Originally Posted by Fountainhead

Guys,

Cell phone batteries are mated specifically to the phone, therefore the phone has a unique regulator/temp sensor/voltage monitor that controls every aspect of the phone's battery. You can't "overcharge" or "overcurrent" the phone battery.

Each battery has a discharge curve, that curve is managed by the battery IC in the phone, there is a maximum that cannot be exceeded.

Each phone also has a charge curve, that curve is also managed by the battery IC in the phone, there also is a maximum that cannot be exceeded. This battery IC also monitors ambient temps and the temp of the battery and will decrease charge current based on battery temperature vs. ambient, a certain rise above ambient will trigger a change in the charge voltage/current curve.

Look at the current rating of the phone charger that is the maximum current the phone will use to charge the battery at the acceptable nominal rate AND run the phone and screen etc., so the user can charge and use simultaneously. The 5V USB voltage is used by the battery manager as follows:

Inside the phone the battery manager system monitors the battery voltage, there are these circuits called a buck and boost, they take the 3.7 volts and by means of a switching supply very efficiently increase/decrease the battery voltage to what ever level the circuitry of the phone requires. Usually the core of the SOC requires about 1.8 volts, other ancillary circuitry requires 3.3, and other parts - the radio transmitter etc., may require more than 5 volts.
What you also cannot do is drain the battery down too far (all phone batteries are 3.7 Volts, and when the voltage of the battery is down to say, 3.1 or 3.2 volts the phone will turn off and the battery is "drained" to the customer but actually the battery is far from dead.

Each Lion/LiPolymer battery has a certain number of full complete charge cycles in is lifetime, when those are exceeded you will notice that the battery may charge but won't last as long, this is because the depth of charge decreases as the charge cycles erode the chemicals in the battery, the chemicals only last so long so after a while the chemical reaction stops at a level below what the phone battery monitor will accept.

Based on my experience, it's not good to leave the phone connected to a charger continuously, the battery seems to decay much faster as the chemicals in the battery cease their reactions and actually build barriers between each other, so I charge my phones during the day and leave them off the charger at night. Conversely, other people charge all night and are off the charger all day. YMMV.

Sorry to be so long winded but I wanted folks to understand charging - the car battery has nothing to do with the phone battery longevity.