its designed to come off, but you aren't supposed to know you can top them up
inside AGM is what looks like white tissue paper (the glass matting) it shrinks back as it dries out and I guess the plates oxidise and its game over... the holes are 5mm across so you can't see in, based on my experience with others I was going to just fill till the top of the plate was damp. As I was filling with a small syringe its easy to suck and know where you are.
The incredible thing was how much each cell needed. I've done it on a 26kg Merc battery and they took less than 100ml each. The odyssee one took 140ml each cell on a battery that's 1/16 of the size. Wait 24 hours and check again for good measure
from being dead unable to take a charge and unable to crank the bike.... as soon as it has liquid in its as good as new putting out 13.2v and tripping to maintenance mode on the charger within seconds
the merc one is good as new 4 years on, in fact it got better over time (say six months from topping up)