Also, you can use a nitecore i2/i4/d2/d4 via solar (can be had new for ~$10-30). They have a DC input (2.1x5.5mm barrel jack) meant for a cigarette lighter adapter that they sell. It's rated at 12v 1a. You should be able to connect this to an appropriately sized panel that has a 12v output (some of the folding panels offer this).
I made an SAE to 2.1x5.5 cord and connect the charger to my deep cycle camping battery with it. The big battery is fed by solar.. works well and allows me to charge up multiple flash light and ecig batteries quickly. Just another option if you want to charge multiple batteries at once