NiMH rechargeable battery pack

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I have a piece of test equipment that uses a battery pack that consists of 4 AA NiMH 2000ma batteries. The unit has a built in charger. The battery pack has a thermostat and what appears to be a thermistor that is strapped against the batteries. What I would like to do is replace the pack with a regular holder to accept 4 individual NiMH cells. So my question is: Would it cause any problems to transfer the thermostat to the new holder and use this way?

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Don't know but NiMH cells are finicky if you want to get a good life span out of them. Maxim has chips and application notes that explain it all. :( I did use a max chip some 10 years ago but can't remember anything about it.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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I would think not if the batteries are the same (ie, have similar heat transfer characteristics) and you place the thermistor in a similar location with similar mounting. The reading just has to be as accurate as it was before and the reading has to MEAN the same thing for the batteries. As long as you attempt to make the setup the same as before it should be ok. I've made simle NiMH chargers without temperature monitoring, but I don't know of a simple way of disabling it (ie, short it, open it, etc) if it causes problems after you transfer it. Good luck.

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I'll give it a try and see what happens. I'll keep an eye on things for the first few charges just to make sure nothing gets too hot and explodes on me :roll:

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Normally, the thermistor only comes into play at high charge rates. On LiIon cells, its around 0.8C or so. Below about 0.7C, the thermistor has little effect. I would expect similar behavior with LIMH.

So, unless you want the very fastest possible recharge time, it should not have much effect.

Jim

 

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Quote:
So, unless you want the very fastest possible recharge time, it should not have much effect.

Actually, it is not uncommon for temperature to be the primary indicator for charge termination for NiMH cells. The controller is looking for an increase in the rate of temperature rise which happens when the cells become fully charged. For NiMH cells, "rapid" charging is the normal mode. More sophisticated chargers also employ reverse delta-V detection in addition to temperature. I would just transfer the thermistor to the new battery pack as suggested above, and maybe wrap a layer of tape around the assembly so the thermistor can best sense the heating of the pack as it charges.

Tom Pappano
Tulsa, Oklahoma