Studio display charges MacBook permanently

Hi. 

I am using a new 27" Studio Display as an additional monitor with my MacBook Pro 15". It's connectd by firewire on the respective slot. When using this setup, The Display permanently charges the MBPro up to 100%, which in my understanding is not very healthy for the battery. In the instructions it says there is an option in settings that allows to regulate that, but I cannot find this item neither in "Battery" nor in "Displays". 

Is there a solution to this permanent charging?

Thanks.