Battery life is shorter than it is shown in display

At my camcorder, after I fully charged the battery, in the display it is written that I can record 100 minutes, but after 20-30 seconds, the value changes to 99 minutes. In the end I can only get 30 - 40 minutes recording insted of 100 minutes. I have both BP-808 (original and old one), BP-809 (new but not original one) batteries, the problem appears on both of them. If someone can help me, please. Note: I'm charging batteries by directly camcorder.