Take the 18650 cell out of the power bank and fit in the battery compartment. Then take the power bank end and remove the USB A socket that you would normally plug your USB device into. Then use the inductor and capacitors to make a LC power line filter on the charge PCB. Cut the end of the power bank off and fit into the scopemeter in place of part of the battery compartment door, so you can plug in a regular micro USB phone charger ( any semi decent one, just has to provide 5V at 500mA) to charge the cell. The original 4V8 4 cell power pack connectors are now connected across the capacitor you added, so the scopemeter now runs off this filtered 5V supply from the power bank.
Charge the unit with the cell charger and when full you unplug and use as normal. Runtime will be somewhat longer, but there will be no real low battery indication, it will just turn off after the cell protection disconnects the load.