Another interesting idea for charging those 18650 (or etc) lithium batteries (and the like) Only 20.5 grams.
Weltool version, Folomov version from approx US$12.69 (Feb 2018)
To convert it to a 3600 mAh power bank you would attach it eg with cable or connector to something else, say your phone.
See: https://www.amazon.com/dp/B075F4MD1G
NB the 18650 batteries are usually 3600 mAh (eg Panasonic version) and may be as much as 4000 mAh (claimed). This is a reasonably sized Powerbank which will charge most electronic devices.
The batteries could be charged by a solar device such as this one, or this.
Here is another one which charges only: Olight Universal Magnetic USB Charger US$9.99/ A$13.95 (Feb 2018) This one will also charge Nicads however.
Review here: http://lygte-info.dk/review/Review%20Charger%20Olight%20UC%20UK.html
You will have to decide whether it has greater utility than the Nitecore I reviewed here. It is certainly lighter, The single cell Nightcore is 30 grams without cable. Mind you, you are likely to have a cable anyway. As an additional point the dual battery Nitecore (at 46.5 grams) has a capacity of 7200 mAh making it able to charge much larger batteries.
See also:
http://www.theultralighthiker.com/ultralight-usb-cables/
http://www.theultralighthiker.com/powerfilm-usb-aa-solar-charger/
