1999 Porsche Boxster battery drain

Everything was running just fine until one morning I go to start it, the battery was dead. I get a jump and it was good all day. Until the about a week later. Same thing. And I keep it in a heated garage. Not sure if it’s the alternator ( highly doubt it ) but it might be just a bad battery because I checked all the fuses and it’s all good. But now, when I turn the starter, there’s no juice at all. The instruments gauges won’t even come on and the battery is good. Any ideas?

How do you know the battery is good?

Take the battery out and have it checked . Or if you can start it with a jump go have the battery and alternator checked . Eliminate the simple things first.