I usually disconnect my cellphone from the charger some time after it's fully charged because I'm afraid the battery could lose capacity over time if I don't. However, I believe modern electronics should be able to handle this kind of scenario and automatically stop charging the battery once it's full. Does this apply to modern Android phones? Or do I need to continue disconnecting it all the time?
As njd pointed out, most cell phone batteries are Lithium Ion now.
Although constant charging cannot hurt, it looks like you may want to periodically run the battery all the way down so the digital circuits can correctly calibrate:
Although lithium-ion is memory-free in terms of performance deterioration, batteries with fuel gauges exhibit what engineers refer to as "digital memory". Short discharges with subsequent recharges do not provide the periodic calibration needed to synchronize the fuel gauge with the battery's state-of-charge. A deliberate full discharge and recharge every 30 charges corrects this problem. Letting the battery run down to the cut-off point in the equipment will do this. If ignored, the fuel gauge will become increasingly less accurate.
As for battery life, temperature is apparently a factor -- the hotter the environment, the more capacity loss over time. And storing the battery at 100% charge is actually unhealthy for Lithium Ion batteries!
Great set of Lithium Ion battery use tips here: http://www.batteryuniversity.com/parttwo-34.htm
Avoid frequent full discharges because this puts additional strain on the battery. Several partial discharges with frequent recharges are better for lithium-ion than one deep one. Recharging a partially charged lithium-ion does not cause harm because there is no memory. (In this respect, lithium-ion differs from nickel-based batteries.) Short battery life in a laptop is mainly cause by heat rather than charge / discharge patterns.
Batteries with fuel gauge (laptops) should be calibrated by applying a deliberate full discharge once every 30 charges. Running the pack down in the equipment does this. If ignored, the fuel gauge will become increasingly less accurate and in some cases cut off the device prematurely.
Keep the lithium-ion battery cool. Avoid a hot car. For prolonged storage, keep the battery at a 40% charge level.
Consider removing the battery from a laptop when running on fixed power. (Some laptop manufacturers are concerned about dust and moisture accumulating inside the battery casing.)
Avoid purchasing spare lithium-ion batteries for later use. Observe manufacturing dates. Do not buy old stock, even if sold at clearance prices.
If you have a spare lithium-ion battery, use one to the fullest and keep the other cool by placing it in the refrigerator. Do not freeze the battery. For best results, store the battery at 40% state-of-charge.
Modern cell-phones all use lithium-ion batteries, which work best with frequent top-up charges.
They have circuitry to monitor the input voltage and prevent over-charging.
Some chargers get quite warm while connected to the mains supply, and that's just wasted energy; so you might want to disconnect the charger from the mains once your cellphone is fully charged, but there will be no damage to the battery if you leave it charging a few hours longer.
All devices should stop charging once they're full. However, they might start recharging when the charge drops down to 80-90% depending on the manufacturer.
If I'm rights most batteries should be rated for about 1000 charge cycles before serious degradation occurs. If you charge the phone at night, in the car and at work that would be about one year. After that you might as well buy another battery since they are getting really cheap. Just dispose of the battery environment friendly.
Here are the real expert answers from Electrical Engineering, our sister site:
Not directly related, but also a nice read:
Tricking the charging circuit considered harmful
TL;DR: The charging circuit is safely engineered to be connected to the charging circuit 24/7.
Modern electronics will not overcharge battery, and phone should be able to handle being constantly plugged in. However keeping battery constantly in high state of charge and charge cycling in high voltage will increase battery degradation. It's best for battery when phone is minimal time in full charge.
My personal experience is that my android phone SGS1 handled overnight charging all well so it's not wise to get too worried about this, but if your battery never last long keep eye on that.
Some portable devices sit in a charge cradle in the on position. The current drawn through the device is called the parasitic load and can distort the charge cycle. Battery manufacturers advise against parasitic load while charging because it induces mini-cycles, but this cannot always be avoided; a laptop connected to the AC main is such a case. The battery is being charged to 4.20V/cell and then discharged by the device. The stress level on the battery is high because the cycles occur at the 4.20V/cell threshold.
Chargers made for operational readiness, or standby mode, often let the battery voltage drop to 4.00V/cell and recharge to only 4.05V/cell instead of the full 4.20V/cell. This reduces voltage-related stress and prolongs battery life.
To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible.
Most Li-ions are charged to 4.20V/cell and every reduction of 0.10V/cell is said to double cycle life.