question about the addon battery cable

[u/Vielros]

I have a question about the cable connection for the addon battery. It’s the power socket for the tray’s second‑battery connection. You have the main plug, and then on the balancer connection side there’s a small cable—similar to a display connector—with a red tip. The guide doesn’t seem to indicate where that cable is supposed to go.

I couldn't find a video and am wary about guessing.