Battery chargers are not designed for such high instantaneous power.
You're looking at a switching supply similar to what you get in computers.
The difference here is that you'll need two stages, one to control the voltage and the other to control the current.
At such low voltages/high current you'll probably have to go with high frequency controllers ($$$), as you wouldn't be able to get the control from bulkier devices at low frequency.
If I'm not mistaken you can get these off the shelf.
Converting an AC welder might work with 2x 200 Amp half-bridge diodes, and a big fat cap, mounted on a big heatsink.
The problem is the instantaneous current of 100A might burn the junction of any power diode. You'll have to play with different rated power diodes.
Treating the caps badly like this would also cause them to prematurely pop, so it'll be an idea to have a bank of many capacitors.
Again the price of building this might be more expensive than off the shelf DC welder.