Are you sure that the RTC chip uses Phase Change Memory? Never heard of that before. Normally RTC chips, in addition to the RTC function, have a small amount of static RAM that is held up by the battery (nearly always, 3V, CR2032).
The static RAM normally holds various system configuration settings.
An error with these settings can cause absolute havoc and prevent the computer from booting, as I experienced recently on a laptop. Strangely, a keyboard fault can do the same thing. I'm sure the people who design computers have a sense of humor.
The only cure for the former is to remove the RTC battery, wait about 10 minutes, and then reconnect the RTC battery.
The cure for the second is to fix the keyboard, or fit a new one.
I do not know the answer to your question about PCM, but static RAM does need a battery to maintain its contents.
Normally, when the host is off, the RTC chip runs off 3V from the RTC battery, when it consumes a minute amount of current. When the host is powered on the RTC power line changes to 5V typically, which is sourced by the host.
spec
PS: interesting blog