
    Charging off a PSU requires constant monitoring.

    Aside: Best way to deal with a lipo fire is to suck the fat out of the room - there's a special name for it that escapes me right now....
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