I need connect a RTC backup battery to the board (2Ge8G-IND, Rev. E) and I'm kind of lost ... Should it be connected to GPIO1 pin4 (+) and pin2 (-) or how is it supposed to work?
No. Backup battery is on separate pads named "BACKUP". These are two pinholes available near the UART0 (serial debug) pins. Refer to the schematic.
The battery should be 3V. Check on the AXP209 datasheet for the requirements and properties of the backup RTC battery.
Edit: this comment is not applicable for A64 boards, it applies for A20 board. I misunderstood.
I can't find anything like that in the schematic (https://raw.githubusercontent.com/OLIMEX/OLINUXINO/master/HARDWARE/A64-OLinuXino/1.%20Latest%20hardware%20revision/A64-OLinuXino%20hardware%20revision%20E/A64-OlinuXino_Rev_E.pdf) nor do I see any pinholes (besides the UEXT ones) near the UART0 (three pinholes on the lower right):
I also need RTC, how do i connect the battery, there is neither a "BACKUP" pad nor pin holes near Debug UART, can you please help with proper reference?
Sorry, as you can see my previous comment was about AXP209 and A20 boards.
For A64 boards and AXP803 there is only a pad for the RTC battery, it is located under the USB host connector ("big" USB) at the bottom of the board, and the pad is called "3.0V_RTC1".
The battery should be 3V. Check on the AXP803 datasheet for the requirements and properties of the backup RTC battery.