So after a few curses I asked for advice from a robot-building friend
familiar with chargers and the dark magic of electricity. He
recommended a _power supply_. While the goal of a battery charger is to
fully charge a battery then turn off, a power supply will churn out
electrons so long as it's plugged in.


Great idea. Here's another. Get one of those timers you hook up to lights.
I'll bet there's a kind that will turn on only once. Then just set it for
the amount of time you want to run.

Cheers,
Eric