The AM Forum

I am getting close to starting a new project. I have several tools and other devices that need new, rechargeable batteries. In the past I have taken battery packs to a battery warehouse neighborhood store and had them rebuilt. At first (in the 90's) the rebuilds came out close to, if not better than the original packs but quality has gone way down over the years and now they won't even try. What bugs me is high quality cells ARE available for all of my needs but one thing holds me back.

Anybody that has opened a rechargeable battery pack has probably seen that the cells are connected to each other by metal strips that are spot welded to the cells. I need a small spot welder to weld the nickle alloy strips to the batteries.

It shouldn't be very hard to put something like this together. I'm thinking of charging a capacitor bank and using 2 copper electrodes to do the welding. I would think a variable DC output of, say, 60-150 volts  at maybe an amp or less might do it. Use two copper leads to keep the positive and negative close together so the jolt won't impact the cell.

I am hoping maybe somebody here has some experience with small part spot welding. Where would you start to build something like this? Maybe an analog dc supply, Transformer, rectifier, capacitor bank and trigger or maybe a switching supply?

Lots of testing would go first. Those replacement cells can cost a bunch. The neat thing is you can find nickel/metal,hydride replacement cells for the older Ni/cad cells that are the same size, and voltage but have up to 4 times the storage capacity.

Mike

I think you want very very low voltage and high current, not 60v+ as a discharge...

I'd consider maybe an old linear 5v supply, one that was like 100amps, and
maybe it will have a CT or even better a 110/220v primary, so you can drop
it down even lower...

Funny you posted this because for absolutely no reason whatsoever was thinking about this EXACT thing earlier tonight!! What the heck was that??

                           _-_-bear

I've welded thermocouple leads with a 65,000uf cap charged to 30 volts. I used a thick drafting pencil lead as a carbon rod to limit current. works great.

I think you're a bit high on the voltage, Mike. My little 120V powered wire feed machine runs at 18V for the arc under load and only 28V OCV.

I'm no spot welding expert, but http://www.powerstream.com/ (http://www.powerstream.com/) makes some capacitive discharge spot welders and one of the apps is "battery tab" welding. Unfortunately, I didn't see a voltage listed, but you might be able to get some ideas from the site.

Good luck,
ldb

Use some serious caution with homebrew spot welding solutions for batteries.  A sudden increase in cell pressure is not going to have a good outcome and with some types of cell construction a lot of heat poorly applied could cause an internal short with a rapid increase in cell temperature/pressure.

And on another safety note for your viewing pleasure is a Darwin award attempt from my hometown of Gulfport MS: 

http://www.sunherald.com/2012/01/19/3698410/explosion-injures-man-military.html

While I understand the desire to rebuild, reuse etc.  the current tools are so much lighter, longer lasting etc. perhaps its time to buy new. (just a thought)

Eric

Good points all.

Frank, I have worked with industrial piping installation for close to 30 years now. High chrome/molly alloy materials require pre and post weld heat treatment. This is done with resistance mats wrapped around the pipe but first a series of thermocouples are spot welded to the pipe to carefully control the temperature. The spot welders used are small and crude but very effective. I wish I had one.

Larry, Thanks for the link and your input. I work with welders and will see if I can find one that has some first hand information I can trust. I like the spot welders offered on that site but $3100 would buy all of the packs I would ever need.

Roger, If I go through with this I will test it on metal samples and not batteries until I'm relatively sure I'm safe. It wouldn't hurt to take some common sense precautions when it comes time to weld a battery like a pair of welding gloves, face shield and Nomex. A fire extinguisher close by would be smart as well. All of which I have. I'm not going to try this with Lithium ion cells. A Li/ion cell can become a self sustaining fire bomb. When the tabs are welded the electrodes are very close together and the pulse is extremely fast so the heat effected zone and conductive areas are kept to a minimum.

Eric, I love to fix stuff and I absolutely hate to toss out a tool that has served me well just because it's old. Just one example of what I want to fix is my collection of De Walt 14.4 and 18 volt packs. I have 5 to fix and I want the best cells I can find so I won't have to do it again for a long time. I had two of the 14.4's rebuilt a few years ago and they didn't last even half as long as the originals. Doing it myself I won't have anyone to blame but myself.

I know how to care for Ni/Cad's and do my best to avoid memory problems and never store a discharged battery. Upgrading to Nickle Metal Hydride I hope to add to the life span and end the memory pitfalls.

I have a few filament transformers here to choose from and a 120/240 veriac. I should be able to dig out some suitable rectifiers and an assortment of caps to try. What should I use as a switch? It would have to be fast and able to handle a high current load. Maybe a large schottky diode or SCR?

Mike
 

Hang on.

I have rebuilt battery packs using copper strap and solder. One has to file off the nickel plating on the battery surfaces first, I pre-tin both sides then solder. Works fine so far.

Best to use a solder with good wetting... a highly activated core is probably better...

They make an extra low temp solder, if ur worried at all about the temp.
Often used for SMT servicing soldering/desoldering

I have done this with NiCad, NimH, and AAA lead-acid cells.

Perhaps no need to weld at all??

                               _-_-bear

I have also slobbered them together for many years without ever having a failure.
Pre-clean well, use slobber with good rosin, pre-tin, and most important of all................. USE AN EXTREEMELY HOT, failly high capacity iron.

the high temp iron is so important so you can "hit and run" quickly without transferring much heat to the cell. Been doing it for years and never had a problem yet. I've done tool battery packs, HT battery packs and a few others.

second the hit and run technique. used to do it at the TV shop!

Years ago I was a field service tech and one of my larger customers was a large local horsepistol. The electrical shop where I turned in my paperwork was next door to biomedical engineering (where they fix all of the fancy schmancy portable stuff)

Their NI-CAD recycling can was right out in the hallway, and one day I asked them if I could "steal" a few pulls out of the can. they told me to help myself as they had to pay to dispose of them, and I was doing them a favor by taking them. The medical pulls were changed strictly by time intervals and usually not very old. I used to have a free lifetime supply of nicad cells and packs.

Needless to say, I rebuilt every nicad pack I had. Why not, I got the cells for nothing. Sometimes I miss working out in the "field".....................

I tried to solder my own together. The heat opened the negative side of the battery, and it wasn't really excessive heat. I am guessing that the heat drew the metal cap away from the chemical and it never went back. I wasted two good batteries.