What is the effect on battery life of enabling glonass support on garmin edge 25? I'm considering the purchase of a new GPS. The Garmin Edge 25 has most of the basic features I need. However, high accuracy is important to me as I do trail mapping for my local MTB club. I thought that getting a unit with GLONASS support would help improve accuracy, but I was concerned about runtime since the manual indicates it will reduce battery life.

What is the impact on battery life of enabling GLONASS support on the Garmin Edge 25?
You're only going to experience roughly a 10%, (maybe 20% max) loss of battery life. Unless you're planning on frequent century or all-day rides, the difference is relatively negligible. The average use case, a commute to and from work, or the average 4 hour group ride, won't see any difference because the battery life will go from about 8 hours on a full charge to 7 hours.

If you do plan to take it on century rides or night rides, you might want to upgrade to a medium tier unit, 510/520/etc, as the medium sized units have nearly twice the battery life. Using the GPS+GLONASS mode doesn't have a significant effect on battery life at all in this case because we're looking at about 16 hours of battery life in GPS-only mode vs 15 hours of battery life with GLONASS. Unless you're living on the edge (no pun intended), the GLONASS mode isn't going to make it or break it.