Posted 10 October 2012 - 11:52 AM
Here's a quote from FoxKat over at droidforums. It's a response to a similar question I had asked him a few months ago:
You've discovered the unique characteristics of how Lithium Ion batteries need to be protected while charging. During the charge cycle, the charger's metering system is watching both voltage levels as they rise, and also current draw as it decreases while the battery fills. Once the voltage approaches peak optimum voltage, the charger switches from rapid charge (Stage 2 charging) to a lower rate of charge (Stage 2 charging) and then then continues charging until current draw reaches about 3% of capacity and then a very slight drop in voltage occurs. It's this unique set of level changes that signals the battery has reached its safe charging capacity. At that point the charging system shuts down and it then simply monitors the battery levels. If it didn't shut down the battery would continue to draw power but it would be stressing the battery and shortening its lifespan.
While in the rest state, the battery naturally discharges slowly over time. A typical LI battery will lose about 10% of capacity over the initial 24 hours after charging completes. The rate it loses charge is highest right after charging stops and tapers off as it gets closer to the 10% drop, so in the first hour or two it's not unusual for it to lose 2% or 3%, and can lose 5% in only a few hours - even with the phone off. If the phone is on this obviously happens much faster.
Once the meter determines the battery has lost 10%, the charging system resumes the Stage 2 charging to top off the battery until the battery once again reaches 100%, at which point the charging system shuts down and the whole cycle starts all over again. If the phone is off during charging this may never actually happen over the few hours from reaching full capacity to when you remove it, but if the phone is on, it can happen several times overnight. This is one of the two main reasons WHY it's so important to charge with power off at least while doing the training. It is also why so many complain of their phone "losing power" immediately after being removed from the charger. Imagine if it had lost 9% by the time the phone is removed, but the owner is not using a 1% meter. They turn the phone on and in just a few minutes the additional 1% loss from the phone using power causes their 10% meter to drop from 100% to 90%, seemingly having used up 10% in just those few minutes. This is how this happens.
The second reason why it's so important to charge with power off during the training is because with the phone powered on, the "parasitic load" of the phone (using power while charging), can cause the characteristic drop in voltage that can fool the meter into thinking the battery has reached capacity even if it's far from being fully charged. This can result in a battery only reaching 70% or 80% charge level, and that is why many times people think their battery is bad. Nothing could be farther from the truth. If this happens several times back to back, the meter can get so confused about the levels that it doesn't show accurate levels and the potential is that the meter will allow the phone to continue using power even well past where it should have shut down at 0%, causing a deep-discharge of the battery and potential damage or permanent destruction of the battery. Doing the training every 2-3 months will mitigate this and prevent deep-discharging.
