[NJARC] safely weld/solder Lithium battery cells?

[Pete O]

AC hammer drills aren't expensive.

      From: Matt Reynolds <mattr04 at hotmail.com>
 To: NJARC Reflector <njarc at mailman.qth.net> 
 Sent: Wednesday, July 27, 2016 9:41 AM
 Subject: [NJARC] safely weld/solder Lithium battery cells?
   
 <!--#yiv5462599110 P {margin-top:0;margin-bottom:0;}-->All,
I took a gamble at a flea market two weeks ago and bought a contractor grade Milwaukee 1/2" battery hammer drill.  It was in the case with two 28V (yes, 28, not 18) Lithium Ion Batteries and a charger, for 30 bucks.  The seller said the batteries were good, but didn't last long.
Turns out, they are so dead they've gone dormant and won't charge.  I'm going to bench test the drill with a bench power supply to see if the drill at least works.  If it does, I will work on making it cordless again. 
Unfortunately, I've found that these batteries are about 100 bucks each (!) and it turns out (per Batteries Plus\Batteries & Bulbs) it's illegal to commercially rebuild Lithium Ion Battery packs in NJ.
I've been investigating what my options are, and I'm considering rebuilding the packs myself, but from what I've read, Lithium cells are very heat sensitive and it's very dangerous to rebuild them via soldering.  Evidently they are generally spot welded together by the tabs at the factories.
I have at home what I was told is a "thermocouple wire welder" which I believe uses an arc (presumably like discharging large caps) to weld metal together.  Is this a "safe" way to weld lithium cells?  Can you discharge caps into lithium cells to weld without harming them?
I would be fine with converting the cells to Ni-Cd, but I'm sure the protection circuitry of the pack won't permit that.  The charger that came with the drill can charge 18v Ni-Cd or 28V lithium, but I'm sure the pack circuitry is different.  I might be better served hunting down one of those 18v packs and rebuilding one of those instead.
I just thought I'd shoot this out on the reflector in case anybody else has had experience with this.  The packs have 7 3.7V "moli cells".
Regards,
Matt

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