See this post for further documentation.
Even if documentation is interesting it lacks ( at first sight ) two things
- what is the reference level for this i2c communication ( is the negative pole or the positive pole )? Doing it wrong means frying a $500 battery
- are pullup resistors included ( I bet no but... )
As I understand there are 2 packs of 12V batteries inside and two separated i2c circuits.
How are thes 12V packs connected? Are completely separated or have a common connection or what?
P.S.
additional information at ultralife
NOTE
Connecting the wrong way you risk damaging the battery logic circuit