Those sound like signs of a very weak or dead battery or charging system.
Whats the date code on the battery. Whats the CCA rating?
Whats the resting voltage?
Whats the cranking voltage?
Once its running, whats the voltage at 2500RPM?
A battery "tender" is NOT a charger and even worse, they are known to kill batteries if left on 100% of the time. Someone on here (Lunatic?) suggested putting the tender on a lamp timer and thats been working excellent for me