In my latest spin, I traded the LPC8N04 for the NHS3152 so I can make use of the added analog features. The added ADC will allow for reading voltages, so this business card will be able to act as a volt meter.
The NFC Forum defines NFC Data Exchange Format so NFC devices know how to communicate with each other. Using one of the standard formats allows phones to recognize the data and act accordingly. The business card uses a standard v-card type to share contact information, but there is no standard type for a volt meter. I could create an app to implement a custom NFC protocol, but there are some other standard formats that can be utilized without requiring a custom app.
One very flexible format is the URL type. This format exists so that you can tap a tag that will direct you to a website. To enable reading the voltage without an app, I will create a voltage display web page and pass the data to the page in the query string of the URL.
This way the business card/badge will also serve as a volt meter that doesn't need batteries and never needs to be recharged.