I too have a similar problem with the 2797, it seems there are multiple USB charging regimes and the adapters come in 1 and 2 amp versions to suit the trickle and fast charging requirements, however there is an upgrade in the plugs from USB 2 to 3 (the blue vs white ones) as well. The 2797 also has mini USB on the mount but micro USB on the case so it's great to have standards, there are so many to chose from.
I haven't nailed it yet but my 2797 says about my standard USB 2 cables "The attached power cable cannot charge your device. Use the one that came with your device."
The supplied cable is certainly heavier but I have it semi permanently installed in my motorhome so I do not wish to remove it.
Somehow the GPS runs on the PC's USB supply, though not exactly friendly to use connected to the PC and does not charge.
So don't get fooled into a USB adapter/charger without addressing the cable two.
Kevin H