Notebook goes to sleep because of low battery while pluged in

Hi! I have a Levnovo Ideapad 700 and lately it started to go to sleep randomly, it happens a few times a day. The event viewer says it’s because of battery (see image, sorry that it’s in Spanish but in general it says “Description: The system is entering sleep. Sleep Reason: Battery), but the thing is that the notebook is plugged in with 100% battery most of the time, though it also happens when is in battery mode with the same frequency even with >90% battery.

Before all this started, I had a weird issue in which the battery percentage would go from 100% to around 15% and then 100% again in a second, but I just procrastinated solving the problem (dumb decision, I realize now)

On additional notes: The battery is not removable in this model (I haven't opened it though). The battery it’s almost two years old, is the original that came with the notebook in January from 2017. I understand that it’s kind of old but this is just too much.

Thanks in advance, i usually avoid asking things becaue it seems that everythig is already answered somewhere but this time i couldn’t find a solution : /