|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...