For what its worth, I'll add this. The small battery, a Group 50 seems to not last long. Mine goes out about every year and a half. When it goes out, the starter does this exact thing. So far I've only found Oreilly's selling this battery and its their lowest quality battery. Let them check it on their scanner and you will know right away if its bad or not. Remember to disconnect one of the cables when testing since you don't want it to read the other battery too. Its been said and absolutely true, clean the cables with a battery terminal brush and make sure they are tight when put back on.
As a side note, last month when my battery failed, it shorted and would drain both batteries overnight. I thought I had other issues, but nope, just the low quality Group 50 battery failing with an internal short.