NiCd and NiMH charging (and Dewalt)

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.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

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.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

(shrug)

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.

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

If it's for a tool then you're talking about fast chargers. The NiMH is more susceptible to lifetime reduction due to overcharge than the NiCd, and sophisticated algorithms using the terminal voltage inflexion point rather than deltaV/deltaT(ime) have been developed to overcome this. The terminal voltage inflexion is where the slope of the slope of voltage with time turns negative, this also requires a temperature rise qualification and an oversampling onboard DSP. An older NiCd charger won't do this, and will almost certainly overcharge the NiMHs, resulting in reduced lifetime, and enough of a reduction to justify using a charger specifically designed for MiMH.

Is there such a thing as a charger advertised to fast charge both NiCd and NiMH? I recently surveyed the literature to include manufacturers application notes, patents, and peer reviewed research literature on charging the NiMH, and no such animal was forthcoming in fast charge form factor...Link us to the Dewalt charger specs if you can.

The DW9117 charges NiCd and NiMH in 15 mins (allegedly):

But on the UK site it says NiCD only!:

The UK DE9130 is a 30 min charger:

I thought that all the Dewalt chargers used the inflexion point method. I know cyclists often modify Dewalt chargers to use with their lighting packs. In the UK all the NiCD only chargers are black and the NiMH capable ones are yellow. Even the NiCD chargers have a third contact for a temp sensor, but I've only seen NiMH packs with a matching contact.

How do they tell the difference in the chemistries in practice?

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. 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.

Jim

Okay- thanks for links, I'll check up on them. Now that you mention it, I do recall one or two papers on battery chemistry identification. That

15 minutes is fast as hell.

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

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.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

That's not even close-all the NiMH are at 90% capacity at inflexion which is well below the peak before negative dV/dt. You can't use absolute voltage thresholds at high charge rates above 1C or more, it's too variable, and well above the intrinsic 1.4V/cell.

As a standalone universal charger then Ansmann do quite a few.

How they detect each battery chemistry is not stated, but in practice they accurately charge with negligible cell heating.

Bosch power tool chargers are also capable of charging NiCd and NiMH I haven't taken one apart but as they have 5 or 6 terminals I suspect they use the additional contacts with sense components.

There are lots of products advertising this capability and they are inferior. This for example:

is an inferior methodolgy for fast charging NiMH despite what TI claims. I'm not sure about NiCd, but for NiMH there is a rock solid correlation between state of charge and terminal voltage inflection. If this is the case with NiCd also, then the charger can switch from fast to something like a 0.5C rate for a timed interval of 2x(1-p) hours, where p is percentage capacity at inflection, e.g. p=0.9 for NiMH makes for 12 minutes additional charge time and is acceptable for fast charge. The life reduction of NiMH is exponentially dependent on rate of charge in overcharge and 0.5C may put them in the negligible region. There could be other correlates such as magnitude of the temperature temperature gradient. Most of the cheap plug-in chargers are fixed time duration at 0.2C for 8 hours which is dead-on the 67% charge efficiency for this chemistry.

I did a quick and dirty test: discharging and charging an old Dewalt

12V NiCd battery pack. I would like to understand more what is going on in the graph:

During discharge the sudden dives are presumably individual cells giving up. Similarly during charge. I stopped the charge (at about

18:20) when the NiCd started getting hot. I was expecting an S-shaped "flat spot" near the end of the charge cycle, but don't really see it.

What was your charge and discharge rate?

Discharge was a 5W bulb, the current was actually about 315mA, charge was 2A. The pack is 2Ah