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HP Forum Archive 14

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N-Cell NiMH Charger
Message #1 Posted by Darin Ingimarson on 30 Aug 2004, 10:12 a.m.


I've been looking around for a charger specifically for the GP 500mAH N-size cells and so far have had no luck. Note that I am not specifically concerned about the mechanics of stuffing an N cell into a AA charger, I am more concerned about the correct charging rate, fast-charge rate, and trickle charge maintenance to properly maintain these cells. Does anyone have any leads here?

Thanks, -darin

Re: N-Cell NiMH Charger
Message #2 Posted by Jon on 30 Aug 2004, 6:37 p.m.,
in response to message #1 by Darin Ingimarson


I have built a charger based on the max712 that works fantastic. You could charge all four cells at the same time, in less than one hour and safely. If interested I have all the schematics and instructions for it.



Re: N-Cell NiMH Charger
Message #3 Posted by Darin Ingmarson on 30 Aug 2004, 7:14 p.m.,
in response to message #2 by Jon

Thanks Jon, I also had put together a quickie design for a battery charger (I am an EE), based on the MAX713 -- I prefer the negative slope method. MAXIM has some great parts and decent white papers on how to do this.

Then I got thinking, hey, why don't I make it programmable with a PIC microcontroller, so I can charge other types of cells/packs, and monitor the progress, blah, blah, blah. So I expanded the design a somewhat to do this.

Then reality set in and I realized that there was very little chance of me getting the time to lay out the PCB and build it right now (maybe by Xmas). If I get it done I'll post the PCB pattern or offer pieces of the board run on the forum if anyone cares.

Even if I do build my charger, I'd prefer to use the NiMH batteries sooner than that, so I bailed and started looking for a commercial charger :(

Re: N-Cell NiMH Charger
Message #4 Posted by Tracy Vermeyen on 3 Sept 2004, 12:39 a.m.,
in response to message #1 by Darin Ingimarson

I have a HP manufacturered N-Cell Ni-Cad battery charger, I have never seen one like it. It is not a rechargeable battery pack. It was meant to receive rechargeable n-cell batteries. The charger is called a Plug-In Battery Charger (HP 92266A) and the output is 5.8V DC (15 mA). It says it is for NI-CD batteries only. I think it will work of NiMH, but I would like a forum member to confirm that.

Let me know if you need any more information about this device.

Re: N-Cell NiMH Charger
Message #5 Posted by Darin Ingimarson on 7 Sept 2004, 12:48 p.m.,
in response to message #4 by Tracy Vermeyen

Tracy, I am familiar with the unit (some sold recently on ebay). I can't tell you for sure if the charger would work on NiMH batteries, as I don't know what the charge rate or end-of charge mechanism is for that device.

Typically, there are several methods for determining when a NiMH battery is charged; you have time (at a given charge rate), dV/dt, negative dV/dt, temperature. Plus you have trickle-charge specs for long-term maintenence of your NiMH charge

The fast charge modes for NiMH and NiCd batteries are terminated under different parameters (NiMH terminates when dV/dt=0, and NiCd should be terminated when dV/dt goes negative). Note that if this is the method of charge termination used by your charger, then it could conceivably damage your NiMH batteries.

If your charger is time-based (i.e. you leave the batteries in it for x hours and then they are charged), you could theoretically measure the current of your device by putting a DC ammeter in series with a set of NiCd batteries and measure the charge current. This should allow you to charge NiMH simply by timing the charge operation (i.e. if your charge current is 50mA, you would charge a 500mAh battery for 10h). All standard disclaimers apply, if you cause a fire it is not my fault, etc. etc. ;-)

Even as an EE I would not tempt fate with my vintage HP N-cell charger by sticking batteries in it with a different chemistry that that for which the unit was originally designed...


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