Your car battery is 12 volts (more or less).
What battery are you referring to?
Car?
Yes, if you keep charging your mavic while the car is off. You'll drain your car battery. Which shouldn't be an issue because you'll have a freshly charged mavic battery.
Will it fry you mavic battery?
No
Chargers have built in voltage regulators.
The charger will only draw the foliage it requires.
For example, my phone requires 9v to charge. That's what the charger draws.
Also, your car recharges the car battery while driving. That'ds why it's ok to charge while driving.