Thank you for contacting the Intel Embedded Community.
The information that may help you can be found in sections 3.3, 3.2 ,and 5; on page 15, 12, and 19 of the Intel(R) I/O Controller Hub [Intel(R) ICH] / Platform Controller Hub (PCH) Family Real Time Clock (RTC) Electrical, Mechanical, and Thermal Specification (EMTS) - AP-728.
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Thank you for your answer. The link to the document of the Intel I/O Controller Hub was useful to understand the RTC in more detail. In the latest revision of the E3800 family datasheet (Rev 3.9) I found this :
A measurement of the RTC_VCC did show a max. value of 2.8uA and with the typical value of 1uA I will do my calculations for the backup battery.