re the three points

1) during charge, sometimes there's lots of reaction that gasses in some cells, whilst others are happy and normal, the gassing throws the water you added back out... I've done this a few times and you end up with the perfect dampness in most cells but one or two go dry again and the plates remain visible... if this fizzing is happening whilst in the car all the corrosive mess doesn't make things pretty - where it touches carpet it will vanish and bare metal will rust - in fact on my bike this went over the chrome exhaust and has stained it a funny yellow in a few places (its old), but if on your pride and joy this isn't bringing a saving vs a new battery. It never happened on the Original AGM I have, nor on my Yuasa replacement AGM - and then didn't on the Odyssey AGM when I first did it - but did second time round a few years later...

2) the lids are unnoticed under the sticker (peel it back) and there they are just like a normal battery - but to make it hard to check inside, they deliberately used a really silly security type, one way design to the lid - so tightening is easy, but to undo is really difficult - then as it has two sealing O-rings per lid and they fit them dry, with time they stick so they are almost immovable - as that didn't stop us, the later ones now add glue or heat to bond a few of the lids in, just to ensure the battery dies unnecessarily

3) I believe its mostly water that evaporates, rather than the entire mix of the electrolyte - so quite often you just need distilled water (these days wrongly referred to and sold as deionised water - which is something quite different). Note: AGM batteries are not meant to have liquid in there - just damp tissue paper - it swells up and draws it in, so 24hr later often needs as much again. Then I guess the action of the compounds mixing together during use and with some desulphation occurring over a few months it recovers (I presume because there's no actual liquid sloshing about to mix things up correctly for the process to happen the in 24hrs you want).

thanks for the clarification.
on point number 3) did you manage to restore your AGMs by just adding water? If yes How does one know how much to add?

point 3 YES - you can add too much (but try not to) and suck out excess with a pipette

unbelievably...

the OEM monster Merc one needed circa 120ml per cell at around 10 years - but originally I put in 60 to 70ml but then in the morning it has all been sucked up and looked bone dry.
the Odyssey tiny bike one is less than 1/5th the size of the merc one, yet needed 140ml per cell !!!!!!!!
and I got 140ml per cell in the Yuasa replacement in the back of my 221 when just 2 years from new - NOTE this battery has cell lids that come out properly with a 1/4 of the fight you get with a merc battery

thanks.
The one you refer to as "Monster Merc" which "in the morning it has all been sucked up and looked bone dry." was that an AGM and hence water absorbed into the Fiber Glass Mat ? If not what was the reason for its drying up and did you have to top off again?

yes the OEM AGM one it was made with - and yes the distilled water you put in soaks into the white tissue (glass fiber matting) - its the moisture that gives the electrical conductivity that makes lead acid batteries work... all the ones pictured in post 15 and your non OEM one on post #8 are basically all the same type of battery - chunks of lead with a mix of sulphuric acid and water... works the same way as this - get a lemon and stick two slivers of copper 3 inches apart in the body of it, then stick your tongue across the two bits of copper - hey its a battery

you realise my time writing all the info and you reading it could have been put towards labour that paid enough for us both to buy a new battery each each ? - just in case this is an AI attempt to use my time... I think we have enough to be going along with...

I measure the pleasure of learning and the fun of sharing and learning valuable experiences in terms of "invaluable me time", how I see it is this is de stressing time for me without having to spend money for entertainment although I have 18 batteries to deal with now and in the near future (11 Vehicles, 7 Generators, Many more UPS) however I am not going to use these batteries in my Mercs they are getting new OEM, Bosch and Varta AGMs / Std, but the fun of learning all this and doing the experiment on my own recovering disposables brings a sense of satisfaction. So thanks for sharing all your experience