Ring Spotlight Cam

I realize there are two battery slots in this device. I bought a second battery in order to have coverage. My question: what is the purpose of the second battery slot in this device? is it to store a 2d battery? If I put a 2d battery there will it provide added time? or what??

When one battery runs out the second one will take over so you have to charge the dead battery without loosing coverage

Thanks. I was concerned having the charged second battery in the slot, it would drain the charge before it could be used.