I understand this is an old topic but also a very good topic. I wish Davis would implement a way to see on your console how much ISS battery life is left, how much is used each day/night and such once the supercap is deprived of energy. Would be pretty cool and nice to see, and it wouldn't catch you off guard.
If you study the Davis Serial Communications Protocol, and the Loop command, you will see that the station battery indication is binary: OK or LOW. This is how almost every hardware or software product gets battery level info from the ISS and other stations.
Similarly, if you search the forum for those that have sniffed and decoded the wireless transmissions, I believe that you will find that the battery level sent from the station to the console is also binary (OK/LOW).
Now then, a quick survey on BatteryJunction.com shows CR123A capacities of 1300-1550 milliamp-hours, a difference of almost 20%. Since Davis can't control which CR123A you put in your ISS or how old they are, any life calculation is a crap shoot, especially adding in temperature considerations.
So, changing the data protocols to add the data you desire will probably break the data connection to almost every hardware device or software program currently getting data from a Davis station, and perhaps make new ISSes etc incompatible with existing hardware.