The battery is 300 cycle rated so wear will happen. Just be sure it's the charger that came with the unit.
If you want to be sure call in your concern now, do not delay.
My Acer R 11 Chromebook is new; purchased on 12/26/17. The first couple of days that I used it, it recharged to 100% overnight. Now, it only charges to 96% overnight. It isn't an "issue" in the sense that I run out of battery prematurely. After a full day of use, there is still about 25% battery remaining. My concern is that this might be a precursor of an issue down the road. Thoughts? Thank you for your input.