Starting last night I drove the pack down until it was only around 200v. Then pulled it into the garage for discharge maintenance. I made up a system using six 2000w, 120vac water heater elements to discharge the pack in large groups rather than just two groups.

Here is the pack attached to all of the water heater elements. A 5 gallon bucket of water was used for cooling since an experiment with air cooling caused the elements to glow red, which is an ignition source. The water level dropped about 3/4″ when the pack was done discharging.

I used my cnc to machine 6 holes in these two pieces of 3/8″ thick plywood. They held the heater elements 3″ apart and 3″ from the sides of the plastic bucket.

After the overnight discharge was complete, I put some jumpers across about 5 cells that were still showing some voltage (not shown). Shown here is the 240vac variac and a bridge rectifier mounted to it’s heat sink. It takes me about 45 minutes raising the voltage by hand, to about 320vdc. After that, the on board charger is used to finish up the charging.

Here is the finished spot welder. I sure like the way the LCD display is shining through the smoked acrylic. When it’s off it’s hidden. With the foot switch jack is in the rear and the LCD behind the smoked acrylic, the front bezel is kept very clean. The knobs blend in nicely. This will be the smallest chassis this welder has even been built into. The tight confines did create some EMI issues however.

Here is the waveform across the fet source and drain with the probes held firmly against the work, making a good weld.

Notice the 40.8v spike on this waveform. It was caused by the probes not being held tightly against the work. The fets have a 75v internal diode that should protect them. But it has been recommended that a schottky board be installed per the note from the designer, to attenuate these spikes to help better protect the fets.

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