I would expect/hope a sat nav would contain it's own current limiting circuitry in order to protect itself.
PC USBs aren't designed as charging points, you are simply taking advantage of it's ability to power up things plugged into it, so are variable in the power it can provide. Properly designed chargers should be better (but in my recent experience, there are some crep chargers being flogged out there - for example I have a third party USB C charger for my tablet, and it often needs replugging into the mains more than once before it actually manages to charge !).