I have been designing the circuits to measure the High Voltage stuff. It's challenging because the battery pack is isolated from the car chassis (i.e. it 'floats' relative the car ground.) But the A/D converter is referenced to the computer ground which is connected to the car ground. That means if any part of the battery wiring shorts out to ground, the current will try to flow through the DAQ - not generally good for it. A simple way to do it is to tie together the reference on the A/D and the (-) side of the battery pack through a high value resistor (like 500,000 ohms). That will limit any current should a short circuit happen, and will skew the measurement a few millivolts under normal conditions. Still need to do the math to make sure everything is right, but I think it will work. A few MV is nothing when measuring on a 300 V scale, so I don't care about the skew. The alternative is an expensive signal conditioner that I really want to avoid buying.