I am running the NUC from a 12v lead acid battery, want to monitor the voltage and if possible shut pc down when it gets too low. The desktop software shows me internal power supply voltages but not the battery input voltage. When I try to get the battery monitor to appear via control panel, the link is there to turn the feature on, but the control is greyed out.
(Its the feature a laptop would use)
In mobile systems (laptops, etc.), there is a EC (Embedded Controller) included on the motherboard that provides battery management that Windows can query. The NUCs are considered desktop systems and thus do not have an EC. I have seen UPS-class products that provide a USB interface to support Windows monitoring the battery's state, but I have never seen a battery that does so.
What you can do is monitor the internal 5V and 3.3V rails. Seeing voltage droop on either/both of these will provide a reasonable indication that the battery is running down. Though not officially supported, Intel Desktop Utilities (IDU) 3.2.8 will install and can be used on the D54250WYK/B and D34010WYK/B boards (note that previous generation NUCs are definitely not supported, however).