NiMH Battery pack charger that charges via USB and powers Nano?

Hello All,

I have a Nano project that I would like to power with 4 NiMH 2000mA EnLoop AA batteries. I know that Lithium is a better choice for power density and that there are plenty of Lithium chargers available for cheap but I am concerned about using cheap Lithium batteries and charger in a enclosed and un-monitored project. I am on a tight budget and would prefer to spend less then $4 for the charge controller.

The project has a HC-05 bluetooth module, a relay, a small oled display and a solenoid (which draws 300mA but can be connected direct to batteries). So, I think the project will draw about 40mA on average, the solenoid is almost irrelevant since it will likely be used once a day for 5 seconds. The USB power source will be plugged in to charge every night. The batteries will only be removed from the project enclosure for replacement when worn out.

So, I need a USB powered charging circuit for 4 NiMH AA batteries wired in series that includes a 5v regulator and a charge state LED indicator. I have looked all over the internet for the last day and have not found anything suitable. I am okay building my own but I am not an electrical engineer. I have built plenty of simple circuits but never engineered a PCB before. I am fine with using the Nano to control the charge, keeping in mind that my project will be using the following pins: D2, D3, D4, D5, D6.

Does anyone know where I might find a suitable charger or design? Or have a better idea?