I charged my iPhone 7 (iOS 11.3.1) from car for about 3-4 hours during the ride and Waze app was on all the time. Next day I noticed that phone discharges really fast. Power saving mode helps, but also the problem still appear. And the problem is not in apps running in background that could take power too much. Now it also gets very hot when charging, like never before. What could be the problem? Could it be that the battery life is almost over?
If the USB Car charger doesn't have the MFi (made for iPhone) logo then it is probably a cheap charger with minimal over-voltage protection circuits inside, even if it says Philips on the outside. This can cause damage to the charge circuit on your phone's logic board and ultimately your battery.
Waze really sucks the juice out of a battery and having that charger on for 3-4 hours straight may have damaged your phone. Try running your phone inside and charging with an Apple original wall cube to see if that makes a difference. If not, then you will need to have that phone looked at by either Apple or an independent micro-soldering repair shop.