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Thread: NiMH question

  1. #1
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    NiMH question

    Hi all,

    I've built my first set of bike lights (hurray!). It's a quad XR-E R2 set up, run off a DX multimode driver (SKU26106) with a simple custom circuit on one side to light some LEDs if the temp gets too high or if the battery voltage gets too low.

    My problem, however, is the battery.

    The driver takes anything from 3V to 8.4V. I bought 2 7.2v 3800mAh NiMH batteries and on my rough calculations each one should last around 2 hours (the driver is 800mA on max).

    However, after only around 30 minutes, the driver starts switching to lower modes and the battery voltage (under load) sinks to below 3V. As soon as the load is disconnected, the voltage of the batteries is 5.6V and after an hour or so it recovers to just over 7.2v.

    I'm draining the batteries at something between 1.5A and 1.8A which is around 0.5C or less. Is this too much to expect from a NiMH battery? If so, should I just switch to a lithium set up?

    Cheers!!

  2. #2
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    "7.2V 3800mAh" - I assume these are sub C cells? Sub C cells are usually built for the power tool market and easily handle 10C discharge rates (38 amps), so 1.5 to 1.8 amps is a cakewalk.

    5.6V for a 6 cell NiMH after immediate disconnect is a totally discharged battery.

    Do you know how old the battery is? If it's very new, it may need 3 to 5 "conditioning" cycles to break it in. The best way to do this is with a slow charge of C/10 for 14-16 hours, followed by a normal discharge. Do this 3-5 times and see if the capacity improves. Some of the better NiCd/NiMH hobby chargers can perform the conditioning cycle automatically.

    What charger are you using? If it's a peak detect type charger, then a new battery may generate false peaks and prematurely terminate the charge. In the next charge, monitor how long it takes to charge up and calculate how much charge went into the cell. A correctly functioning smart charger will terminate when about 110% to 150% of charge has gone in, when starting from a flat battery (it's greater than 100% because charging is not a 100% efficient process).

    If it's an old battery (>2 years), then it's probably past it's useful life.

  3. #3
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    I suspect the batteries are 4/3A 3800mah nimh cells. Where did you get the batteries? A lot of places, batteryspace.com is a good example, sell really poor quality nimh cells. They also sell really poor quality chargers for nimh packs.

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    They're off ebay - looks like I might have made a mistake.

    However, the cells are brand new so perhaps they do just need 'breaking in'.

    There may be a chance that the cells weren't fully charged, but we'll see (I've only got a dumb charger) so I've left them in for around 10 hours and I'm now retesting...

    Cheers!!

  5. #5
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    Radio Shack

    They have 9.6V and 7.8v RC Car battery packs and chargers. $19.99 for one setup, so get 2 and they have a warranty. Mine have worked great. Each pack gives me 2.5 hours with 2 Cree XRE at 1000ma in series. The 7.8 is a back-up as it won't give the full forward voltage needed for max output. Old mistake, but how else do you learn?

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    Seems to be getting better. Last night I got 30 minutes on high and another 70 minutes on medium out of the battery before the voltage went into freefall... Will recharge and see if it gets even better.

    I had to turn from high to medium after 30 minutes as it seemed to be getting a little too warm (indoor test, no airflow).

  7. #7
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    Quote Originally Posted by jonboy001
    Seems to be getting better. Last night I got 30 minutes on high and another 70 minutes on medium out of the battery before the voltage went into freefall... Will recharge and see if it gets even better.

    I had to turn from high to medium after 30 minutes as it seemed to be getting a little too warm (indoor test, no airflow).
    You shouldn't run NiMH completely flat, they don't really like it! For a single cell generally 1V is a healthy place to stop... I think your pack is 6 cells so time to turn the lights out at 6V.
    DIY LED Bike Lights:
    A few Dynamo builds and some Small battery lights

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    Quote Originally Posted by jonboy001
    There may be a chance that the cells weren't fully charged, but we'll see (I've only got a dumb charger) so I've left them in for around 10 hours and I'm now retesting...
    What's the current rating of the charger? Have you verified the charge current with an amp meter?

    NiMH (and NiCd) start warming up noticeably when they're towards the end of a charge cycle. If the battery isn't at least a litttle bit warm, then it's probably not fully charged yet. Side note: many embedded Ni-xx chargers use a temp sensor to shut down the charge, in addition to the -delta V method.

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    It's a really basic charger that says its output is 100-220mA... So yes it will take a while to charge a single 3800mAh pack...

    I actually took it out for a ride a few nights ago, but using two packs in parallel. All seemed to work perfectly. On my next ride, I'll probably just plug one of the batteries in and see how long it lasts.

    The temperature sensor LED I have on it seems to work well - although it never lit up on my ride. Airflow REALLY helps heat transfer...

    I might do a bench test for duration in front of a fan when I've got a few hours to spare.

    On full, the light is way to bright to ride on the road, and I need to dim it to its lowest setting (sometimes using strobe for where there is lots of traffic and I get worried about not being seen).

    On high, however, it's really good where there is no traffic and no street lighting. Only downside became evident when I came around a sharp corner and found a herd of deer in my path. I've told my wife I'm building a helmet light next - she wasn't impressed....

    I'm thinking of a quad XPE with some of the 10mm optics....

  10. #10
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    Quote Originally Posted by jonboy001
    It's a really basic charger that says its output is 100-220mA... So yes it will take a while to charge a single 3800mAh pack...
    Charging nimh batteries is a fairly inefficient process. So using standard charging efficiencies a 100-220ma charge rate means it'll take 53-24 hours to fully charge a pack.

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