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Old Jan 28, 2004, 11:20 PM
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Australia, WA, Warwick
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Question
NiMH discharger circuit

Hi gang,
does anyone know of a circuit for building a discharger for NiMh cells.?
I'm looking for one that will cut out at the low voltage point (1 volt?) automatically.
Being a simple circuit, I'd like to put a bank of them together simultaneously to level out a 7 cell KAN 1050 pack.
Any ideas at all?
Phil
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Old Jan 29, 2004, 02:47 AM
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N. Staffs, UK
Joined Jan 1997
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I'm confused about what you're looking for here. For a start what exactly is it that you want to discharge and why ?

If you're intending to discharge individual cells you don't need to stop at 1V. It's only when they are in packs that you stop at 0.8V-1V PER CELL.

If you're discharging a pack you only need one discharger not a bank of them.

But if you just want to balance a 7-cell pack you don't really need to discharge it except by using it. Just slow (C/10) charge it every so often. That's all it takes.

Steve
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Old Jan 29, 2004, 03:31 AM
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Cheers Steve,
Sorry for the vagueness.
I had been told that to get all cells balanced, they need to individually be slowly discharged to around 1 volt & then charged slowly as you say @ C10 for the pack.
Seeing as I didn't want to bust up my 7 cell packs to do this & could not sit around & watch the voltage on each cell, I was looking for a device that could do it.
So just by using the battery pack & letting the charge run doen- it conditions it?
I have a suspicion that I have a dodgy cell in a pack as it behaves unusually.
I uausally run the packs down to BEC point whilst flying.
The packs are 7/KAN1050 & std 7/600mA 2/3AA cell packs
So what I'm doing should be fine?
Cheers - Phil
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Old Jan 29, 2004, 06:25 AM
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Yes you're o.k. It's the slow charge that does the little balancing that is needed.

BTW this is one time when it is actually a good idea to overcharge. So if you're charging at C/10 (100mA or 60mA for your batteries) leave it on charge for at least 15 hours.

BTW it's correctly written as C/10 (one tenth of C) not C10 (which would mean C times 10) .

Steve
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Old Feb 01, 2004, 02:06 PM
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Take a look at this circuit

Simple discharger,drops about ~1 volt per cell.
http://es.geocities.com/aerogallecs/
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Old Feb 01, 2004, 07:33 PM
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Thanks very much JMVC,
I'll give that a go!
Looks like it will do the job & very cheap to make
Cheers - Phil
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Old Feb 01, 2004, 08:44 PM
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Edgewood, NM
Joined Jan 2002
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Hmmm..

I have been thinking of making a discharger for my Ni-Cd's. Perhaps a good-size power resistor(s) and a MOSFET. Use a PIC to drive FET with PWM, and one could vary current to suit his needs. A voltage divider and an AD conversion channel of the same PIC could control cut-off point.


Hmmm....
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