Project Help - USB Charger AA batteries

So i wanted to try and pick peoples brains about this and surprisingly my web searches havent really returned anything fruitful on the subject.

I have an electronic device that uses 4 AA batteries in series (6v) in a standard arrangement. Due the relative newness of the product and the way the battery compartment is shaped, its basically impossible to replace them with a 6 volt pack and its equally tiresome to use rechargeables as the battery compartment is screwed shut and this is designed to be a portable device. Ideally what I would like to do is remove a section of the case and embed a micro usb socket and setup a charging circuit to the existing battery compartment and stick some long life (lion maybe) AAs in there then just leave the compartment shut and charge via the usb port. Maybe add an led indicator for charge level, i dunno. 

However, while i can solder with the best of them, the actual engineering part of laying out the necessary circuit etc is wayyyyy beyond my skills. would anyone be able to assist? Would it be easier to just scrap the battery compartment entirely and stick a battery pack inside the case that was 6v and many many many mah and hook the charge circuit up to that and its leads to the wires going to the compartment and leave it empty? theres plenty of hollow space inside the appliance itself.

Second, the device itself doesnt have a "hard off" switch, it has a momentary to boot it up and a reset button. The momentary is constantly being hit in travel and turning the device on, draining the batteries. Theres no internal memory to worry about being wiped, so id love to slap a slider switch on there to kill power to the whole device unless its physically toggled, especially if im cutting a spot for a usb port as well. 

Anyone willing to help consult to bring this to fruition? Ill happily give you full credit and buy all parts and what have you, and do all the solder work, just need someone to help me plot out the circuits, decide best course of action, and design a nominal breadboard to slap it together for testing.

 

Anshu AR12 days ago

Basically, you want a USB charging setup for your 4 X AA batteries.

And an off switch?

xBenedictx (author)  Anshu AR12 days ago

yes, but there needs to be overvoltage protection for the device and a few other things, i think i sorted it, we will install a cheap cellphone power bank and wire it to a dead switch then to the device.