I am usually hard pressed to recommend SuperCaps for backup of RTC chips. The concept always sounds
good to have no battery but there are other considerations as well. SuperCaps have definite length of life issues,
temperature sensitivity, loss of capacity due to ageing under bias, are extremely sensitive to failure due to over
voltage spikes and they cost an arm and a leg.

This chart compiled from current DigiKey data shows the cost comparison for some Panasonic GoldCaps and
correspondingly sized Panasonic lithium coil cells.



The cost factors alone should be convincing.

Note also that the PCF8563 has a nominal backup current of 0.25 µA and the lithium cell has a 190 mAH rating.
The simplest of first order calculations shows that the coin cell can backup the RTC for many many years, provided
the support circuitry is carefully designed. I think it is safe to project a capacity of the battery to backup longer than its
normal shelf life!! In one product we predicted the battery life at 8 to 10 years!!

Michael Karas