NiCd batteries

Discussion in 'General Electronics Chat' started by strantor, May 29, 2012.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    I've got a bunch of dead cordless drill battery packs. I've got the bright idea to take them all apart and salvage the good cells out of them and make a mondo battery pack out of them. So far I've taken one pack apart. When I tested the terminals beforehand, I read 0V. I seperated each cell and they all read 0V. They read about .3Ω ±.05Ω. I figure they're probably all hopeless, but I've got one on the power supply right now trying to charge it. my questions:

    What's a good set of parameters for charging? What I've read so far, recommends charging at 1/10C. I don't know what the Ah or C rating of this pack is, so I'm charging at 200mA, with a voltage limit of 1.2V. Is it safe to go >1.2V if the current is still <200mA?

    does having 0.0000V acrosss the cell mean that the cell has 100% met it's demise?
     
  2. MrChips

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    Oct 2, 2009
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    As someone said in another thread, do not measure with an ohmmeter. You risk killing your meter.

    I assume these are NiCd cells. I usually don't separate the cells from the pack unless I know for sure they are bad. Reading 0V on a cell is not a good sign. This usually indicates a chemical short. I try to zap each cell individually with a high current. Sometimes you can break the short and the cell starts charging. You can tell fairly quickly if this is working.
     
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  3. #12

    Expert

    Nov 30, 2010
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    Ancient notes say:
    C/100 is a slow charge
    C/10 is a fast charge
    C/1 is damaging to the cell.
    Absolute Max voltage is 1.39 per cell
    1.345V = 99%
    1.29V=98%
    1.26V = 96%
    1.25V = 94%

    and no, 0.00 volts does not prove anything.
    Do not zap first and charge later. See if the cells will take a charge without risking the damage of zapping them.
    Zapping can be done with 10,000uf to 20,000uf charged to 20 volts. No current limiting resistor.
    This allegedly "burns the whiskers off" but I've never got a good, long lasting cell out of anything that needs to be zapped. Still, you're free to try it and see if you get a week or a few months out of a sick cell.
     
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  4. strantor

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    While you guys were replying, I was in the shop zapping cells with the welder. I zapped 5 of them; first 2 were a learning opportunity and the other 3 seem to be behaving more like batteries now, so I'm trying charging them again. If that works, I'll zap the other 6.

    About measuring with the meter, I verified 0.000V before taking a resistance measurement. Even if they weren't directly shorted out, my meter is a fluke - idiot resistant.
     
  5. MrChips

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    A welder seems rather aggressive. I don't use the capacitor approach. I use a bench power supply and just touch the leads from the PSU to the cell for a second or so.
     
  6. strantor

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    yeah I tried that first. Didn't seem to do anything. My bench supply only puts out 3A.

    i don't know how long I need to wait for an indication of health before I call it dead. I've had one battery hooked up to the bench p/s (post-welder zapping) for 10 minutes now with a 200mA limit and it hasn't come up above 7mV. I
     
  7. #12

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    Doing an amp-hour test for capacity is a useful method. Charge to 1.345 volts (99% full), let it rest to see if it is going to self discharge over only 10 or 20 minutes, or over-night, then, if it holds steady, less than 1% self discharge per day, attach a known load and count time. 1.05 volts = 1% full. The question is not, "Is it as good as new?". It won't be. The question is, "How much of the original capacity is left". 90%? Great! 30%? won't last a week under normal use.

    ps, the capacitor zapping allows very high amperage for a millisecond or so. That blows whiskers off better than 3 amps for a minute and doesn't risk heating the healthy cells. Remember, try charging before zapping. No need to risk the damage from zapping if the cells are going to behave well without it.
     
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  8. strantor

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    Ok I think these thinks are dead. I I zapped the other 6 and they didn't ever act like a battery afterwards, still just like a dead short. One of them showed about 400mV after zapping and by the time I got back to bench supply to try & charge it, it had dropped to 80mV. When I hooked it up, it went back tup to around 500mv but as I leave it on the charge with a constant current, the voltage goes down and down, now less than 100mV. Should be going up! CRAP! oh well, got a few more batt packs to break apart and investigate.
     
  9. strantor

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    I'll do that, if I ever get a cell that's not absolutely departed. I think one of the ones I have at my house had some juice when I measured it, maybe it's good.
     
  10. #12

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    At least you are learning what a bad cell acts like :D
     
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  11. strantor

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    say, in these batt packs there's 3 wires. one each for the 2 ends of the series string of cells, and then a 3rd one that's connected to the center. Can I assume that this 3rd wire is how the charger knows when you've got a bad cell? It's at the center of the voltage divider (voltage divider formed by battery cells), so if the voltage on the 3rd wire isn't exactly 1/2 of the pack voltage, then a cell is shorted or something.
     
  12. #12

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    Makes sense to me, but the third wire that I have encountered were connected to a temperature sensor. Can't take a cordless drill out of a 125 F truck and charge it because the overheat detector has already detected.
     
  13. strantor

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    I don't see a temp sensor here, but these are cheap-o packs. When I get home and open my dewalt packs I bet I'll find some.
     
  14. bountyhunter

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    Sep 7, 2009
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    The 'good" cells are not really good, they are old cells that haven't died yet but probably will in the near future.
     
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