The parts for the charger arrived a few days ago. I have let the magic smoke out of some of them, but I made some progress.
The diode I chose (STTH6004W) comes in a DO-247 package. While it is rated to 60A, the terminal legs are rather small, and did not last long when soldered to 14ga stranded wire.
I lost some IGBTs when I thought the varistor across would protect them. Or the fuses would help, but nope - IGBT goes bang!
However I can push 10A through a string of 4 batteries.
