Litesmith have recently utilised the 21700 Lithium battery to make perhaps the most compact and lightest (at 73.5 grams) 5000 mAh power bank. These batteries are available to 5800 mAh so expect some improvements still.They are selling for US$22 (Sep 2023).
This size should allow you to charge (once) the largest mobile phone around (usually having a battery capacity less than 4500 mAh) with an Ultralight Charging Cable at 4.5 grams – a total of 78 grams for your charging system.
Size is 21mm x 76.5mm.
Nitecore have an almost identical product the NPB1 (but 105 grams – only theirs is claimed to be waterproof, which might be handy). It is available for A$60 in Oz. They also have a waterproof NPB2 (10,000 mAh) also waterproof.
If you need that little extra ‘Oomph’ Litesmith also has the Nitecore® NB10000 Gen2 Ultralight Power Bank (ie the 10,000mAh one ) which weighs 152 grams for US$60 (Sep 2023).
This may not be your cheapest or lightest option depending on your need/budget but it is pretty good. My ultralight Fob Pocket Phone has a smaller battery of around 2000 mAh for example.
An 18650 3500 mAh battery as a single power bank (total 64 grams= total charging system 72.5 grams) will charge that easily as well as my camera and a couple of my wonderful Nano Torches for as little as A$10 – or less than $2 if you get the batteries for free. See last two posts below.
PPS: Battery cases are already available to make your own power banks using the 21700 battery. This two cell (10,000mAh) one from Amazon is available for US$12.11 (supply your own batteries) and also this four cell (20,000 mAh) one for US$12.21.
You might even ‘rescue’ some batteries from a dead appliance. I have one today, a dying Dyson V6 battery which I will break open to see what makes it ‘tick’._
See the posts below.