What do I need the batteries for?

In the main menu, tap successes. There you can see in the area Everyday learning At the bottom, how many batteries you have.

If you forget to study for a day, your streak would actually be lost — i.e. your “winning streak” would be interrupted. You can also see these in the “Daily Learning” section.

This is avoided if you have a battery: It is then automatically used on your lost day. However, you can only bridge it for one day, and your streak is then interrupted on the second day.

‍