I had to jumpstart my 2008 Jetta last night & this morning & mid morning. After the mid-morning jumpstart I drove 2 hours (short day trip to a town 2 hours away) & immediately stopped by an Advanced Auto when I arrived (My battery is 2 year old & has a 36 month guarantee). When I got to the Advanced Auto, my car restarted with no problem.
They tested by battery & said the battery was fine, just needed charging... and said my alternator was only reading 11-something (sorry if I messed up the technical terms). But then the AA guy said my alternator likely was Not bad... since I had just driven 2 hours after a jumpstart with no issues. I also drove 2 hours back home with no issues.
other background... I did leave my trunk open for 4-5 hours 2 days ago (a bag of ice melted, so was airing it out)... but not sure if there was a light that stayed on in my trunk as a result..and that night it cranked without any problem. Also, 1 week ago (which may be completely unrelated) my check engine light came on & car started had little power (code said issue in chamber #5)... and so I replaced coils & spark plugs in chambers 2-5, which fixed the problem ... I had a similar issue 6 months ago with chamber 1... at which time I replace its coil & spark plug).
So my ultimate question is why did the Advanced Auto test indicate my battery was not charging (Alternator reading 11-something), yet was able to drive for 4 hours immediately after a jumpstart? And is this likely a simple issue dealing with my trunk being open for 5 hours that somehow ran my battery down... and the Advanced Auto machine simply somehow misreading the alternator only charging at 11-something?
Any thoughts?