NiCd and NiMH charging (and Dewalt)

[Neil Kalo]

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?

 

[Tim Wescott]

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.

--
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

 

[Vladimir Vassilevsky]

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


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

 

[Tim Wescott]

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

(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

 

[Fred Bloggs]

(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.

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.

 

[Fred Bloggs]

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?

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.

 

[Neil Kalo]

Is there such a thing as a charger advertised to fast charge bothNiCd
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):

http://www.dewalt.com/us/products/attachment_detail.asp?productID=1889

But on the UK site it says NiCD only!:

http://dewalt.co.uk/attachments/productdetails/catno/DE9117/

The UK DE9130 is a 30 min charger:

http://dewalt.co.uk/attachments/productdetails/catno/DE9130/

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?

 

[James Beck]

(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.

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

 

[Fred Bloggs]

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

http://www.dewalt.com/us/products/attachment_detail.asp?productID=1889

But on the UK site it says NiCD only!:

http://dewalt.co.uk/attachments/productdetails/catno/DE9117/

The UK DE9130 is a 30 min charger:

http://dewalt.co.uk/attachments/productdetails/catno/DE9130/

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?

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.

 

[Vladimir Vassilevsky]

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

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

 

[Fred Bloggs]

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.

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.

 

[Mike]

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.

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

http://www.ansmann.de/cms/products/charging-technology/fast-chargers.html

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.


--

 

[Fred Bloggs]

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

http://www.ansmann.de/cms/products/charging-technology/fast-chargers.html

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:
http://focus.ti.com/docs/prod/folders/print/bq2005.html
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.

 

[Neil Kalo]

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:

http://neilkalo.googlepages.com/batterycharging

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.

 

[Richard Henry]

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:

http://neilkalo.googlepages.com/batterycharging

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?

 

[Neil Kalo]

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