battery voltage goes up and down while car is parked
...Sounds like the alternator may be going bad, bring it to Auto Zone and have them do a load test for you (IT'S FREE)........
How much? Could be just your AC cycling on and off at idle. A slight movement does not mean there is a problem but a big drop indicates something is probably adding a load to the system or your alternator (with internal voltage regulator) is the issue. Like "joemom" said, have it checked first.
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