[rohrmann]

If you read 14.4 volts, you just happened to be checking voltage when the converter was in it's every 21 hour cycle for 15 minutes to prevent sulfation. In your Progressive Dynamics manual for your converter, this is what it says:

STORAGE MODE: When the converter senses that there has been no significant battery usage for 30 hours the output voltage is reduced to 13.2 volts DC for minimal water usage. When in storage mode the microprocessor automatically increases the output voltage to 14.4 volts DC for 15 minutes every 21 hours to help reduce sulfation of the battery plates.