If you want a remote control solution, battery life would be the last thing I would worry about. I do not think a mechanical solution is desirable, I would keep the S50 always on and throttle the USB power to keep the battery level at 50% if that really worries you. If you mount the S50 in a fixed location to remotely control it, it would probably stay there until it dies of old age, so battery life is more or less irrelevant. The battery is replaceable anyway if you are still concerned about that.
If ZWO would provide the automation and remote control features I have been advocating for this entire year I would immediately mount my S50 in a fixed location much better than my heavily light polluted one to remotely control it and I would buy a second S50 for mobile use. But it looks like ZWO doesn't need my money, that's why I preordered a Dwarf 3.
