6 CIM robo-rio brown out

[Chak]

The RoboRIO brown-outs at 6.8V. What does that translate to in Amps? Based on my limited high school physics knowledge, I think it depends on the battery's internal resistance, but is there a set number for that?

If not current, how else would we know the voltage drop caused by the current draw of a design? I see warnings about 3 CIM drivetrains everywhere, but is there a way to calculate/estimate the voltage drop and mathematically prove that 3 CIM drivetrains will brown-out? It would be interesting to see whether a 3 CIM drivetrain would work under certain situations, or if 3 CIM drivetrains are just not possible.