CAR96 said:
Battery is about 12-15 months old
There are a lot of junky Chinese batteries out there today, particularly the house brand batteries at most auto parts stores and Wal Mart. Buy one made in Mexico or the US. I prefer AC Delco. Most of the Chinese batteries are good for 12-24 months before they go bad. Today's cars with continual low amperage draws while "off" don't help battery life either.
A proper battery test puts a load on it while the voltage is cracked. There is also a recovery time for the voltage to come back up after the load is removed. All these these things tell you more than just checking voltage across the posts.