Paul,
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).
...Scott