I also put a full AA Maxell (ER6C) in my CPS2 in 2006. It is still working. According to spec it is 1800mAh.
Since then, usage of the game has been very low so the battery had plenty of time to discharge.
I think it's time to change... The Maxell seems no longer in production, i will go for the AA Tadiran SL360/P. The rating is 2400mAh, i don't know if the increased density is a good thing (durability, leakages...).
There is also a AA SAFT (LS14500CNA) with 2600mAh !
Maybe i will try to cut the current battery pins and solder the new ones on them, to avoid heating the pads again.
(Although these round pins are easy to remove, unlike some troublesome flat pins.)
Actually, at the time, i also replaced my CPS2 and Naomi fans by the Papst mentioned above. I wonder if i read this topic 15 years ago.
** unrelated tip : if you have a Naomi, make sure to remove(or replace) the supercapacitor ASAP, before it start to eat the board.