I think what Rukbat suggested about turning the device off if you can't charge it when it gets to 40% is somewhat drastic, but I look at it as more of a guideline. As a general rule of thumb, I try to get it on a charger once the phone gets to the 30-40% range.
The 5% of actual battery charge left when the device reads 0 is a safety net -- but if the battery continues to drain further until it reaches true 0, then my understanding is that it's irreversibly discharged, so that's another thing to be cautious about.
The 5% of actual battery charge left when the device reads 0 is a safety net -- but if the battery continues to drain further until it reaches true 0, then my understanding is that it's irreversibly discharged, so that's another thing to be cautious about.