Onboard Voltage meter/batt life display

[JThiessen]

I'm looking for a small device to wire into a system that will show remaining battery life. Its for a non RC application, but we will likely be using cheap rechargable Nimh batteries - something we can readily purchase that is enclosed in a case. Low voltage, but around 5K mah.
This meter (and the battery) are going to have to be able to handle the abuse of an industrial environment, and be plug and play (no diagnostics - if it dont work, we'll just replace it).