I have a 2001 non-ABS 1150 and an Odyssey PC680 which appears to need replacing as my chargers report it as being "charged" even though there's no oomph in the battery to turn the engine over anymore. It's been off the bike for a few days and is still showing over 13 volts, but fails under load ...yes, connections are good.
There are a few 51913 batteries under £50 inc. acid and delivery, just need confirmation the positive terminal is on the RH side.
The PC680 is only a couple of years old, but I think the way it's been used/charged has helped it's demise. Apparently Odyssey batteries (and possibly other AGM ones) are quite fussy about charging and need a high power (7 amp) "approved" charger to fully charge them, or a long "revvy" ride to be fully charged by the alternator. A standard "fit and forget" OptiMate III etc. apparently won't do it.
As my bike mainly had low-speed runs across town with dipped beam running and has been left on a low amp trickle charger when not used (e.g. OptiMate III), I think the battery never really got FULLY charged and deteriorated more rapidly (sulphation?) when left with a low amp trickle charger, trying to fully charge it instead of just maintaining a full charge.
Wet (non AGM) batteries like the one on my other litre-twin bike, seem quite happy being charged up by the low amp chargers (eventually) and seem to fire up the bike easily after sitting on a trickle charger for weeks/months at a time. I guess it's possible that the battery doesn't get very low because whenever the bike is started, it's usually taken out for a proper ride ...either way, the batteries always last a lot longer than in the GS
I'll probably be getting as cheap a battery as possible for the GS as the more expensive AGM ones (Westco and Odyssey) never last more than a couple of years anyway and my current

augie) chargers should be sufficient to charge and maintain a cheaper non-AGM battery ...I hope!