The cabling sequence is --
AC Mains ---> PSU ---> BBU ---> HG612
|_________|
240VAC into the PSU, 12VDC out of the PSU.
12VDC from the PSU into the BBU, keeping the rechargeable cells trickle charged, 12VDC out of the BBU.
12VDC from the BBU into the HG612. If the mains input fails, the cells will keep the HG612 powered for a finite time.
The BBU and HG612 will "talk" to each other over the telemetry cable (the lower link in my ASCII art, above). It is via that "chit-chat" that the data is provided which then populates the
Status --> Battery page of the HG612's GUI.
[The
Huawei HG8240 ONT is powered similarly but I haven't yet mentioned it in my
(currently stalled) experimental examination of such a device.]