Hi all I'm looking to buy my first dslr a Nikon 3500D and after reading into it I noticed the supplied charger runs straight off the mains but cannot be used via usb. Is it possible to charge the battery via the usb on the camera from a portable battery bank? thanks Josh.
You're thinking like people who use portable pocket or wearable technology like mobile phones, MP3 players, smart watches, etc instead of a proper camera in a camera bag.
Charing up via USB is often done for pocket and wearable portable machines that people just charge up via USB instead of carrying spare batteries. (Some mobile phones can have the batteries removed and replaced but who carries spare batteries for a mobile phone? Most just go home and charge up via USB.)
A camera like Nikon D3500 have a bigger battery and is designed to be changed around rather than be directly recharged, thus you could buy spare batteries for your camera bag. They would be charged up from the mains, one put into the camera, the rest put into the kit bag, and during the day, in the middle of the photo shoot, you just change the battery.
Plus USB charge at a low rate while a camera battery holds a lot of power. Thus charging up a DSLR battery form a USB would be a waste of time, you may as well try having a go recharging your car battery from USB. That's why the mains charger is capable of charging up a lot faster, because it pumps a lot of power to the battery.
And someone already mention it, so I agree with
@Bobsyeruncle that on DSLRs, the USB is mainly and mostly used as data transfer for moving the photos form camera to computer.