I apologize if I've missed the answer to this question in the many posts I've read about battery charging, but why is 15% the magic number? For example why not 10%?
If I remember reading his post correctly, Fox said that the lower it goes below 15%, the more chance you have of it not accepting the charge. I very well could be incorrect though
Rogus, the phone's charging system is set up to recognize 15% as the reserve tank, so to speak. It's related to the way a Lithium Ion Polymer battery's voltage decreases during use. All batteries have what's known as a discharge curve, essentially a plotting of the voltage of the battery at given time intervals while being used, a load as it is called (think of you carrying a 20 lb bag of potatoes as a load and the extra energy you would expend carrying it), in this case the load is the phone's cellular radios (1X, 3G, 4G)
, WIFI, Bluetooth, GPS, apps, etc.
A fully charged LIPO battery will have a voltage of somewhere between about 3.9 and 4.2 Volts, depending on how recent charging was completed (voltage will drop slowly after completion of charge as the battery balances itself out). While being used the voltage will also drop but LIPO batteries don't decrease in voltage at an even rate through it's discharge cycle. It's the three unique curves (rates of decrease) that indicate the battery is in the top 20%, middle 60%, our bottom 20% of usable capacity. The top and bottom portions see increased voltage level change during discharge whereas the middle 60% displays a slower, more gradual voltage decrease with the same load.
It's the middle section that causes the meter to become confused about the charge levels since even though current is being used up, the voltages aren't dropping as quickly as they do in the top and bottom ranges. So the meter uses the sudden increase in voltage drops that happen within the last 20% of usable charge to determine when the battery is approaching complete discharge, and at 15% of usable capacity is when the meter sets the low battery flag.
And yes, jackiescivic, there is risk of the battery becoming unresponsive at levels approaching 0% of usable capacity, but those risks are minimal if the meter is calibrated properly. However, those whose phones have become unresponsive after a discharge to 0% of usable capacity were victims of the meter errors I've tried so hard to help everyone understand how to avoid.
Sent from my DROID RAZR MAXX using Xparent ICS Tapatalk 2 using Google voice to text translation. Please excuse any minor spelling, punctuation, capitalization or grammatical errors.