70 batteries in series, with a relay between each ten, around 900 volts because they were Optima Redtops which fully charged are 13.1 volts. We had five of those that we put in parallel, so we could dump each of the five into a single load, which was
this launcher.
The load was an ESR of about an ohm so each bank dumped about a kiloamp into the load, five kiloamps through the load in total. Each of the five banks was switched into the load with an IGBT, and the output of each bank had a big flyback diode in series to keep the banks from trying to dump into each other if their output voltages didn't match. We tried to keep the banks charged with a 1000 volt power supply, which taught me some very hard lessons about series charging multiple batteries, which is to say, you can't do it! Maybe if you had two or three batteries which were all in an exact same condition of charge and sulfation, but 70 batteries? No way, it can't be done. You get one or two bad apples in there and they dump waaaaaay more than their share of the voltage, now you're cooking the sons of bitches so I would go up and down the lines with a battery tester and if one had more than 16 volts across it I would slap that thing on there and suck a few hundred amps from it for four or five seconds to knock it back in line, it was a crazy ass game of whack a mole and though it worked obviously it was completely impractical as a long term maintenance solution.
I also built up a kick ass desulfator that would dump 50 amp pulses at a five percent duty cycle at a thousand hertz, I found I could recover fully discharged Redtops, which is to say, Redtops that were down below a volt, in about 24 hours with that thing. The problem with commercial desufators is they're too wimpy, you've got to hit a sick battery hard if you want to recover it in less than a month...