Voltage and "charging banks"

[vindibona1]

I've never studied electrical engineering at all in the slightest and any electrical/electronic work I've ever done has been "paint-by-number". I can solder, but just follow directions. I'm just curious what voltage(s) are needed to charge some/most/all of the DJI batteries, particularly if the batteries are inside of the DJI charging hubs?

What's piqued my curiosity is that recently I was searching for a 9v external battery solution to running guitar pedals and realized that there was a battery pack that houses 6 AA 1.5v batteries, that when ganged together had an output of 9v. I had to reverse the polarity on the connector to make it work, but it was pretty convenient. [I'd like to know how many mAh of power that pack has with 6- 2800mAh batteries installed?]

I know I can use the DJI charging hubs to charge devices at 5v, standard USB charging, but how do you know when to use 5v, 9v or 12 v?

 

[DougMcC]

These batteries are wired in series to create 9 volts with a total mAh of 2800 in that configuration at that voltage. In the RV world a pair of golf cart 6 volt batteries wired in series will power a 12 volt circuit but the mAh at 12 volts is defined by the individual battery mAh. Batteries wired in parallel will increase mAh but at the voltage of the individual batteries.
I have never slept at a Holiday Inn or what ever so an electrical expert may counter this but I’m sticking to this or I’ll learn

 

[vindibona1]

These batteries are wired in series to create 9 volts with a total mAh of 2800 in that configuration at that voltage. In the RV world a pair of golf cart 6 volt batteries wired in series will power a 12 volt circuit but the mAh at 12 volts is defined by the individual battery mAh. Batteries wired in parallel will increase mAh but at the voltage of the individual batteries.
I have never slept at a Holiday Inn or what ever so an electrical expert may counter this but I’m sticking to this or I’ll learn

Thanks. That sound pretty reasonable. I don't usually do Holiday Inns. Four Seasons is more my other half's speed .

 

[Kilrah]

but how do you know when to use 5v, 9v or 12 v?

You read the specs of the device you're trying to power.