[Galaxy S II General] LightspeedGalaxy:
This is pretty normal, Li-Ion-batteries can´t be charged fully under 3h. Devices that claim to charge the battery in just 1 or 2 hours will charge it only to about 80% and then tell it is fully charged.

You can charge a Li-Ion-battery very fast from 0 to about 75-85%, if the power-adapter delivers enough energy this can be done in about 1h.

But to fill the rest up to 100% you need another 2-3h.

Li-Ion-cells are charged in 2 stages.

In the first stage charging is done with a current between 0,5C and 1C. C in this case means capacity, so our 1,6Ah-battery will be charged with 0,8-1,6A.

The voltage of the cell rises from about 3V when completely flat (you shouldn´t discharge a Li-Ion-cell below 3V or it will be damaged permanently, when discharging below 2,7V the cell becomes unstable and could explode when being charged, therefore a safety-circuit should prevent any further charging when the cell-voltage was too low) to about 4,2V.

When the cell reaches 4,2V it is charged to about 75-85%.

If you would charge further at this rate, the cell-voltage would rise even more and significantly reduce the life span of the battery.

The supplied power-adapter delivers 0,7A at 5V, so with some quick calculations it will not be able to charge the battery up to this stage below 1h.

If all the power from the adapter can be used for charging (which means no other phone-activities which will consume power) it can reach the saturation-stage in approximately 1,5h.

To completely charge the battery stage 2 follows, this is also called the saturation-stage.

During this stage the charging-circuit will hold the cell-voltage constant and reduce the charging-current accordingly.

If you want to charge the battery completely the rather weak power-supply doesn´t matter that much (at least as long as you don´t use the phone otherwise and take a significant amount of power away from charging). The faster you charge in stage 1, the longer the saturation-stage will last. The whole charging-time is not influenced a lot.

To fully charge the battery you will always need about 3-4h. Also the difference would not be great whether you begin charging at 0%, 20% or 30%.

Charging the first few % will always be very fast and gets slower the fuller the battery already is. The saturation-stage will take about 50-75% of the overall charging-time, so there is no significant difference whether the battery is completely flat or still has some charge left.

Are you just charging the phone or using it the same time? If you are just charging it, it definitely shouldn´t get hot, charging Li-Ion-cells has a very high efficiency-factor (about 95-99%), so there shouldn´t be much heat generated from the charging.

If you are using it the same time, it is pretty normal that the phone is getting warmer.

It is also important what is getting hot. Is it really the battery (the lower half of the phone) or rather the SoC (which seems to be placed just below the camera)

If it isn´t the battery, it can´t be in direct relationship with the charging. If the whole phone is getting warm, it is an indication, that the SoC is doing some work and generating the heat.

If only the battery gets hot (and not just slightly warm) this can be a problem. Either the charging-current is too high (quite unlikely, with the weak power-adapter) or something else is wrong, which will at least cause a shorter life-time of the battery.

It is normal that the battery gets a bit warmer than the surroundings, especially if the battery is in the early phase of stage 1, where it is charged pretty fast.

While the charging will not produce much heat, the battery is in a plastic-shell, so it can´t be transported away that fast.

But if it gets really hot you have a serious problem. But I doubt this, the phone actually has a protection, that will not charge the battery above a certain temperature-level.