Can I use different NiMH capacity AAs together?

Discussion in 'General Electronics Chat' started by moeburn, Aug 17, 2013.

  1. moeburn

    Thread Starter Member

    Aug 16, 2013
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    Hi! Can I use two 2450mAh NiMH AA and one 2200mAh NiMH AA together in a circuit that calls for 3 AAs? I have a multicell charger, so I can charge them all back to 100% individually no problem.

    <csb>
    So I bought one of these premade DC-DC boost converters specifically made for charging high-drain USB devices like iPads and tablets and such. Here's the specs:

    Well it only supplied 200mA with two fully charged NiMH AAs (2.4-2.8v), but with 3 NiMH AAs, I get the maximum current my device can draw! (850mA). So obviously I need 3 for this to be a decent charger. Unfortunately, I don't have any matching sets of 3 AA NiMHs, because I always bought them in pairs. I have a matching set of 4 AAAs, but they are only 1000mAh, not really ideal for a mobile charger, despite the incredibly high efficiency rating (and its accurate, my batteries barely even got warm when charging at 850mA).

    So I'm aware that one will die before the other two. That's fine. But is it safe? Is it bad for the batteries, or the circuit? What about when that third battery dies, but the other two are still running, and the device tries to draw high current, will it damage the battery some how?

    Also, quick second question: The 3 batteries only got warm, but the boost converter IC got pretty darn hot. Finger started to hurt after pressing on the IC for about 10 seconds, which I think means ~50-60°c. Do you think I should put a heatsink on it? I dont have any special IC heatsinks, but I do have old copper CPU heatsinks and a dremel to cut them down, how would I go about attaching a piece of copper to it? Just thermal paste?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,220
    If one dies ahead of the others then it will be reverse charged until the other batteries go dead. Reverse charging is bad for battery life so it's preferable that you buy 3 or 4 identical batteries to use in series.

    You might try some thermal epoxy to attach the heatsink to the converter.
     
  3. moeburn

    Thread Starter Member

    Aug 16, 2013
    31
    0
    Thanks for the help guys!

    Yeah, I read about 'reverse charging' too. And since only one of them would die, while 2 would still be alive, that would mean that the one would be getting reverse charged with 3 volts, not 1.5 volts, which I think might kill it.

    Does anyone know at what point this would happen? I'm pretty sure NiMH don't just go from 1.1v to 0v instantly, so what voltage is considered 'dead'? What voltage would it start getting reverse charged by the other two? I ask because I could just only use 90% of the smaller battery's capacity, without letting it get reverse charged.

    But either way I'm probably going to get a new set of 4 anyway.

    Do you guys think I need a heat sink? I have no idea what the datasheet specs for the DC boost converter are, I couldn't find them by typing in the writing on the IC. But the ebay page lists some specs, and they don't say anything about requiring a heatsink. It is after all designed to produce 1.2A at 5v, I would think it would come with a heatsink attached if it couldn't.

    But I might just attach one anyway, if for no other reason than it makes it look cool :D I don't have any thermal epoxy lying around, but I do have thermal paste. I suppose I could just use thermal paste for the flat parts, and then go around the edges with some superglue or something.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Once NiMH batteries reach about 1V they rapidly go to 0V with further current drain. When the voltage reaches zero, the voltage will reverse with continued current flow and they will start to reverse charge.

    If you monitored the voltage of the three cells in series you could shut off the device when the voltage drops below 3V. That would insure that none had dropped to 0V. The worst-case would be with two cells still at 1.2V each, leaving the weakest cell at 0.6V.

    You probably don't need a heat sink, but it certainly won't hurt and should increase the operating life of the unit.
     
  5. moeburn

    Thread Starter Member

    Aug 16, 2013
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    0
    Alrighty, thanks for all the help! One more quick question; my local electronics parts store (Active Surplus) also sells these lithium ion packs that look like 3 cells wrapped in white tape; I assume these are the kind with their own protection circuits built in? How difficult are they to learn how to charge safely?
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    Lithium ion batteries generally require specialized charging circuits to avoid damaging the cells (or, worst-case, causing the batteries to burn or explode). They may or may not have protection built in. So you need to determine the particular charging requirements for the cells they are selling to provide a proper charging circuit.
     
  7. moeburn

    Thread Starter Member

    Aug 16, 2013
    31
    0
    Yeah, I'm not sure I'm ready for Li-ion yet, seeing as how I know someone who is a total battery expert, and knows all about CC and CV charging Li-ion (he's a flashlight aficionado), and yet still somehow one of his li-ion batteries exploded while in a sealed metal flashlight, sending large heavy metal shrapnel flying everywhere, strong enough to dent his metal fridge, and take chunks of his hand with it.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Boeing will probably hire him.....:D

    Li-Ion batteries can be very dangerous in enclosed spaces....
     
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