Can leaving a cell phone hooked up to the charger harm the battery?

[Rigamarole]

Usually when I am home, I leave my cell phone in the charger all the time even after it has reached its full charge. (yes I know this wastes energy but that's not my question)

It was suggested to me that this can make the battery "dependent" on the charger and reduce the amount of charge capacity it can hold, effectively damaging it. I was skeptical since this theory makes it sound like the cell phone needs to go into rehab, but I just don't know for sure - can a battery really become "dependent" on a charger and become damaged in this way?

[IAmNotSpartacus]

The electricity can't be stored by the cells, it has to go somewhere. That somewhere is heat, which will eventually cook your battery. Most cell phone batteries today are Lithium Ion, overcharging the battery will definitely shorten its lifespan.

[Oakminster]

Well, I've been doing that to my cell phone every day for the last 5 years, with no problems at all. Original battery.

[seodoa]

Originally Posted by IAmNotSpartacus

The electricity can't be stored by the cells, it has to go somewhere. That somewhere is heat, which will eventually cook your battery. Most cell phone batteries today are Lithium Ion, overcharging the battery will definitely shorten its lifespan.

Modern charging circuitry will reduce charge to a negligible trickle when the battery reaches full capacity, preventing overheating. Overheating is more of a serious issue with laptops, which when running usually generate a considerable amount of heat even when not charging. That being said, when storing lithium-ion batteries for long periods of time, it is best to store it at partial charge rather than full charge.

This is a great website that answers all kinds of questions about proper battery usage and care: http://www.batteryuniversity.com/

Also, another anecdote:

I have an older cell phone (about 3 yearsish) that has, for the past year and a half, served as an alarm clock, plugged in virtually 24/7. Even when I used it as an actual cellular phone, it remained plugged into the charger whenever I was in my apartment.

The other weekend I unplugged it to lend the charger to a friend and the phone's battery lasted for four days before dying. I'd say the battery has suffered very little more than usual expected wear.

[chromaticity]

Yup, Like seodoa says, trickle charging keeps the battery topped up when plugged in.
Also most phones check the temperature of the battery and go into a wait state if it exceeds a threshold.
If the battery temperature cannot be monitored for some reason (unsupported or damaged batteries), the charging is done until a "safe" threshold which would not cause too much overheating (hopefully).
Of course all this is not foolproof, but reliable enough not to worry about IMHO.

Older batteries had problems with hysteresis which is another kettle of fish, not such a big problem with newer battery technology.

Probably most people change their phones before they need to change their battery..

[kanicbird]

IIRC Li-ion batteries have a shorter life span if kept at 100% charged, their longest life will be at 40% charges, and shortest life at 0% charged.

[bouv]

Originally Posted by kanicbird

IIRC Li-ion batteries have a shorter life span if kept at 100% charged, their longest life will be at 40% charges, and shortest life at 0% charged.

So you're saying you're best off stopping your charger when the battery reaches 40%? I'm pretty sure this isn't true. Modern Li-ion batteries and chargers are designed to work best when the battery is at as much charge as possible, and being re-charged as soon as possible (ie, not waiting till it gets close to 0%, like you would with Ni-Cad.)

[susan]

I have a new phone that aparently has a sensitive battery. There isn't a car plug charger, and it enjoins me to unplug it as soon as it's charged.

[IAmNotSpartacus]

Originally Posted by bouv

So you're saying you're best off stopping your charger when the battery reaches 40%? I'm pretty sure this isn't true. Modern Li-ion batteries and chargers are designed to work best when the battery is at as much charge as possible, and being re-charged as soon as possible (ie, not waiting till it gets close to 0%, like you would with Ni-Cad.)

This sounds like SOP for storing Li-Ion batteries, not for daily use. Running a Li-Ion battery all the way down before recharge, on a constant basis, will shorten the lifespan of the Li-Ion battery.

As for trickle charge, Li-Ion batteries do not mix with trickle charge cycles at all, in fact Li-Ion batteries cannot absorb overcharge at all (and this is why you must use a charger designed for Li-Ion-- using a NiCD/NiMH-only charger will cook the battery). Proper charge procedure for Li-Ion is to go full steam until the battery is fully charged, generally 4.20V per cell.

Also most phones check the temperature of the battery and go into a wait state if it exceeds a threshold.

Not so much. The circuitry monitors the voltage of the cells. There are thermistors built into the circuitry to deal with temperature issues, and generally there will be a pressure device as well that will open the circuit if the cells expand beyond a certain point.

There are "smart" batteries and chargers out there, that will communicate with one another regarding cell health, maintenance cycle, battery age and so on, but you will rarely find any such thing in the cellular phone market. It is becoming very popular in the two-way radio market, however.