Jimw, that adds some insight. It looks like the voltage is dropping low enough to reset the RPi. It runs on a 5V rail, which is served by a regulator. The regulator can slice off up to 1V, so if your source power on the BRickPi drops below 6V, that can be leaving the Rpi with less than 5V. A strong motor draw can cause the voltage to suddenly drop like that.
Tested using an 8 x 1.2V 2400mAh power pack today (9.6V), and the test runs smoothly without reboots.
Hey Oystein, awesome and thanks for letting us know. Sorry about the problems. I want to test this a little more extensively but I think it’s a great solution to the problem. I just want to be sure that nothing is getting damaged. Can you let me know if anything happens to your motors.