How to Add USB Power and Rechargeable Batteries to 9V Colloidal Silver Generator Safely

Hi All
Thank you for taking time out to read my request.
I build simple 9v battery generator for producing Colloidal silver, very basic design, X2 9v batteries , i would like to change these batteries to rechargeable and be able to connect my device to a laptop or charger, please can you tell me how i would wire in the usb connector on my device design..I am a bit stumped, its needs to be extra safe in that it will have a fuse and any thing else to stop it burning out or overloading, the device can be on for over 2 hours at a time.
So the device when connected using a usb will run the device and charge the device at the same time.
Please help thank you.

Paul, we'd need a bit more information here.  How much current does your colloidal silver process need?  The light in the diagram will limit the current a bit, but what kind of light is is, LED, Incandescent bulb...?2 9V Batteries will provide 18V in series as you have wired them.  USB will provide 5V only.  So you would need a step-up converter to step the 5V up to 20V or more to charge the batteries.  And most USB supplied can only provide 1A safely, so when you have stepped up the current available for your process and to charge the batteries would be in the region of around 250 mA max.Please reply below with some more information as above as  to what you want to do, and we can maybe assist you.

Hi DavidThank you for replying, my generator needs at least 18v, it has a LED light and a simple toggle switch, I am thinking about using rechargeable batteries, but have no understanding which are the best and hold their charge the longest, i would like to be able to charge those batteries some how, but it must be safe and have some type of protection from over charging  etc..