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 | DIY Capacitive Discharge Battery Spot Welder |  | Ian Hooper, 6 January 2012 | |
Many batteries come in a cylindrical format with no inherent means of interconnection (such as screw terminals). Two common examples are the 18650 and 26650 sizes of lithium cell. The best way to connect such cells together permanently is to spot weld nickel strips between the terminals. (Soldering to battery terminals is not recommended due to the amount of heat that gets into the battery - potentially damaging the internal chemistry.)
A common method for doing this is with Capacitive Discharge (CD) spot welding, which basically involves dumping a pulse of energy stored in a capacitor through the nickel strip, causing localised melting of the nickel and welding it to the battery. (Critically, with battery spot welding the weld current must pass through two points on the same side of the battery terminal - not through the battery!)
Commercial CD welders typically cost several thousand dollars. But with a little ingenuity, you can build your own for a fraction of the cost. Here are a couple of examples of people who did it before me (and even more economically!)
Basically you just need a large bank of capacitors, a method of charging them up, and a method of discharging rapidly through heavy duty cabling and electrodes into the nickel strip. |
Example of a spot-welded battery pack
Close-up of spot welds
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With a quantity of K2 26650EV cells sitting in my workshop, orphans from a cancelled project, it seemed like a good idea to build myself a CD welder such that I could assembly these cells into packs suitable for use in my various electric vehicle projects.
Completed CD spot welder |
My completed CD spot welder is pictured to the left. A box for it might be nice, but the naked electronics has it's own charm too. I won't go into explicit detail about the design because everyone is likely to do things a little differently, but will review my own component choices.
The heart of the welder is a bank of six Maxwell Ultracapacitors , each rated to a whopping 650 Farads, at 2.7V. Running six in series results in 108 Farads at up to ~16V. These capacitors feature very low internal resistance, so can deliver very high discharge current. Based on research, I learned that a typical battery spot weld requires approximately 200 Joules of energy. With this figure you can work out how much voltage and capacitance you will need for each weld, based on the formula for energy stored in a capacitor:
E = 1/2 CV² |
Hence at 15V, I would need about 1.8 Farad per weld. Clearly 108F is much more than necessary, and unlike the other designs above I need to be able to start and stop my pulse (not discharge the entire capacitor energy in a single weld), but having such a large capacitance does allow for many welds in quick succession, if desired. (I got these capacitors cheap off a friend. If I had to buy new, I'd probably opt for something smaller.)
Right is a labelled diagram showing the layout and control board on top (click to view larger).
The pulse timer is based on an Atmel ATtiny13A microcontroller. The same function could also be done with something like a 555 timer, but I like the precision of a microcontroller, and it actually involves a lower total part count. This fires an IXYS IXDD4141PI gate driver, to switch a bank of MOSFETs.
I also installed a 100ohm bypass resistor to slowly discharge the capacitors. It's a bit of a waste of power while running, but avoids any surprises lest you come back a day later and the capacitors are still charged. |
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To switch the pulse current on and off, I'm using four IXYS IXFX120N20 MOSFETs, pictured left. They're actually part of an old motor controll assembly I had lying around. They are rated to 120A continuous or 480A peak current each, so blowing them up is fairly unlikely. I imagine it's overkill, but sometimes it's nice to have a bit of safety margin.
All the high power wiring is 8AWG, which has a resistance of around 2mohm per metre. Including electrode leads, I have about a metre worth in the welding circuit. The electrodes are made from solid copper rod with tapered ends. |
To charge the capacitors and power the logic circuit, I use a dual-output laboratory power supply. Separate power supplies for capacitors and logic circuit must be used because the ultracaps charge very slowly (about a minute to reach welding voltage), and the AVRs don't particularly like such a slow ramp up of supply voltage.
As can be seen from the pictures at the top, the welds are quite small but hold on tenaciously and should be good for about 10A per spot. A good way to verify weld penetration (on a test weld) is to try tearing off the nickel strip. It should leave part behind on the battery, and a hole in the nickel strip where the weld occurred.
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Nickel strip can be purchased from Powerstream or Sunstone Engineering. For those outside the United States, shipping is pretty expensive (if available at all). I'd be interested to hear from anyone who knows of a good source of nickel strip in Australia. And feel free to contact me if you have any requirement for battery pack assembly!
~Ian Hooper, 6 January 2012
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