If I leave my cell phone with the charger plugged in all the time, would this weaken the batter, and why? I've heard that you should only charge the cell phone when you receive a 'battery low' message and leave it off the charger at other times?
The cell phone has got a Li-Ion battery.
All newer phones use Lithium polymer batteries.
To decrease their aging they are intended to be stored at 40% charge. This means when you receive your phone it should be at 40% charge, otherwise they will have aged your battery for you. (you are probably used to the effects of aging, like a 2 year old phone seeming to have very short battery life). When you get your phone you can use it until it is discharged, but they normally say 'charge it' because people will not notice the partial charge.
You should not fully worry about fully discharging, this is superstition to earlier battery technologies. Fully discharging a lithium battery is one of the best ways to make it fail. below a certain charge they will have their overcharge protection circuitry fail and you cannot charge it at all. I have seen studies that show that this makes up more than 75% of "failed" lithium batteries.
Lithium batteries have a set number of charge discharge cycles before they fail. This might be a number like 500 cycles. You actually get more like 1000 cycles if you only discharge to 50% before recharge. Lithiums really do not like a deep discharge, I cannot stress this enough.
If you would like more information about lithium battery technology let me know, I can get you many links, just drop me a comment. I have a few answers on the electronics and robotics stack exchange about it.
For example, my Lenovo laptop will not apply a charge to the battery unless it is under 97%. When it does charge the battery it charges directly to 100%, then stops until the battery sags below 97%. Many laptops did not do this, on most just applying charge if it is not 100%. This would put the battery through thousands of charge cycles in a week when you are not using the battery. This ages a battery quickly.
If your phone maker took the time and paid the extra cash then your phone will stop charging once it reaches full charge and just power the system from the wall outlet. It is significantly more likely that your phone is charging your battery on a short cycle and aging it thoroughly.
Some people have some confusion from some of the myths that go about. The primary one is memory. As Battery University will say, this is mostly extinct, and actually applies to nickel-cadmium batteries. As was stated in a comment about crystals Battery university has in reference to nickel-cadmium:
With memory, the crystals grow and conceal the active material from the electrolyte. In advanced stages, the sharp edges of the crystals penetrate the separator, causing high self-discharge or electrical short.
Now, talking about Lithium batteries, which your phone uses, there is even more difference. To quote them battery university directly from their simple guidelines:
I understand how this may go against what you have been taught, but I am someone who not only has research this but uses lithium batteries in my day to day work as an engineer.
It will contribute to weakening the battery, but not for obvious reasons, and maybe not as much as you might think.
The classic problem with cheap battery chargers used to be that they overcharged the battery. Lead-acid, Ni-Cad, and NiMH all tolerate some amount of overcharging. Lithium-ion cells must not be overcharged. Overcharging raises their voltage to the point that they break down internally (usually they will catch fire). Therefore, when a Li-ion cell is fully charged, the charging stops.
Where we run into a problem is in the fact that the life of the battery is affected by its charge level. The breakdown of the electrolyte inside happens gradually, but it happens fastest at high voltage--namely, at full charge. So, this is where leaving it on charge all the time is going to reduce its life. The actual effect of this varies with battery chemistry and with its relation to some other factors. I will mention these briefly:
Age: Cells deteriorate with time. A 3-year old battery has been gradually deteriorating for 3 years, and there's not much we can do about this.
Cycle wear: This translates directly to how much the battery is used. It's not really how many times you've charged it; it's more like the total power that's been removed and put back in (as if the battery had an odometer).
Heat: Heat accelerates the breakdown of the electrolyte. If your device keeps the battery hot, it will deteriorate a lot quicker. Note that I said the battery itself isn't being charged when plugged in at 100%. But my Macbook gets plenty hot just when it's on and that means the battery inside is being kept hot. On my Acer, the battery sticks out the back and it never gets hot.
So the life of the battery is affected by all these, and if one is taken to extreme, that will predominate the life calculation. Otherwise, it's kind of a variable mix of things.
I did mine research and found out that lithium based batteries don't suffer the memory problem but allowing your battery to drain to much can in fact do more harm than good, you can top up your phone anytime you wish, it's always advised to keep your battery between 40% to 80% charged, as for leaving on the charger, yeah it's safe as I always do and have no problem and it's over 2 years I have mine phone
