Why is my keyboard battery not charging?

The computer started having some weird issues where the both the tablet and keyboard batteries were used until the tablet dropped to 80%, then the keyboard battery took over until dead. One day, I let the keyboard battery drop all the way to 0% and after that, it never charged again. I can only charge the tablet and the keyboard works only when plugged in. Could it simply be the battery or something wrong with the motherboard. The computer is only 3 years old and I checked with another charger and nothing happened.