Don't batter yerself 'bout batteries Bergs!
My XJR I have had for 20 years this month. It was 5 yrs old when I got it, ready for primary school, like its owner.
In 2008 or 2009 the original battery played dead. It showed your perfectly decent 13v odd after a charge, but that doesn't mean much. It didn't have the cojones when current was needed and you could see the voltage drop off in front of your eyes as soon as it was loaded up. A goodish test is to leave a 60W bulb on for a bit to see how quickly it dims - and maybe measure the voltage every few minutes to see how it's doing. Or not doing, which was the case with mine. (And 60W ain't a lot compared to the starter motor's greed.)
I replaced mine with a not-as-horribly-expensive-as-maker's-spec clone that fitted the hole for half the money, against the advice of the bloke in the Yam shop, who said 'oh well, it'll do for a bit I suppose but don't expect, etc . . . .'
So I haven't 'expected'.
Nor have I touched it apart from cleaning the terminals now and then. It is still good. I don't ride it that often because I prefer oldies but it always starts, year round, with a press or two of the tit. Even after gathering dust for weeks. I don't even use a trickle charger or 'maintainer' thing.
So I reckon you should do a rough test on yours and probably just get yourself a medium-price new one that'll fit your hole. Then, if you think like I think, it'll probably be your kids' (or the donkey sanctuary's) problem next time it needs replacing.