https://forum.arduino.cc/index.php?action=dlattach;topic=462485.0;attach=203963
I remember now why I didn't want to use power transistors. I'd need to filter the output side of the FET to get DC from the square wave that the switching would produce and the size of the cap I'd need would destroy the time constant. Charging and discharging would take so long that I wouldn't be able to get high frequency pulses when I desired them (which for some batteries is desirable) but have smooth DC when I wanted it (desirable for other batteries). The only way I could think to do it was to control the waveform in front of some isolated current source, such as a linear amp.
...and I'm willing to be proven wrong but every piece of literature I've ever read about amps since someone here pointed it out to me, has said that you can't achieve rail voltage with an amp. There's a dead zone of a couple of volts for even the best amps. I can't live with that. I'd need to get down to at least 0.8V which means on the - side I'd need to at least feed -3V or so into my amp to get 0 at the output reliably.