QGC reads 14.1 volts after charging to 15.5 volts. Is this normal?
Can you confirm the battery voltage with a multimeter? What is the difference between voltage displayed in QGC and voltage displayed on the multimeter?
Anyway, you can adjust voltage scaling from the Power tab in the Vehicle Setup page (click the gears icon). Choose ‘other’ for power sensor, then adjust the voltage multiplier until the voltage readout is correct.