I checked the battery after charging during this weekend:

open circuit 12.49V
connected and lights on 11.69V
trying to start the engine 10.something V
after trying the starter 11,29V

...so it looks like it's over.

Which one do you reccomend to buy? I can't see any GS Yuasa GT-9B-4 available now at the moment here in Poland. They show only YT9B-BS.