So two potential things going on here and not sure what the OP is referring to. I assume its in reference to a potential parasitic draw on the battery when connected to the car. If that is the case you must first measure for a draw (using a multimeter), and then secondly, start disconnecting fuses until you isolate where the draw is coming from (giving you further evidence on what circuit or component is causing the draw). Youtube has many
videos demonstrating how to test this.
If the battery just isnt holding charge when completely disconnected from the car, that is a different issue likely related to the battery. Since the battery is almost 5 years old, that starts to reach the end of life of many batteries.