Moose without electricity

Moin together! I need your advice again. I was away for 4 weeks now and my wife moved the moose only once for a short distance < 5km. Yesterday I wanted to drive a lap, everything was dead. Not even the ZV worked. I bought the battery (min. 58ah) in May. I just removed the battery and connected it to the charger. The charger has 3 modes: 6V, motorcycle, car, truck? When charging my previous batteries I was able to set on modes "KFZ". I can't go anymore, stays on 6V. And the charging indicator stands on "full" after a few seconds "half full" was displayed. I suspect now that the battery is in order. Unfortunately, I don't have a measuring device or the like. Do you have any advice?