I found this in the picaxe manual 2 and I found it works well.
The calibadc command reads the value you would get if you were to read an analog input at 0.6V. the second line recalculates the present voltage for the picaxe (times 10). So if you feed the picaxe 4.8V -> b0 will have a value of 48.
Off course: if you use a voltregulator (like me) you will allways get a steady 5.2V until the battery output is so low that the voltreg doesn't do anything and the voltage to the picaxe drops below 5V.
If you run your picaxe directly from the battery these commands should work well.
I really like the concept. If your bot needs 4.6V min, you could use a U-type attached to an LED which would go out when your Vbat dropped below 4.6V. Or use it as an input to your MCU to cause a light to flash at the danger level.
This would be no good as a standalone battery checker, though. Certainly, it could be used standalone battery voltage checker, but that information might not ba any use to you. This device only draws 1uA at 5V, so it won't tell you what your battery voltage will be once you attach a 500mA robot to it. You'd need to build it into each robot.
My personal solution is to use an ADC input to my pic to constantly measure the battery voltage. Of course you need a voltage reference - select an MCU with an inbuilt one.
...or read up on brownout detection...