[Al Skierkiewicz]

Win,
Your biggest issue with 50% rule is determining the 50% SOC point. If you were to accurately determine the current output of the battery and added a fudge factor for battery life, you might be able to predict the actual charge state. Under current robot rules, you cannot add anything between the battery and the PD except the main breaker and associated wire. Due to the nature of the DC source, loop current monitoring really doesn't work. You can however, measure the voltage drop across a known length of #6 wire. This resistance will change very little with temperature but will give you an indication of the current via Ohm's Law. Maxim makes a chip set to monitor battery charging in laptops and other devices made especially for this. The only issue is getting enough resolution over the current swing you would expect. Start current on a four CIM drive could easily exceed 400 amps and with the robot resting, current is likely less than an amp or two. As Don pointed out, battery current is affected by the internal resistance but more important, that resistance has a voltage drop. At nominal internal resistance of 0.011 ohm, 100 amps will drop 1.1 volts. 400 amps will drop 4.4 volts giving you an output of only 7.6 volts. When you add in the drop in voltage for a depleted battery, you are now approaching the point where the power supplies start dropping out. It is this effect that gets most robots into trouble. That is why the field monitors robot voltage in addition to other things (like packet loss, connection, etc.). If we see loss of connection or packet loss at the same time we see a drop in robot voltage, we know what is at issue.
To put this in perspective, loose connections (and bad crimps) on the main wiring, long cable runs and the main breaker can all sum to the same resistance as the internal battery resistance. So at 100 amps you have a 2.2 volt drop at the input to the PD instead of a 1.1v. Remember that includes the red and the black wire together since all robot current flows through both wires.
If we come to Milwaukee next month we can talk further.