18650, charge and supply at the same time

Question

I am planning to use a NodeMCU with a couple of encoders to determine the position a telescope is pointing at. While running, the device will be connected to wifi all the time since it needs to interact with an app (or maybe on AP mode so a tablet/phone connects to it).

In the future I may also add a barometric pressure (BMP280) and temp/humidity (DHT11) sensors, gps, and a touch lcd. Probably wont have enough pins for this, but maybe using a MCP23017 could work. Anyway these are all just ideas, to start I will only use the 2 encoders. I haven't even received any of the devides yet, so in the meantime I'm reading and thinking of stuff I could do.

The device will have to be mobile since it may be used in places where there's no electricity, so I am thinking on powering it up with some 18650 batteries I have. I want it to work for about 4-6 hours before having to recharge it, and the device itself must have a charging (usb) port. I also need a 5v line since the encoders work with 5-24v (I choose to use the min 5v since it would be easier to use a logic level converter for input in the nodemcu). So I thought of doing something like this:

Pinout is not important right now, just the concept.

Would it even work? What will happen if I turn the switch on while the charging usb port is plugged in? Any simpler solutions? The charger protection board would be this one.

Answer 1

Turning the switch on and off is fine, all this does is add a load to whatever source is a higher voltage, be it the charger or the battery. The way this setup will work is that the boost converter will attempt to reach it's programmed output voltage and fail if the cells are below it. The boost converter will then instead reach it's maximum duty cycle and begin to charge the battery, which may or may not upset your USB ports maximum current limit depending on the boost converter used. An inline resistor between the boost and cells is a simple fix for this problem. Alternatively you might can find a boost converter that has a low enough output current to avoid this. The truly correct way to go about it would be to put a charger IC/module between the USB boost supply and the battery pack.