USB battery charger?

is there some way to charge one of the lithium ion batteries with a USB?

i want to start bringing my D5200 with me when i go backpacking and recently bought a solar panel to charge the electronics, but unfortunately the D5200 doesn't except AAA/AA batteries, so i am looking for a USB adapter for the Lithium ion batteries. anyone have any ideas?

alternately i could go buy a couple more batteries but they are expensive.

In order to charge a battery, you need to force more that its rated voltage onto the battery... Your car's 12v battery needs a 14v charging system. Nikon batteries are 7.4v.... USB voltage, as a standard is only 5v

I have a cheap one off ebay there is a voltage converter (inverter) in it to take the 5v from USB to 8.4v for the charger
Seems to work OK I use it as a travel charger and it worked just fine of a mains USB charger I carrel for my phone /tablet how well it will work of a Solar panel I not sure

Also powered it with a back up power battery pack that worked OK