Charge one battery completely from another Raspberry Pi (Zero)
You need two (or three) modules. These can be DIY using input from elsewhere - application notes from IC manufacturers is usually a areasonable source of these. Input from You Tube videos and websites in some cases works well but can also be very unsatisfactory.

You need a boost converter that will run from 4S LiIon in any state of charge that outputs enough voltage to operate a 4S charger. The 4S hub battery can be as low as about 12V (4 x 3V) and as high as about 16.8V.

You need a 4S charger. This will need about 18V or more.

As a separate module or included in the charger you will need a 4 cell BMS.


4S chargers with or without BMS, separate 4 cell BMS units, and 12-17V  in / 18V out boost convertrer are avaiable commercially or you can roll your own.

EXAMPLES ONLY
4 cell BMS here

or, an example of an IC that would allow you to &quot;roll your own&quot; here.