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Charger for Li - can use with NiMh?

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Charger for Li - can use with NiMh?

Old 11-17-10, 03:07 PM
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Charger for Li - can use with NiMh?

I have a 36V charger for a defunct LiPo battery. I was wondering if I can use it to charge a 36V NiMh battery. Any thoughts? Thanks.
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Old 11-17-10, 04:28 PM
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A 36v lipo charger (NOT lifepo4) puts out 41.35 to 41.5v so if your nimh pack is Balanced AND when fully charged, it is at or above that voltage, then it might work as long as you don't drain the nimh pack to low...
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Old 11-17-10, 04:57 PM
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A LiPo charger that puts out an appropriate voltage would indeed charge your NiMH cells ... but it wouldn't know how to turn off properly and probably would cook your cells if you didn't remove them when they were full.

If the voltage wasn't appropriate, it probably would refuse to charge your NiMH cells, thinking that either 1) they were already charged, 2) they're not the right batteries for this charger or 3) they're damaged somehow as the voltage is too low.

In short -- it might work under certain conditions, but it's a recipe for ruined batteries.

A better bet would be to try to trade the LiPo charger to somebody who wants to upgrade to a LiPo battery pack.
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Old 11-17-10, 05:06 PM
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NiMH chargers generally have a thermistor based temperature sensor embedded in the pack, and the temperature rises rapidly at the end of the charge cycle - at which point the thermistor signals charge termination to the charger - charging them any further could result in cells exploding...
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Old 11-17-10, 05:33 PM
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Originally Posted by chvid
NiMH chargers generally have a thermistor based temperature sensor embedded in the pack, and the temperature rises rapidly at the end of the charge cycle - at which point the thermistor signals charge termination to the charger - charging them any further could result in cells exploding...
If there is a thermister there, it's just a safety precaution. In general, NiMH and NiCd chargers detect that the battery is fully charged by looking for a small decrease in the charging voltage.
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Old 11-17-10, 05:53 PM
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Good point...I guess the charger can "see" the voltage drop that signals the end of charge...so I suppose if the charger could continue to pump charge into a battery that was already at the correct termination point if it couldn't sense the drop...anyways, I'm glad there is additional protection on there.
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Old 11-17-10, 06:25 PM
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My lipo charger charges until 42v and then stops altogether, cuts voltage and amperage. (all tested with a meter accurate to .0001a). I'm running that on a 10ah 36v lifepo4 pack. The pack is balanced and gives about 9ah until LVC. When the pack gets 8ah or less, I know the cells are unbalanced and then I charge them individually to 3.6 each and start the process over again.
This usually happens about once a month. (it's actually on my sons bike)
Every charger, charges at different rates and diff voltages and different cutoff values.
If you can test out yours, (which you can't currently) you would be able to use it, if small cautions were followed and the nimh pack was the correct voltage (within reason)
The lipo charger, charges to 3.5 per cell on the lifepo4 cells so that's within .1v on the safe/low end going .1v over, I wouldn't do it.
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Old 11-17-10, 07:14 PM
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Originally Posted by chvid
Good point...I guess the charger can "see" the voltage drop that signals the end of charge...so I suppose if the charger could continue to pump charge into a battery that was already at the correct termination point if it couldn't sense the drop...anyways, I'm glad there is additional protection on there.
The drop will be there. The battery is never *completely* charged unless it's being charged right now -- so if you take a fully charged battery, it's really just 99.99% charged or whatever -- you hook it up, and it charges for a few seconds, sees the dip and stops.

The smarter chargers will stop charging periodically for a short period of time, to give it another chance to peak, and if they're matched to a specific battery pack they'll often have a limit on how much total current they'll put into the battery pack or how long they'll charge. They've had many years to work all this stuff out, and it works.

NiMH and NiCd batteries tolerate overcharging, where LiPos do not. If they're overcharged at a low rate, no damage will happen, though if it's done for weeks at a time you'll see voltage depression (people often call this memory, but it's not.) If it's done at a high rate, the excess energy is released as heat -- so for a few minutes it's fine, but if done for hours it'll damage the battery.
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