Hi, I'm not getting why it is that when I put one probe of a voltmeter on the positive end of a battery and the other probe on the negative end of another battery (which isn't in circuit with the first battery), it only measures about half of the voltage that it does when I put the second probe on the same battery. Can anyone help?

 

What do you mean "which isn't in circuit with the first battery"?

How are the batteries interconnected?

Show a circuit diagram.

 

Ah, here it is.

http://imageshack.us/photo/my-images/33/75885098.png/

 

Doesn't work like that.
The two batteries must be connected.