Thanks for all the replies, though I guess I wasn't completely clear about the situation. You can't just go without an alternator just until you get out of the Mode C airspace. The FAA rule that lets you have a plane without ADS -B (or even a transponder, actually) is for planes that NEVER had an engine-drive alternator from the day they got their airworthiness certificate up to the present. The plane has to have lived a totally alternator-free life.
That's why I was wondering whether "engine-driven" meant the big engine in the front or any kind of engine. If an alternator driven by some smaller engine is allowed, then I could have power as needed without a wind generator, which is a small generator or alternator out in the airstream with a little propeller on it (If I faired the generator itself, I could make one with a removable propeller so I could get rid of the drag when I didn't need the juice, but I don't see how you could put it on or take it off without landing). The idea of a high-tech battery sounds interesting, but I wouldn't be needing it just for the avionics but also for the engine. I plan to have a regular VW engine, probably Great Plains, and they normally have one magneto, plus a secondary ignition that's powered by the alternator. So either I'd have to fly on one magneto or have a battery system that's able to put out enough juice to run a VW secondary ignition system for a whole day's flying. Maybe a lithium battery could do that, but I'm not even sure how much juice a VW ignition system uses (or where to find that out). Finally, concerning the charging finickiness of Lithium batteries that Tony mentioned, there's an easy solution to keep charging systems from spiking the voltage too high -- a zener diode. Older British motorcycles like my '72 Norton Commando used this system since before I was born. A zener diode connected to ground is an insulator until the voltage rises to a certain level Then it dumps all voltage above that level to ground. So old British bikes had primitive alternators with voltage that varied widely with engine speed, but they also had a zener diode that dumped any voltage higher than 13.75 V (or something like that) to ground. There are two problems with using a zener diode this way as the only voltage regulation. It's wasteful of power compared to a more sophisticated system that actually regulates the alternator's output. Plus, if it has to dissipate many watts it gets hot, so you need a big zener diode connected to a good heat sink. The Lucas zener diode found on an old British motorcycle is as big as a 3/4" nut, is made of copper, and is bolted to a large aluminum plate out in the airstream. But those systems had to be overbuilt because they're the only voltage regulation the system used. If you had a modern automotive system with occasional pulses high enough to harm a lithium battery, you could use a smaller zener diode with its zener breakdown voltage a tiny bit higher than the one the regular system is supposed to maintain. It would then stop any pulses higher than the breakdown voltage, but the zener wouldn't have to work too hard or dissipate much heat. I assume a system designed for lithium batteries already has something at least this good, but if you plan to drop a lithium battery into a system designed for lead-acid batteries, something like this would help. Mike Taglieri
