@Clockwork- yes the AGM is sitting on the concrete and it would be better if it wasn't but hasn't been a problem. With a good multimeter you can actually detect the voltage across the outside of the battery especially if there is any amount of dirt/moisture film on it. The AGM is totally clean plastic so I'm pretty sure it's a lot less of a problem but I haven't measured it (Yet.) Leaving the oil cap off is a good idea and even better would be to take it off while the engine is still running. I put a wool blanket or towel over both out M54s for the winter (take the mech fan off the X5) and helps a lot.
You will be a good test for the AGM battery especially if you have been keeping track of voltages with the lead acid battery. I would track the voltage on the 7 min. trips and to see if it holds without using the maintainer.
I've been a big Delco battery fan-boy for about 40 years so that's what I bought for the AGM
I didn't know there is different chargers for AGM though ??