12.6 volts. Looks like a good test.
Battery ok at rest
13.6 is better.
Charging system working properly at idle
Slowly roll on throttle and watch the volt meter. Battery voltage should rise to 14.5~15 volts then stop rising, If voltage does not rise the rectifier is the most likely cause. If the voltage continues to rise over 15 volts the Voltage regulator is faulty. In either scenario replace the regulator/rectifier unit.