I’m in the process of building a portable power bank that can recharge at least 8 Mavic 2 Pro smart batteries & maybe up to 11 depending on other usage, it will comprise of 18650 Li-on cells in a 4S setup in up to 26P.
I’ve also managed to source a small USB board that is powered by 6-32v that can output fast charging capability that will fast charge the DJI Smart Controller, tested it today on a small portable test pack & it worked great, this can also fast charge any other device such as smart phone or Alientech Duo etc.
An added addition is going to be if I find the time to add a built in wireless Qi device so just placing your smart phone on top will charge that as well, it will of course have BMS & LCD display to show curent status.
I would imagine it would be ideal for flying in remote locations where multiple charges of packs would be needed over an extended flight path/profile.
Just waiting for the final bits to arrive from Aliexpress to begin.
Here’s an earlier project I did on a 48v setup.
This is really cool Tony 
How many £’s will this set you back to build?
Not too much really, I already had the supply of 18650 cells & they would have been the most costly, to be honest lately I have been more interested in projects like this than flying really, the extra parts are quite cheap on Aliexpress, the case is the most expensive piece followed by the BMS.
How long to recharge from lowest point?
How much it all cost.The 500 w potable power batterys are all coming down in price £400
Charge time depends on charge rate, would expect around 2-3 hours, again less time if charged at a higher current.
Quick update, I have wired the 96 x 18650 cells in 4s x 24p BMS is wired to 50% of the cells for initial testing & voltage readings, all within correct expected parameters, setup on my test bench charging the DJI Smart Controller which was down to one LED, quick charge works, & was up to 3 LED within no time, tested one DJI Smart Battery at 50% to 100% charge at 17.4v 3amps, hardly any drop in primary pack voltage/charge status ( only using 50% of the pack for testing), so need to add wireless phone Q Charger & the shunt meter/display next with main external battery posts & add x 4 sockets for connecting DJI Smart Battery charge cables.
Great project @Tony92 
Quick update, been busy with work so not been able to put much time into the portable pack yet, in saying that I wouldn’t fancy carrying it around for any great distance on foot, not unless it was in a comfy backpack, anyway, here’s some testing pics, just finally need to add 3 sockets for plugging in three DJI Smart Batteries along the front, found the sweet spot to be 3amps @ 17.35v for 2 and 4amps @ 17.35 for a quicker charge for 1 battery when needed, as a bonus the wireless phone charger worked better than I anticipated, really did kick a fast charge into my Samsung S9.
Great project @Tony92 - thanks for sharing the pics 
How much did that cost to make?
Or would it cost to make from scratch if you didn’t have any of the components to hand?
Well done 
fantastic project, I have enjoyed following it. Could you make a smaller/lighter version without losing capacity?
Just asking because I got one of these
Never needs charging and runs for 4 hours on a litre of petrol.
You just plug the charger and fast charger into it.
Or use an extension lead and do them all at the same time.
Not really sure what the going rate is on 18650 battery cells is at the moment, I know they are expensive when using as many as I did for this project, but all my cells where salvaged from 2nd hand laptop battery packs - which believe me was a LONG process to actually get the packs apart & then the even longer testing of each cell etc, so apart from the cells I’d say around £100 for the inners & case, my next project may be to rig up a solar panel to charge the pack in the field to enable extra long flights.
That looks ideal to be honest, if you don’t mind the noise that is, its something I would consider as an extra backup & at £189 from Screwfix it wont break the bank.
Mate I had it going at the big meet and no one noticed. It’s very quite.
I got mine a while ago and had a trade voucher for screwfix. Cost me £160
Charged 6 of my DJI FPV batteries, 3 MA2 batteries 3 M2 batteries and how ever many of @Howard78’s batteries (not sure how many) and only used a litre of fuel. It’s a suitcase inverter generator so it ups and downs with the demand. Very quiet indeed
Well chuffed with it, I would recommend it to anyone