Re: NiCd and NiMH charging (and Dewalt)



James Beck wrote:
Vladimir Vassilevsky wrote:
Tim Wescott wrote:
On Tue, 19 Feb 2008 13:08:51 -0800, Neil Kalo wrote:

Is there much difference in a circuit designed to charge NiMH versus
NiCD batteries? How does the circuit tell what is attached? In
particular Dewalt brought out a completely new range of NiMH/NiCd
chargers when they introduced NiMH batteries. Was there any real need to
do this?

Yes, they did. NiMH batteries are more touchy about charging than NiCd batteries. If you charge a NiCd battery using the optimal NiMH procedure you'll get a slightly less than optimal charge; do it the other way around and you'll get a damaged battery and possibly a fire.

I think this is an urban legend.

BTW, the Lacrosse BC700 battery charger does not distinguish NiCd and NiMH. It is a very good charger indeed.

http://www.discovergadgets.com/product.asp?itmky=492702

It may be an urban legend, but it was splattered all over the design rags, with endorsement and encouragement from the NiMH battery manufacturers, when NiMH batteries were first coming out.

As I stated in my post, there's no reason that you couldn't make a battery charger that would do fine with both types, but a peak charger for a NiCd wouldn't necessarily work well for a NiMH. It looks like Lacrosse did just that.

Agreed. Also, the peak charger for NiMH wouldn't necessarily work well for NiCd.

I don't have a gross of each type of batteries, or the will, to pull out an old NiCd charger and prove it one way or another.

Well, I can tell you from experience that (depending on how aggressive the charger is) setting a universal quick charger to NiCD and plugging in NiMH can sure pop them.

The results can be the opposite: setting the universal 1 hour charger to NiCd and plugging in the NiMH results in the ~50% charging of NiMH. The charger is Duracell CEF80NC. Perhaps it has the different limits on the max. charge amp hours for NiMH and NiCd.

There is also considerable variation in the charge/discharge curves between the different makes of the MH batteries and even from one cell to another if both are from the same make.

I have a charger that you can select the delta V in 4 different ranges. 2 for NiCD and 2 for NiMH. I had it set for the wrong cell and popped off an 8 cell 2700mAH A size NiMH pack. I think what really happened was the cells got hot enough to destroy the individual cell covering and the cells just shorted together and off it went, but that's just my best guess because the unit was destroyed and there were flying cell cans going everywhere.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
.

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