I want to use the RTC so, What should I do to enable it? I just found that connector 5 pin 5 is called VBat.
Does it need a battery between this pin and ground?
Is an external circuit required?
What kind of battery do I have to use?
Remember that the Supercap is only designed for short term power off use. This can range from hours to x days. After this time the clock will not be backed up.
The coin cell provides a much longer backup period and can be years depending on the current draw and battery capacity.
Here’s a handy little calculator from Maxim for the supercap.