looking at the BPI-M1+ schematics it seems that the A20 and its power management companion chip PMU AXP209 already have all is needed to run the RTC included in the A20 even when the power is down.
So probably you don’t need to buy an external module.
It seems that the PCB battery (BAT2 component in the schematic, type NC\XH414H) is simply not populated.
Moreover there should be an external RTC battery connector (BAT1 component in the schematic) which is as well not populated in the board.
If you look on the bottom side of your BPI-M1+ you should find soldering plates that in principle can host the battery and the external battery connector, they are placed near the SD card slot.
Soldering two wires to BAT1 and using a coin cell battery should do the work from an hardware point of view.
I don’t know which is the sunxi A20 kernel support for internal RTC, I’ve never tested.
I was interested in RTC too but I didn’t have time to experiment this.
Can someone from sinovoip check this from hw and sw point of view?