You can "technically" let the battery drain to 0% (where the phone powers down itself), however if the meter isn't properly calibrated to the battery (not the other way around), then there's a possibility that when the phone eventually shuts down the battery may actually be close to a critical level that will fail to respond next time you try to charge it. There have been many "bootlooping", "white light of death", "black screen of death", "power cycling" threads where they all originated from a phone that was allowed to drain and power down on its own. For the record, I've left my phone drain to 0 and power down a number of times with no negative consequences, but did so just to see if it would fail as others' have. Of course, my meter is properly calibrated to the battery so I felt confident it would respond as it did. I attribute the failures to the meter inaccurately representing more power at the nearly completely discharged level than the battery actually has, so the phone doesn't shut down soon enough to prevent deep discharging. So a healthy battery meter will result in the auto-power down executing at the right voltages and leaving enough power to allow for connecting to a charger later and being able to boot into charge only mode. Excellent deduction, Watson... The phone indicates "Low battery" at 15% (10% for Jelly Bean), for three very good reasons. That's when the voltages start to drop faster than it does over the previous 70% or so, and that's the indication that the meter is looking for to determine when it's approaching the cutoff voltage for full automated power down. That's a good level to warn you of impending doom (like the Low Fuel light on your car's dashboard), essentially telling you that you are now running on the "reserve tank". This way you have time to save any work you're actively working on, say goodbye to the person you may be on the phone with, finish that last text or email and send it, make the emergency call to warn the wife/husband/BF/GF that the phone is about to die, and get to the nearest source of power to replenish the phone's battery. It sets the flag for "Low battery" at that level, and then with that flag and the one it sets at 100% charged, can now make fairly accurate estimations of how much power is remaining anywhere along the discharge curve. Without those two flags, it would be guessing (inaccurately), and would begin to stray from the actual numbers farther and farther as time and power/charge cycles are completed. This is not so much a meter that's not calibrated as it is a battery that's not falling within the ranges that were spec'd for the phone, or in other words, a battery that's simply not 100% compatible. It may power the phone, but as you and others have found out, it will probably provide terrible level indications.