Hi C,
He has a total of 3 wires, Post #1.
Two of them he is using to power the UNO.

update;
The yellow wire has approx 4v on it, my guess it is the centre tap of the two Li-ion calls
E

 

So, yeah, after some tests without the Arduino, I'm pretty sure it's as Eric suggests: a center tap. Still not sure why the A0 on the Arduino was off; I considered whether it was because the battery was charging the Arduino, lowering the red line's voltage and thus the yellow line's voltage, but even when powering the Arduino, my multimeter still read the voltage as ~ 8v. So, battery mystery solved, at least. Thanks for everyone's help.

 

hi these,
If you are still having problems reading with the ADC, post your circuit, the Sketch works OK.
E