charging the phone all the way up to 100% is less efficient than just charging it to say 80&, it's not linear. That reduction in efficiency creates heat, while your phone is also already working really hard navigating, playing music, blasting the screen at high brightness (assuming you're driving at daytime). All the heat can cause your phone to start performance throttling and could cause it to overheat and shut-down, but it will also hurt your batteries longevity