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  1. #1
    mtbr member
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    Testing 18650 cells??

    Hey folks,
    Love all the DIY lights out there.
    Sorry to post a stupid question but here goes:
    I have a few Dell laptop battery packs that will no longer run the laptops. I pried one apart after I noticed that the led battery gauge still read charge. The packs come with 18650 cells. I was thinking of making an extra battery pack for my MagicShine lights but I need to be able to test the cells to see if any are still good.

    What do I need to test the cells?
    Thanks for the help!

    Also where can I pick up the water resistant connectors used on my magicshine light? I am looking at Batterysource and Dealextreme.

    Edit: found some connectors on battery space.com. Any other options?
    Last edited by wormvine; 08-01-2010 at 03:32 PM.

  2. #2
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    Get a voltmeter and a 4 ohm 5 watt resistor. Charge battery, clip resistor across 1 cell, clip voltmeter across resistor, read voltage, note time and write both on a sheet of paper. Wait 30 minutes and note the time and voltage again, continue until battery drops to 3 volts. DO NOT test to a lower voltage. The batteries that stay at the highest voltages are the better ones.

  3. #3
    www.hahntronix.com
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    Make sure you use a resistor that can dissipate the watts you'll burn up putting a 4.7 ohm resistor across a freshly charged 4.2 volt Li-Ion cell. That's about 3.8 watts.

    If you use a 1/4 watt resistor for this test, it will burn up in just a few minutes.

    You can get a nice 5 watt 4.7 ohm resistor from digikey (http://search.digikey.com/scripts/Dk...name=4.7W-5-ND) for about $0.35.

    You can also build up a suitable resistor out of the more commonly available 1/4 watt resistor by putting a lot of them in parallel. Which is fine if you can get twenty 100 ohm resistors for free, and you like to solder a lot. Otherwise just buy a 5 watt resistor.

    A 4.7 ohm resistor is probably a good choice as it will cause your cell to draw around .9 amps when the cell is freshly charged and will drop to around .68 amps when the cell is down to around 3.2 volts. A decent brand new cell should take more than 2 hours to go from 4.2 volts to 3.2. volts driving a 4.7 ohm load. Any cells that discharge to 3.2 in much less time should be recycled.

    Mark
    Nimium est melior!

  4. #4
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    Thanks for the info guys. I'll see what I can drum up...

  5. #5
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    where did you take the battery? http://www.battery4us.com ??

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