T520 battery starts charging when over 96%

I have a problem with the battery in a T520, if the batter is at 99%, and I connect the charger, it starts charging, even though it shouldn't start charging til the charge go below 96%.

When I set custom power values in power manager, for example, I set it to not start charging when below 95%, it respects those values, but then when I disconnect the AC and battery for few seconds, this result in the values being reset, and it starts charging when connecting at any value below 100%.

This is a...

T520 battery starts charging when over 96%