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Weird charging issues with known good batteries are typically alternator or voltage regulator problems.
I had a a similar problem last spring. I suspected Voltage regulator and was correct. My car finally through a code that indicated an overvoltage condition ie. bad voltage regulator, most likely caused by a bad diode (or two).
The regualtor is a stock bosh (or equivalent) part available for aobut $50.
1. Have both of your batteries checked at an autoparts store.
2. Wait for the problem to reoccur. It will.
3. Pull your alternator. When you remove the belt, spin the alternator and check for smoothness.
4. With alternator out you can have it checked out. At this point, you can;
A. Replace the voltage regulator (on the back of the alternator) and have it rechecked.
B. Have your alternator rebuilt.
C. Replace with new or rebuilt unit
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