battery voltage question

The voltage you show is either not charging or the battery has a shorted cell. This IS for sure an ammeter and not a voltmeter? How far to the right is it going? Bring up RPM to simulate low to medium cruise

"Normal" and "warm" should be 13.8--14.2 optimum, not below 13.5 and not above 14.5

It can go below 13 under conditions such as yours, IE low battery, depending on how large the battery, and how much you've charged it