I want to enable filesystem check when on battery..becaus after skipping almost 30 filsystem checks when on battery, it takes quite long to check the filesystem when rebooting next time connected to power.

thanks in advance

I don't have an answer for you offhand, but just to make sure you understand what you're trying to do - first, how many checks it's skipped due to being on battery has nothing to do with how long it will take. It does the same series of steps in a check on the same data. Second, there is a very good reason why they configured it to behave the way it does. If a fsck is interrupted - due to your battery running out (you really can't tell how much you have with the system in that state), getting dislodged, anything - there isn't much limit to how badly your filesystem could be broken. I'm sure there's a way to disable that check somewhere, but there are very good reasons to accept the developers' decision here.

of cource iam ignorant abt the process..but sometimes it does take a lot more time than normal, some 2-3 min for checkin the filesystem.

If you're interested in exactly what fsck is doing check out this link - old, but still relevant. Towards the bottom, it details the actual steps involved in fsck (though you may need to read the rest of the page to understand that part). That said, 2 to 3 minutes ain't much - I have a 1500 GB drive that had to be fsck'd yesterday, and it took about 10 times that.