Many Samsung Galaxy Tab 8.9 and 10.1 users who have flashed custom ICS ROMs report problems with battery drain. Some cases have been so bad that the battery has been draining from 100% down to 0% within just two to three hours.
Many users were speculating problems in the ROM or kernel with high CPU usage causing the drain, but it has been found out to be an issue with the battery calibration. Luckily, this can be fixed by calibrating the battery again. Keep in mind that re-calibration will require some patience on your part, as it will involve letting the battery drain, sit idle, charge and sit idle again for fixed time intervals, and this timing is important.
The method for the Galaxy Tab 10.1 comes from XDA Senior Member Vlad_z, and can be found in the original thread. The Galaxy Tab 8.9 guide thread is based on the method used in the previous thread, and is brought to us by XDA Senior Member pseudoheld.