It is a great light. As others have said, it just cuts out dead when the batteries reach a certain level. It is a good idea to do an experiment with the rechargeables that you choose in your kitchen , rather than on the bike. Hope's burn-time figures might be a little optimistic. One thing that you must be aware of is that , with LEDs, you will not get double the burn-time if you double the battery capacity ( eg the 2000 mAh figure stamped on the battery). Google will tell you why far better than I can explain.
Fabulous front light.
Edit: You will actually get more than double. Oh go on! Take a lesson in physics from Google......