Another interesting idea for charging those 18650 (or etc) lithium batteries (and the like) Only 20.5 grams.
To convert it to a 3600 mAh power bank you would attach it eg with cable or connector to something else, say your phone.
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.
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.
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.