I think any modern smartphone running a navigation app will struggle to actually charge over USB.
I always make sure if I plan on using my phone in the car, it starts with a full charge as (even with a dedicated 12v USB adapter) it can actually slowly discharge.
When the screen is off, it'll charge OK through the glovebox albeit at a slower rate. I'd be interested to know if anyone has tested the glovebox port, but the standard output for USB is 500mAh with most wall chargers being 1000mAh or even a little higher...