RTC preservation across power off?

Is the RTC preserved across power offs?

I’m trying to keep a clock running while the MangoPi is powered off and I was under the impression that the RTC values would be saved and continue to count but that doesn’t seem to be happening. Is there something that needs to be enabled or am I mistaken that this is possible?

volatile unsigned int *RTC_HMS = (unsigned int *)0x07090014;
*RTC_HMS = 15; // set this and then power of the pi
*RTC_HMS // value read is 0 not 15

I also tried GP_DATA_REGs but those don’t seem to be preserved either.