The manual says on page 156 that the internal battery will charged when the main battery is installed and AC adapter is connected and after 10 hours of charging.
I have done this and still have to reset time and date every time the battery runs our and I have to replace it with a fresh battery.
I still like the features of the P100 but maybe it's time to retire my 6 year old camera. I have take thousands of pictures over the years.