I agree, its well worth having, my only mild niggle it can charge on storage mode (stops at 62%) but won’t auto discharge down to 62%.
Adrian
On the subject of those 737 powerbanks. 2 of them will charge 3x air 3 batteries from storage charge 60% to 100% using one of these chargers and they lose 20-25% from each power bank doing so. So theres some maths for you to do. Nice charger at 200 watts total and bluetooth which I thought would be a gimmick but is actually really handy for monitoring.
I do like a person who labels everything, good job 
Certainly looks the part 
Is it this one John @UAVShipSpotter ?
https://www.amazon.co.uk/LIMETA-Intelligent-3-Channel-Discharge-Batteries/dp/B0CTK9ZJHX/
Yeh that one. Im still testing it but happy with it so far after a couple of trips.
Ive done some rough testing because i never worked this out either. Results may vary if outside, tests done in my warm office using a power bank with 34 charge cycles on it.
Mini 3 pro battery with 43 cycles, charges 20% to 100% while consuming exactly 20% of the 737 power bank, this was in the Yuanley aftermarket charger in fast mode.
I then charged a second Mini 3 pro battery with 42 cycles, again 20% to 100% which consumed the next 22% of the 737 powerbank with the DJI fly more charger hub.
Some sort of increased losses as you go deeper into the capacity of the power bank? Maybe someone thats more of an expert in batteries would know because it doesnt make sense the more basic charger consumes more power without a bright screen lit up the whole time. Or the screen just isnt that accurate. 1 and a bit watts extra used when the battery is full just to keep that screen on.
Id say if youre not landing in the yellow or red then you can make a rule of thumb that this power bank will charge ~5 mini batteries on a warm ish day. Youre flying all that extra time for the cost of 1.5 additional drone batteries. But at some point depending how much you leave the controller on standby you may need to account for putting some charge into that too.
I thought this would be the best place to resurrect this thread which although doesn’t relate to Mini 4 batteries but…
Any recommendations on a power bank for Air 3S batteries. Something that doesn’t weigh like a brick and can be carried around in a rucksack. 
How many batteries do you want it to charge?
Multiply that by ~80% of the battery capacity (you never run them below 20% .. right?) and then look for ones of that capacity or more.
Anker/UGreen probably have the best names .. currently.
Out in the field, 3
reading online it seems you will need like a 50 000 mAh or bigger type of power bank / portable power station in order to charge the Air 3 battery up 3 times ( or 3 batteries ) due to power loss during the charging process - they are quite big boys in the drone
So really something like the Anker Solix C200 would be perfect - bulky but it will more than do what you need.
I have two small 12 000 mAh ugreen 100w PD chargers and that realistically gives me one more battery to fly with - so by the time I have taken the 1st of my 4 drone batteries down to like 10 - 20% and put it on charge it will be fully charged by the time I come to flying that battery again after the others have depleted - I have only ever done this once so far as having like 25min+ on each of the drone battery is quite a fair bit of flying for a day.
Perfect! Much appreciated.
I use one of these.