I am no auto mechanic, so, what I say is not advice in any way.
My experience is that many / most mechanics have a tendency to maximize their own income or earnings and not to minimize your costs or hassle.
So, I have learned to 1. find a reliable and honest mechanic and stick with them until they becomes untrustworthy. I have been using one but they are about to retire.

1. Take it to a place that they do not do more repairs that the one you are dealing with so that they have no incentive to charge you for things that they also do and recommend that you do it. In this case, I would go to an auto-parts store that does not do repairs (auto-zone is one; not Pep-Boy - they do repairs). Auto-zone can help you test and recharge batteries and I am sure that they can help you determine what was wrong honestly.
   Again, that is what I would do.
   All the best!