[werk_ag]

I don't wish to criticise your hard work, but I just found this, 
https://www.banggood.com/5V-218650-Lithi...rehouse=CN

Looks interesting I have one on order & will report what it is like..

Andy

Yes, it looks interesting, however the output current seems in the low limit for a Pro2 Plus receiver, but enough for a Pro2 Compact.

normal load current recommend ≤ 1A, when output 6V ≤800mA, output 9V ≤500mA, output 12V ≤400mA

Lets see what our experts in Li-On batteries will say about this device.
I believe the UPS designed by Engolling have some vantages: Allows greater output power and allows 9V input (I think other voltages can be used too), instead of 5V only. So, for a Pro2 Plus system, it can simply be inserted between the current power supply (9V) and the receiver.