With all this testing it will need a service soon 
I need to buy a scales to answer the weight question accurately, PingSpike, but my impression from the heft is that they are very similar.
Current sitrep; I flew all the batteries out, and the RC is down to 1 steady led & 1 flashing. iPhone is down to 70%. This means that the Phone and RC have suffiicient power to manage 2hours 15minutes of flying time with 3 grey & 3 black batteries all charged fully at the start of the session.
I am currently recharging a single discharged black battery in the hub from my small powerbank to see if it can be done; progress is slow and it might not fully charge, but it might be something useful to do on the bus home from a sessionā¦ When the grey batteries are recharged, I will repeat this experiment with a fully charged 3-battery grey hub. and time it. Next step will be to try to recharge a single grey battery from the fully recharged black hub, which will of course be less capable of the job and slower if it can do it at all, but if it can be done within an hour, that will be an extra 31-minute battery for a session, albeit at the cost of discharging all the black batteries and making them unavailable for further flying in that session.
If it works, it will increase the flexibility. Options being:-
- Fly through all 6 fully charged batteries in a session of 2hours 15mins total flying time, with 5 battey changes requirng landing & take-off.
- Fly through 3 grey batteries while charging the first after itās flight with the black hub, 2 hours total flying time with 3 landings.
- Fly through 3 black batteries while charging the first after itās flight with the grey hub (more likely to be feasible), 75mins total flying time with 3 landings. But pointless when 3 grey batteries will give 90minutes with 2 landings.
Iām getting there. If only I knew where I was goingā¦
Well yes, they will be, that was precisely my point.
There may be a combo which means youāre no longer flying a sub-250g drone.
Fuck me this worse than a soap opera, just go and fly the thing 
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Canāt for the next week, not if the weather forecasts are right! So I may as well kill the time by playing with the batteries and the chargers.
RC took 50 minutes to charge from the mains, btw. I reckon the grey hub will charge all 3 grey batteries in about 90 minutes, then Iāll try to charge a black battery with it. My little powerbank is nearly exhausted and canāt manage more than 2 leds on a single black battery; not worth bothering with for drone purposes. To be fair I originally bought it 4 years ago to charge a smartphone on the go, and it canāt even do that beyond about 65%!
No way, what charger are you using
Apple 20W with folding pins. 90mins was a bit optmistic, gonna be more like an hour 50, but itās already on the 3rd battery and going like the clappers! It is currently (sorry) my best charger, and has a usb c socket.
Iāve just thought of a possible potential issue with using the black batteries; would it invalidate my warranty or affect Care Refresh? In which case the black hub would have to be restricted to use as a powerbank; in this role it has pros and cons.
Pros - I already have it, and it can be used to fly the Mavic Mini.
Cons - It may prove too slow to be effective as a powerbank for the grey hub.
If they fit and it flies then there should be no issue with warranty using a different battery as long as theyre DJI. But too late to worry now anyway because it will already be in your logs that you flew with them
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You need a 36w to fast charge Mini 2 batteries in the hub
Check the specs
Donāt waste your time, hovering indoors isnt real world
If you have to remember to set low battery behaviour to hover
No, I realise that, but the grey batteries seem to be lasting within 2 or 3 minutes of what they are doing outside in the real world, so as a general guide itās not miles off. Iāve learned that the sensble limit is about 25 mins for the 4k grey batteries and 20 for the black ones in the Mavic Mini anyway; these theoretical experimental results reflect what should be possible, not what is wise or probable.
Will look for a 36w mains charger, then!
It needs to output 12v 3a to charge at the fastest rate 
Ok, Iāll check Amazon for that later, but for home charging it is debateable whether speed matters. Right now Iām charging a single black battery in the black hub from 3 fully charged grey batteries in the grey hub with a usb-A to micro usb lead, and it isnāt working out as Iād hoped!
The grey hub will only allow one grey battery at a time to charge the black hub, as it happens the middle one, while the outer batteries are presumably waithing their turn and will also be used singly. Speed is no better than the small powerbank, and this experiment is a failure in the sense that it wonāt do what i wanted but a success in that I now know that it wonāt do what I wantedā¦ There is clearly no point at all in trying to charge a grey battery with the black hub.
At least I triedā¦
And the black hub & batteries add about 40-45 minutes flying time, so are are still worth carrying with if Iām going to be out for a long day. Moreover, they are arguably more useful than the grey for short flights, where using a black battery for a few minutes will save eating into the full charge burn time of the greys.
But I think thatās it for this line of investigation/experiment! Thank you everyone for your support and comments. which have been genuinely appreciated. Further research will involve looking at powerbanks powerful and fast enough to do what I need, and probably coming to the conclusion that I canāt afford them or they are too bulky and heavy, My flying bag, the one that came with the Mini 2, is full already and cannot take any more, and is about as heavy as Iām comfortable with for a long day out.
Its not debatable it matters
This is what I have
What I meant was that, once Iām home where there are wall sockets, it doesnāt matter how long it takes to charge anything so long as it is charged by the time I want to fly the drone again; overnight trickle charge is fine in most cases. Charging while out and away from home, powerbanking, is another matter, this needs to be as fast as I can get it without being a fire risk, and of course within my limited budget. A mains or even a car charger, however good, is just dead weight when Iām out flying.
Iāve ordered a digital scales from Jeff Bezos to check drone weight with the different batteries (and the lights, and the lens filters, and the prop guards, &c). Theyāll come in handy for other stuff as well, cooking and fish medications.
ā¦ maybe you need one of these - will carry all your gear and if you get too tired after your extensive flying program - The Squeeze can push you home in comfort - win/win 
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I have this Anker. 20k mAh and 30W output, and currently on sale for less than Ā£23 when you apply the discount. Think I paid Ā£30 for mine towards the start of the year.
Not bulky, affordable and will get 5 or so batteries charged. Iāve not timed it as I havenāt been bothered to but have it connected to the charging hub while I am out and about.
Or an upgraded version 
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I have put this suggestion to The Squeeze, and her response was some very rude words in Polish. Polish is an excellent language for profanity, very expressive! Kurwa!!!