It should be possible, however, you should investigate different batteries with different specifications to determine which one is best for your project. There are various posts about this, I would suggest you to check the next one: https://communities.intel.com/thread/67372. Two of the users suggest different options, Yodal talks about a battery that he’s using on his custom board, and mpapini suggests using capacitors to maintain the RTC running. I’m not sure if the last option would be useful, I believe you should use something more reliable. But as I told you, you should check different options.
I found a loot of links of how to edit the battery voltage and uA charging, but is it not a easy way to do that? one code/command or somthing.
Also what is the discharge current when only battery is connected to the RTC pin.
Found Sanyo ML1220-HJ1 Litium 3V 15mAh in Norway - Link
From the documentation:
Discharge current (consumption): 8.0 μA
You have to rebuild the kernel to adjust the PMIC settings when you want to change the charging voltage and current.