Frank,
I think I've seen this answer to your question before, but I'll reiterate it. It seems that you are using a 12-bit MIO DAQ board (such as the 6070E) in conjuction with your SCXI-1102. If your MIO board is set to bipolar +/- 10V, then your resolution is 0.00244 volts per bit. Therefore, a 1-bit change on your A/D is .00244 volts, which is what you're seeing. A 1-bit noise factor is not uncommon.
Also, the discrete voltage values of the D/A conversion don't always have an absolute 0 value in bipolar mode. Therefore, a short will give you a reading very close to zero, but not exactly zero. There is a good discussion of this in the beginning of the Measurements section of the NI catalog.
As far as the 1.5V battery, batteries usually read high when not
under a load, so it's not surprising that you read 1.5437 volts, with a 1-bit change of 0.00244 volts (your 1.5462 reading).
I'm not sure what you mean by an open channel reading 5 volts, but keep in mind that any inputs which don't have a signal attached float because the input impedance of your MIO board is very high. For example, if you connected your battery to one of the inputs, several of your inputs may also "track" this voltage until another input of lower impedance is connected to it.
Mark