A electrically actuated clutch pulley would solve that. But, many newer cars already have, and need, a mechanical clutch pulley that disengages the alternator on deceleration.
I assumed it would stop generating electricity (and therefore resistance) when the batteries are topped-up. I didn't even realize they're mechanically disconnected as needed.
Some are, but only on deceleration, to reduce wear on the alternator. It's a mechanical clutch only.
AC compressors, on the other hand, do have electrically actuated clutch pulleys.
So, if you have a car where the alternator has no clutch, you may be able to retrofit an electrically actuated one. You would have to design the part that decides when it's needed as well.
