I am wanting to save a ton of $$ by rebuilding battery packs for my rechargeable drills and other tools. You can get the new battery cells easily, and most come with tabs installed, but you must solder them to other cells to make the circuit. I am worried about the heat factor on the surfaces of the cells. I am also definitely a novice when it comes to soldering, so any advice is helpful.

They are not soldered together, they are spot welded. To get enough heat into the cell to flow solder onto it, you would probably damage the cell.

If, you had cells with tabs on both ends, you could probably solder the tabs together.

I do mine with a spark welder, based on an ultracapacitor.

There are two types of devices that are commonly called “ultracapacitors”.

One type has an internal resistance of ~10 ohms (I have a stack on my desk now that measure out at 10) and are used for memory backup and such. You can get these in lots quite cheaply on eBay, but they won’t work for you application. I call these “supercaps” to avoid confusion

The other kind is commonly used for high power car audio, but be aware that a lot of those are marketed to the “more dollars than sense” crowd – I’ve seen units that use supercaps instead, but add gold plating and digital readouts until they like like a Sci-Fi- fusion grenade, but seem to use a mere supercap inside!

One could easily pay $100 or more at an audio shop, but Google Shopping lists 1-2 Farad capacitors for ~$20. Don’t be swayed by fancy readouts and features on your first unit – you’ll probably have to remove them anyway until you gain some familiarity. Some units will work fine with them attached, but most (esp at the low end) will slow you down: they expect audio range power draws (under 100khz = more than 10 msec) while you ill want a spark in the microsecond range or less.

Another alternative is to use Computer Grade electrolytics – I mean the kind that are used in mainframes. These are bigger and tend to be more expensive, even on eBay, and you may have to wire several in parallel to reach ~1 farad (each connection is a potential point of failure at high current), but I’ve some people recommend them for a first unit, and they often come in higher voltages. Voltage is more important than the actual capacitance, because the formula for stored energy is CV²/2 – so a 10% increase in voltage is better than a 20% increase in capacitance. I’ve seen “computer grade cap” spot welders that used as little as 250.000 or 400,000 μF (0.25-0.4 F) total capacitance (but more is better)

While you will later get fancier, with double-spark systems and adjustable timing, etc, your first sparkwelder won’t need much at all. I used:

1) a 20 VDC power supply (more amps mean faster the rechrage, but that doesn’t affect the actual welding), or whatever voltage your capacitors will take. A charged car battery will work – if you can safely work with them
2) 2 short copper bars or segments from an old 8 gauge battery cable
3) a few inches of spare Romex (household wiring) for my welding tips
4) (optional) a drill press attachment for a hand drill or Dremel
5) any SPST or DPDT switch to take the PS out of the circuit when you are actually welding/. Most PSs don’t like to be shorted out, esp. repeatedly
6) a digital voltmeter is nice

Firmly clamp a buss to each terminal of the ultracap, then attach the other end of each bus to one of the wires in the romex pair. Strip about 1/2” of the far end of the romex, and bend the exposed tops to your desired weld spacing (about 1/4” apart) and square with one another – the spark won’t fly until both ends make contact. You can sharpen the ends to concentrate the current.Jigging it to the drill press makes repeated operation easier. Set things up stable – you don’t any accidental shorts. (This can leave an ugly mark on your watchband, for example)

As a test, connect the PS anywhere on the two power busses, from tip to capacitor (observe polarity!), measure the voltage (or just wait 10-60 seconds, depending on the amperage of your PS), then disconnect the PS. The spot-welder is now charged for a single “weld” jus tpress the two tips against a dead battery, and you should get a spotweld (or close to one) instantly on contact. One nice thing about using romex (aside from the ready made insulation and holding the pair of tips together for you) is that copper rarely sticks to the weld.

There are many easy refinements that make this device more practical for repeat use, but I’ll let you figure out what you want/need. Keep your connections short and the busses thick and solidly connected: you want ALL the resistance at the point of contact with the nickel solder tab so all the heat is generated there. A poor connection could rob you of much of your power

As you get fancier, you may controlling the timing, duration and amplitude of the pulse, or using a double spark (the smaller first spark blasts away surface contaminants, etc), but it will work as described.

Though the voltage is low and hence fairly safe, flying sparks can be dangerous, so wear goggles and gloves – and I am not responsible for any crazy way you may think of yo injure yourself. This is just an outline, not full instructions

The battery doesn’t have time to heat up and the current flows from copper tip to nickel strip to the other copper tip without entering the battery at all, but still, practice on some dead alkaline cells before you start soldering nicads

Sorry. New to the board. Don’t know what I did to cause the strikethroughs, but they weren’t intended

fixing it now :)

We use http://hobix.com/textile/quick.html

to format posts. Apparently double dash makes a strike through…

thx for contributing.

I have always just soldered tabs onto my battery packs. The soldered tabs can carry more current and introduce less resistance. I do agree that there is the potential for damaging the cells if too much heat is applied for too long, but I’m willing to take the risk. (plus I haven’t had a chance to make a spot-welder, although that does sound like fun)

I’ve had success with making 10 and 12 sub-C cell packs with a 45-watt soldering iron and copper tabs cut from a copper sheet. The packs are very rigid as compared to the flimsy spot welded packs.

**The trick here is to use a really hot and powerful iron to heat the solder joint quickly and then let it cool. Using a smaller iron will take forever and you’ll probably damage the cells in the process.

Orpheus,

Thanks for the detailed response. I built one of the capacitive discharge spot welders you mention from plans off the Internet that included a Thyristor that “fires” the welder. I built it for exactly what you mention – to rebuild cordless tool battery packs. I used 2400 maH batteries to replace the stock 1800 maH (?) DeWalt cells in an 18V tool battery pack.

Well, I have had a couple problems I would like to ask your advice on:

1. I used .25” by 5 mil nickel strips to connect the new cells together. I only put one strip between each cell, because that was all that would fit on the positive cell “button” (although the original had two parallel strips between each cell. Anyway, when I load tested the new pack, by stalling out the drill on a large screw, the strips got so hot they changed color and melted the case plastic slightly where it touched the strips. Do you think this is just a combination of the higher capacity cells and using just one strip per cell connection?

2) DeWalt has a plastic cap that goes on the top cell and holds the battery contacts that plug into the tools or charger. The contacts are brass and the negative one is spot welded to the top of the first cell. Well, even though I carefully ground off the old brass connection trying to save a little to spot weld to the new cell, there was no way my spot welder would weld that thicker brass to the new cell. I ended up welding a long nickle strip on the cell and then just bending the nickle strip over the old brass prong that connects into the tools or charger. Thisd works – but I’m afraid it won’t hold up to numerous connection/disconnection sequences AND it is only ONE strip carrying all of the current. Any ideas on how to handle this?

Thanks,

Rick