The batteries are floating signals; thus, when you read them in differential mode, you must use bias resistors (about 47 kiloohms) in order to discharge the bias currents. Otherwise, you might see unaccurate readings. I have seen this problem before, and maybe this is what it is causing to get bad readgins. By the way, the bias resistors should be connected from the negative lead of signal to the AIGND of the NI board. Finally, see if the force sensor signal and the encoder digital signal are floating or grounded.If they are also floating, that might be the issue.
Hope this helps.
Aguila