@lefty: yea, it is understood we measure the battery voltage.. to know the battery is low.. Btw, I've seen an application note at Maxim (an opamp) for current measurement (one op-amp, one pnp trans, 4 resistors - it is an simple i/u converter and it measures the current flowing from the positive vcc rail somewhere). So when using a dual opamp we can measure both and provide a kind of "power management". E.g. you can measure the current consumption of some external devices.. And switch them off when battery is close to die (you will decrease the current, based on its capacity, by switching off/on the shields with lower priority such the battery will last until the mission is accomplished..). Useful when you plan to land on the Mars (Arduino Guidance Computer) 
PS: see the
http://datasheets.maxim-ic.com/en/ds/MAX4240-MAX4244.pdf
fig.11.