..... I only had one 317 to test with and it seemed to have the same problem with open circuit vs load regulation the 7805 had. I only tested this a little though.
I'm trying to keep this as cheap as possible and the 317 has a different pinout than the 7805...right? That would mean some creative mounting inside the case.
So, if I read your note correctly, you have one big DC supply and you charge all the batteries from it by putting a 317 in the middle for each battery. Right?
so I've been looking into a float charger that doesn't let the battery discharge or boil the water out over time.