I knew some others would have a more firm understanding of the exact dynamic involved.
A couple things come to mind on the other stuff -
You mentioned the 'trigger chopper' in the corded drill - Very true, it's good for 'close enough' when you're turning a drill bit, but even though under load it can hold a fairly steady RPM, the ability to fine tune is crap. Just the nature of the beast.
For this, and where a drill motor (or any other 'universal'
series wound motor) is involved, a VARIAC is invaluable. They are a little more rare than they used to be, but short of a full range (pronounced 'high end') variable power supply capable of 0-150V @ several amps, DIY or otherwise, it's really the best way (IMHO) to tame such a motor. As long as the load doesn't change (which during tests it wouldn't need to), you can dial the RPM right on in to exactly where you want it to be and hold it there. If the load
does change, just adjust to suit.
They're not entirely cheap, but well worth the money just to have around for such an occasion.
Note that this doesn't work with any other kind of motor. You'll probably just make heat and some humming.....
The other thing I think of as worth mentioning is that for any kind of scale (and really even just in general), the genny is the device that gets the 'dialing'. While it is entirely possible, and may even seem practical at first glance, to dial in a battery bank's voltage to within a couple of volts of the genny's natural cut-in, this line of thinking will turn on you a little bit on down the road, when you go to
use the energy you've produced.
Almost everything is designed for 12, 24, (some) 36, and 48V nominal banks, and don't tolerate being much outside the normal operating ranges for the specified voltage. Even if you're close, but still not on, ie 22V bank on a 24V inverter, it will not play nice, and (in this example) will cut-off early, because it thinks that the battery is too low too soon, while there would still be usable power left in the battery.
If rewinding the genny isn't practical, your next best bet is what's called a 'buck' converter (if the butter zone is above the battery voltage) - this converts the surplus voltage into current at the lower voltage of the battery so that the genny is better matched to the bank. A variant of the basic buck converter is called MPPT (stands for Maximum Power Point Tracking), and uses a 'brain' to find the sweet spot for you so that regardless of the output from the genny (within reason), it will provide as much power to the bank as possible. One caveat - they are quite expensive. Not 'tinkering' equipment - I don't even have one, and I've been in this for a while now. I use a buck converter (that I built with the help of Commanda and others) from time to time, but a full blown MPPT is a little out of my reach ATM...
For experimentation and learning purposes, if you want to play around with the 'in-between' voltages, this is fine, just keep in mind that they are not standard and you will have hurdles down the road to work around when it comes time to use the juice.
Other than that, sounds like you're gripping the ideas rather well. As the saying goes, "Having fun yet?"
Steve