Some time ago I was recommended by my professor as a subcontractor to a local company for a some project. This is one of a small things that I made for them - Raspberry Pi hat with a battery power backup, connectors to a servomotors and ultra-sonic sensors. For a battary charging circuit I used a famous TP4056 IC that's dirt cheap on LCSC... I also hat to use some fancy power multiplexer circuit from Texas instruments to ensure that backup circuit is switched on fast enough if input voltage drops below a specified threshold. This was also my first encounter with big Raspberry Pi, previously I only worked with Pi zero.