The OpenUPS board on my TurtleBot 2i robot does not work, the NUC does not seem to receive enough power, and the battery is not charging.

You probably need to set up the OpenUPS board again. Start with downloading the OpenUPS software:

OpenUPS software for Linux
OpenUPS software for Windows
OpenUPS software set up manual
OpenUPS firmware

Once the software is installed, click on the "Settings" tab of the pop up window. Click on the "File => OpenUPS" button to load and replace the settings files.