My Lincoln Aviator won't start, is it the battery?

Today's question is following our No-Start Diagnostics theme, an aspect of our cars that often comes with many unanswered questions.

"Hi there. I'm having some problems with my aviator. I just bought it private and it seems to be fine when I looked at it. Then I got it home and the next day I cleaned the inside and then wanted to go into town but it wouldn't start. I turn the keys and all it does is a clicking noise. So I boosted it and it runs fine until I shut it of again and leave it for about 20 min then it does the same thing. I'm wondering if it has something to do with the battery?  I thought it could be the alternator but that would mean as soon as I boost it and take the cables off it would slowly die on me again which is not the case. Please help me before I spend more money on a mechanic.
Thank you so much!"

Dear No-Start-Navigator, 
        Thank you for your question! In this instance, while I wouldn't rule out a potentially dying battery, I would say your alternator is the culprit in this case. You are correct that if the alternator was completely bad, the car would slowly run out of juice after being jumped, but that's often at the extreme end of life for an alternator, and most folks get it replaced before it gets that bad. In this instance, if it were your battery and the alternator were fine, it would start charged up for a couple of days at first, and then for at least a few hours and then slowly get to the point of not starting at all. This would be pretty noticeable in it's symptoms.

Obviously, there are elements that I could only diagnose through looking at your Lincoln myself, but just from your description, I would definitely go to the alternator first, as that's what the symptoms point to it right off the bat.

If you have tools available to you, or someone who does, you can do a basic check in both your alternator and battery using an voltmeter. It should be 14.5 for the alternator and around 12.5 for the battery.

Alternatively, you could also take the alternator and battery into a local auto parts store and have them load test them, most do it for free. It'll cost you a little bit of time, but save you money in the long run.

Let me know how things go!

Yours truly,

The Free Mechanic