A large number of Mac users are reporting that installing OS X 10.8 (Mountain Lion) is causing a major drop in battery life on MacBook Pros and Airs. Those users have been posting about battery life problems in Apple's support forums ever since Mountain Lion's launch on July 25, but it appears that Apple is now actively investigating the issue.

Users claim that battery life reported by their systems is significantly lower after installing Mountain Lion—in some cases, it's nearly half of what was reported under Lion. The batteries themselves also seem to drain noticeably faster, with more than one user claiming that it was possible to watch the battery indicator drop in capacity.

Some potential explanations include file permission errors, increased CPU use while applications are idling, and network access issues. But despite these theories, no clear fix has been identified yet.

This week, however, Apple began contacting users who have reported the problem in its support forums. Support representatives from Cupertino have been asking users to log activity monitor data and gather system information in order to determine a cause for the problem. (Apple did not respond to our request for comment on the issue.)

My own unscientific tests on Friday morning suggest that battery life may indeed be lower under Mountain Lion on a 2010 MacBook Air. Previously, while running Lion, the system would typically report about 4.5 hours of battery life with my usual set of applications open. With Mountain Lion installed, it now reports just under 3.5 hours under the same conditions, a roughly 20 percent reduction. The battery also appears to lose 1 percent charge every 2 minutes, which is roughly equivalent to the expected run time of just under 3.5 hours.

More tests are needed to prove conclusively that Mountain Lion does drain batteries faster, however, and hopefully Apple's investigation will turn something up. Have you noticed a decline in battery life since installing Mountain Lion? Let us know in the comments.