I'm designing a circuit that integrates both USB Power and a Li-Ion battery when USB is unplugged. For the battery charging/protection/boost part, I'm following the schematic given in this Instructable.
The below has been completely edited to provide a more full schematic.
Currently, my project has a representative schematic like below which mostly parallels the GreatScott! schematic linked above. However, the key differences are replacing the 5V/GND terminals with an actual resistive 5V Load that draws 300mA [please assume this]. Additionally, a power line runs from the 5V of the USB [with power filtering using a ferrite bead and decoupling caps omitted] directly to the load via a schottky diode [omitted by accident], to power it when the USB is connected instead of the battery.
Note that BAT+ and BAT- on the image below (right in the middle near the protection IC block) are two solder terminals for a single-cell Li-Ion polymer battery, outputting 3.7V nominal, 4.2V max.
My main concern here is that is actually appears the BAT- terminal of the battery is never explicitly connected to ground. Thus, when USB power is removed, does BAT- ever form a complete circuit in this case? The only instance where I even see BAT- here is in teh Protection IC section [lower middle of the picture].
I'm aware that BAT+ should never be shorted to ground, since that would just be a short-circuit. However, where does the BAT- terminal go? The path of the BAT+ makes absolute sense, going into the Boost IC after a toggle switch. The BAT- connection is confusing me on the other hand.
Clearly, when USB is plugged in, the USB's ground becomes the functional GND for the circuit, and it is complete. However, the BAT- terminal never appears except right in the protection IC. Therefore, I'm confused how BAT+ and BAT- form a complete circuit w/ the load in the event the USB is disconnected.