Yep - def thinks time for a new battery and as others said very simple procedure on a 986.
That said... might throw your C-tek maintainer on another vehicle to ensure it too is not bad.
Had one couple years back make me question if I had a bad cell on a (newer) battery.
Swapped maintainers with another vehicle and problem followed maintainer.
Alas that C-tek had bitten the dust after ~12-15 years which was pretty good in my opinion.
Good luck