The reason is the batteries aren’t just plain batteries, they’re intellegent battteries and each battery has it’s own microprocessor to manage the cell(s). It has a 6 pin connector to the drone and to charger that not only passes power but data relating to the battery. This is why you can see the remaining battery charge % and flight time in flight on the controller screen. The battery state can also trigger an automatic returnm to home when low. Because of the 6 way connector can only be charged in a drone or a dedicated DJI multi battery charge hub.
Another reason for the heavier 6 way connector is that the in flight power to the motor currents are far higher and have 2 + and 2 - pins for reliability. USB just isn’t up to this.
The charger you listed is really intended for the larger Mavic and Air series rather than the Mini under discussion here.
Ooh I would just like to add that I had a small hover indoors up to the point where the low battery warning came on. I then put it on charge on my Samsung 45w charger and it recharged the battery in about an hour.
For mobile use, I would personally buy a second charging hub currently on offer @£49 I believe and add a second cable to allow one battery to be charged while using the others the second battery could go in the other hub and the third battery would replace the first and so on… thus not having to wait for batteries to recharge before using them.
