Switching Lead-Acid battery charger
I think I've got a dead simple solution; just pwm charge. I've wired a high-side switch up to the battery from a 13.8v transformer, and by pwming the gate at low frequency I can get pretty reasonable average current control. I don't need the charger to be too fancy, so this'll be good enough.

It also injects 8mhz of high-frequency noise into the battery leads, at high current, so FCC testing this is going to be really fun.