The mini2 batteries are 7.7V and 2250mAh.
Max charge voltage is 8.8V
Also I’ve got 4 of these batteries, and they’ve been great for Oculus, charging my phone, and also for my goggles power out on the field! Charge them from solar off of my leisure battery!
DJI OM 4 - Specifications - DJI on here (that’s where I got them, heh)
That’s where I’ve been going wrong then!!! My calculations were rather shoddy. I’ll maybe have to revisit the negative feedback I left for them!!
Well 5V compared to 8.4/8.8 is probably around your 25%, heh. Once you know you know, took me a while until my old boss explained it last year to me! So my 12v leisure battery of 70,000mAh, can do loads of 10,000 5V devices (14/15 odd), but 6S batteries at 25V… not as many.
So Volts x Amps (capacity) = Watts. Convert to watts and you can compare. Obviously you never drain a battery completely (or if you do, you won’t get a lipo back!) These cheeky power bank sellers will nearly always use the 5V capacity value 
That mini page actually states 17.32Wh… So you’re 10Ah 5V = 50Wh
This is stuff I should know, im an electrical engineer to trade, albeit a rather lowly NC
The spec I’ve read for an Air 2s battery is 3500mah so that pack should charge at least 3. I had a similar situation, 20000 pack only charged 1 battery. Seller replaced it and the replacement charges all 4 of my Mini 2 batteries.
Interesting. I’ll get on to them to see if I can get a replacement. I thought it wasn’t right, but wanted clarification first! In theory it should charge 5.7 batteries right?!
20000/3500 = 5.7
No, because the drone batteries are not 5V. See my above answers
For the mini2 the batteries work out at around 17.5w, your powerbank at 50w so you should get almost 3 full charges
For those interested I use a wee app from Red Binary for ohms law, but it also does watts which I’ve just used for the first time. Use it all the time for working out resistance or current draw on motor windings. Completely free and no adds
Dont forget that there will also be losses in the (mini2) charger (maybe 80-90 percent efficient) and added to that the output conversion of the powerbank (70-85% efficient) (converting from the 4.2v of the cell(s) in the powerbank to a 5v output) and also added to all that is the fact that charging a battery always takes more power than it’s capacity (the rest is lost in heat).
Tim
Thanks @GunjaFPV. I wish I fully understood this because i still don’t even though you’ve explained it very well. Numbers habe bever been my forte.
If someone can work it out for me id be very grateful.
The battery pack i bought is a 20000mAh 20V 65W
An Air2s battery is 3500mAh 11.5v 40.42wh
Does that help and woth those numbers does 1.5 batter charges make sense from the battery pack i bought?
Lastly, if it does add up to be expected thats no good for me, so can someone recommend a bettery pack that will charge 2 batteries quickly, ideally at the same time aswell and isnthat even possible!
If my battery pack appears to be suitable based on the maths then ill get a replacement.
Apologies for being dumb, as much as ive tried i just cant get my head around the numbers. 
You’re not dumb, your numbers to me look correct. The power bank has 65watts, your air2s needs a little over 40watts for a complete charge. Bare in mind that you wont completely drain the battery’s so you might get away with two top-ups if you left around 20% in each battery after flight. If you’re looking for more charges you will need to move to a much bigger power source. I don’t know how big these things get, or how portable you want it to be?
thanks @DazC So essentially ignore all numbers except wattage! That makes life much simpler!
65/45 = 1.4 so the 1.5 batteries is about right!
Thanks everyone for clearing that up, my expectations were way too high because I didn’t understand, but now I do. This is what forums are for! Thanks again.
Oh just to confuse us all as we get to work it out… Watts out, and watts capacity is different! A battery might hold 300W, but it can only supply that at 50W 
So… Volts x capacity is the Watt value you want
What’s the best power bank to buy for the DJI Mini 3 Pro to charge the batteries on the go or in the field.?
I have just purchased this with the intention of charging my phone and Mavic Mini batteries when out and about… seems like a reasonable price with a 15% discount on Amazon too…
Anker Power Bank, 325 Portable Charger (PowerCore Essential 20K) 20000mAh Battery Pack
Charged it up yesterday, so intend trying it out this weekend… hopefully!!
Ideally, for fast charging, you will need the Mini 3 battery in the Hub (36w Max) a USB-c power delivery PD power bank that outputs 12 volts (12v 3amp), then you can charge at 36 watts
Something like this
