If you want to save as much gas and money as possible, it may not be worth it to charge your phone/tablet by plugging it into a car's USB port because the car would use slightly more gas when more electronic devices like chargers, air conditioner, radio, touchscreens, and lights are powered on. Gas is usually more expensive than electricity because of gas taxes, and gas shortages and war which can make gas more expensive.
I think it is more affordable to charge your phone/tablet at home is cheaper or free if your parents or roommate pays the electricity bill. Electricity is usually free at work or school where electricity is provided to users for free because the boss and school pays for electricity. Using an external portable mobile battery charger may save you more money after a few months to years instead of using your car's USB port to charge your phone or tablet because the price of gas can be very expensive.
Always using your car to charge your phone may eventually cause your car electrical parts like battery, alternator, fuses, and alternator belt to break faster because your car electrical system needs to work a little bit harder to generate enough power to charge your phone.
I think it is more affordable to charge your phone/tablet at home is cheaper or free if your parents or roommate pays the electricity bill. Electricity is usually free at work or school where electricity is provided to users for free because the boss and school pays for electricity. Using an external portable mobile battery charger may save you more money after a few months to years instead of using your car's USB port to charge your phone or tablet because the price of gas can be very expensive.
Always using your car to charge your phone may eventually cause your car electrical parts like battery, alternator, fuses, and alternator belt to break faster because your car electrical system needs to work a little bit harder to generate enough power to charge your phone.