I successfully charged the batteries, one at a time, using the innard from an old APC UPS. I’m not sure how long it took. After two hours, one battery was only up to 12.5 volts (from about 12.2). So I left it overnight. In the morning the DMM read 13 volts even. After letting the battery sit for a few hours it was down to about 12.9 volts, where it has been ever since. I left the other battery charging overnight and it is at 12.9 volts now too. Next time I will try wiring the batteries in parallel and charging them. The car battery charger I used at Ben’s charged both batteries in a couple of hours. But it wasn’t free…
I still need to make a trip to Halted to get a couple of ammeters and a cheap DMM to measure the battery state and the charging and motor currents.