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Charging NiMH battery pack

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Posted by Jim Wilkins on June 17, 2021, 11:52 am
 
I bought a $5 pair of replacement battery packs for my 14.4V DeWalt drill  
that contain NiMH instead of NiCad cells like the original. Amazon reviews  
claim the original DW9118 charger handles them without problems, but having  
been a battery test tech I wanted to know more.

DeWalt says their chargers need a true sine input so for remote job site  
solar+battery use I bought a 300W Bestek inverter which gets decent reviews.  
Mine shows a nice 113V sine wave on a scope and cuts off at 350W. Some users  
wrote that a modified sine inverter blew their charger's fuse or worse. The  
AC input feeds a capacitor rather than a transformer.

I recorded the voltage and current while charging the old NiCad and found  
that the charger ignores the small negative steps as each cell tops off and  
begins generating oxygen, instead it cuts off the current once a minute and  
measures the battery voltage. Charging ends when the zero-current voltage  
reaches 17.0V, or ~1.42V per cell. The charging current of 1.3A raises the  
voltage almost to 18V before the individual full-charge cell drops begin,  
ending at 17.65V. When the NiCad pack was new (or new-old-stock) it measured  
17.13V at full charge.

Internet sources suggest without firmly stating that constant-voltage  
charging to 1.4V~1.45V per cell is acceptable, though the last part of the  
charge is slow.

So does anyone have hands-on experience with replacement drill battery packs  
that use NiMH cells instead of the original NiCads?


Posted by Clifford Heath on June 17, 2021, 11:35 pm
 
On 17/6/21 9:52 pm, Jim Wilkins wrote:

I can't answer your main question, but I avoid NiMH because they go flat  
too quickly when not used, and after three months cannot be charged  
again. They're hella expensive to replace usually as well.


CH

Posted by Jim Wilkins on June 18, 2021, 12:05 pm
 "Clifford Heath"  wrote in message  

On 17/6/21 9:52 pm, Jim Wilkins wrote:

I can't answer your main question, but I avoid NiMH because they go flat
too quickly when not used, and after three months cannot be charged
again. They're hella expensive to replace usually as well.
CH

------------------

Many brands of these non-OEM NiMH packs had lots of very negative reviews. I  
chose a more expensive version with mostly good reviews.

For years I've let NiCads in tools I rarely used self-discharge flat and  
then brought them up when needed with a lab-type power supply until the  
charger would accept them. In contrast I try to top off my lead-acids  
monthly and my lithiums once or twice a year.

https://en.wikipedia.org/wiki/Nickel%E2%80%93cadmium_battery  
"In fact, Ni–Cd batteries in long-term storage are typically stored fully  
discharged."


Posted by David Billington on June 17, 2021, 11:55 pm
 On 17/06/2021 12:52, Jim Wilkins wrote:

I can't help with your query but last year I converted my 14.4V Bosch  
NiCd battery pack to Lithium. I found 4 18650 Li cells was close at  
14.8V and fitted beautifully in the Bosch pack, 3 fore aft, and 1  
across, these were tabbed cells so I made a simple PCB to couple them  
and hold them together, on top of the PCB I mounted a supervisory board  
for over charge, over discharge sensing and wired to the standard  
battery pack contacts. For the charger I used a 4 cell Lithium charger  
wall wart and gutted the original NiCd charger to just leave the bare  
PCB and charging contacts with leads to those and a socket to connect to  
the Li charger plug with its lead going into the original charger in  
place of the mains lead. I've had no problems and it has revived a  
useful tool and the new pack is 3.6Ah as opposed to the original 1.2Ah.


Posted by Jim Wilkins on June 18, 2021, 11:07 am
 
On 17/06/2021 12:52, Jim Wilkins wrote:

I can't help with your query but last year I converted my 14.4V Bosch
NiCd battery pack to Lithium. I found 4 18650 Li cells was close at
14.8V and fitted beautifully in the Bosch pack, 3 fore aft, and 1
across, these were tabbed cells so I made a simple PCB to couple them
and hold them together, on top of the PCB I mounted a supervisory board
for over charge, over discharge sensing and wired to the standard
battery pack contacts. For the charger I used a 4 cell Lithium charger
wall wart and gutted the original NiCd charger to just leave the bare
PCB and charging contacts with leads to those and a socket to connect to
the Li charger plug with its lead going into the original charger in
place of the mains lead. I've had no problems and it has revived a
useful tool and the new pack is 3.6Ah as opposed to the original 1.2Ah.

Thanks. Were you able to determine the battery discharge rating and the  
tool's current draw?

The tabbed 18650s I've collected are in outdated cell phone chargers, so  
they may not have a high enough discharge current capacity, maybe only 2C. I  
bought them to refurbish old laptop batteries and since a few had dropped  
slightly below 3V and self-disabled, I recharged them directly with a lab  
power supply, as I learned to do at Segway. Unlike a smart charger, a  
current-limited power supply will try to recover a fully discharged battery.

I suppose I could measure the drill's stall current with a 20A power supply  
or 12V battery and the 18650's max output (outdoors) with my 1 Ohm 1000W  
rheostat.

I stopped testing the NiMH in the DeWalt charger when the voltage rose above  
19V. They came at 16.5V and charged reasonably quickly at 17~18V 0.5A from a  
power supply, to the endpoint of 0A drawn at 17V. Anderson PP45s fit  
directly onto the battery contacts and I wired the charger contacts to  
external PP45s.
https://www.powerstream.com/NiMH.htm  


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