My 04 cooper s had a dead battery so I bought a new one. I hopped in the car after a few weeks of driving and it was dead so naturally I thought it was the alternator so I took it to autozone and had it tested and it came back working fine but it showed it had a bad battery so I exchanged it for a new one and tested it again once I put it in and it still shows bad battery...it's good out of the car but not in. Is this a cabling issue?

 

Ground not connected to chassis?

 

From what I can see it's attached just fine. No corrosion.

 

Check where it attaches to the alternator as well.

 

You really need to sandpaper all the battery & ground connections down to the bare metal.

 

I'm confused. The act of installing the battery shouldn't make it test bad. How are you testing it when it's installed?

 

You have a ground in the front and the back. Have you checked both?