"-RBS-" wrote:

... I can not find any configuration tab/page in BOINC Manager

The configuration is performed on the Boinc website.  This is one way:

Open the Boinc Manager
On top, click on the Projects tab
Click on the "Your Account" button, which will open your browser to a 
webpage with the heading "Account Data" (progress)

In your browser,
In the blue field on the left side of the webpage, click the link "Your 
Account," which will open a webpage with the heading "Your Account."
Scroll down to the Preferences section, and click on the link "View or edit 
general preferences," which will open a webpage with the heading "General 
preferences."
Scroll down the the link named "Edit Preferences," which will open a webpage 
named "Edit General preferences."  The first preference on this page is "Do 
work while computer is running on batteries? (matters only for portable 
computers)."

That should get you there.

Yes, it is weird that adjusting the battery usage preferences should be 
performed on the website.

-N-