Started off with the starter just making a clicking sound after sitting in the garage for a couple of months with a retainer on the battery. I cleaned the posts and jumped from another car only to hear the starter strain to turn the motor over. Went and got the big charger with a 200amp boost and the motor would turn slowly and not start. Do I have a fairly new battery gone bad or a starter problem. And why won't it start?

 

I'm not seeing my car info: 2002 Jaguar xj sport

 

I would suspect a dead battery first, especially if it's been connected up and sitting for a couple of months. if it's really dead, it will affect any jump start. Try disconnecting it and putting it on an overnight charge, then check the voltage, should be at least 12.5v, or borrow a good battery and try that, a lot easier to swap out a battery than a starter, so worth a try before going down the starter motor route.