Anyone have opinions on why efficiency drops with lower charging rates and if the lower rates help with battery health or balancing?
I believe lower efficiency with low charging rates is due to charging overhead making up a larger proportion of the incoming charge current.
You'll need a bit of imagination for the next part.
I use 120V 15A circuit to charge at home. I get 3-4 miles of range for every hour it is charged. That's fine for me, I'm retired. Anyway my use of electricity isn't as effective as those with 240V charging connections because there is a larger proportion of that charge power is used for overhead, pumps, etc.
I think the slow charge will be gentler on the batteries than slamming power in as I do at superchargers. I went to visit someone who had a 14-50 plug put in for me. There I got about 33 miles of range for each hour spent charging. Until that 14-50 went in, I charged with a 30A clothing drier plug adapter and a heavy 14-50 extension cord.
The 30A 240V would have provided 12-16 miles range per hour of charging if it was at the same efficiency as the 120V 15A. I got 23 miles of range as I recall, so much more efficient use of power. With the 14-50, the miles of range showed even more efficiency. Going from 30 to 40 amps is a 33% increase. I would have got 29 or so miles if the efficiency was the same as the 30 A, so the 33 miles shows even better use of power at 40A.
I don't know the gentle 120V 15A will extend battery life appreciably. I don't think there is any way to tell. Still, it makes sense to me that charging gently is probably better for the battery. It is more expensive, though, I'll pay quite a bit more for the range I get while charging at 120V 15A. Environmentally, I know I'm damaging the planet just a bit more with the slow charging. I am retired so I don't drive much anyway. I have to find reasons to drive.
It's a great car, so quiet, so powerful, so smooth. Oh, yes, that imagination part. Imagine me driving, I'm smiling.
Best,
David