Maybe the battery is on it's last legs...or the terminals are dirty/corroded. Another thing to check is the main cable that goes from the alternator to the battery (the fat one at the alt) - these are known to overheat where they join the alt and the cable crytallises internally leading to eventual failure...I would check this item. Problems like this do not fix themselves - usually they fail completely on a dark wet night in unknown territory so if it was me, I'd investigate and repair asap.
So, inspect the battery cables - if they are not spotless or are not tight then fix. If the battery is more than 2 years old replace it. Batteries are cheap and a tow home will cost much more than a new battery and are one of the major causes of breakdowns. Report back.
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