Your alternator is barely charging, and your battery is low. When you check battery voltage while running, check at the posts, then at the clamps. If the readings are different, you have poor connections. Else, you likely have a problem with the alternator. Check belt tension for a bad tensioner, and pull the alternator to have it load tested.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7