I believe this have a historical reason. In old day, the most commonly use rechargeable battery is Ni-MH, this chemical, will allow minor current passing through it even though it is fully charged. This is seem like it have relatively larger internal self leakage from positive to negative.
In those old days, cell phone battery normally contains 3 Ni-MH cell in series to generate 3.6V (1.2V per cell), while notebook battery normally have 8-12 cell those day. Charging it in small current continuously for long enough will eventually balance all the cell, this is good for those cell in series, so all of it fully-charged at the same potential. This is safe as long as the current is small result in no overheat of battery cell.
If all battery cell in good matching of relative state of charge, it will be able to output the maximum power can be stored, which is same with the maximum power of the poorest cell, for example 3 cell in series have capacity 700mAH, 710mAH, 720mAH respectively, but currently it stored 210mAH, 190mAH, 200mAH respectively. When charge to full, cell 1 have 700mAH (700-210=490mAH), cell 2 have 190+490=680mAH, cell 3 have 200+490=690mAH. If not all cell reach fully charged state, the maximum capacity it can reach is only 680mAH, while the poorest cell actually can store 700mAH. If small current is allowed to continuously charge it for long enough, it will eventually reach 700mAH, 710mAH, 720mAH respectively, anymore charge will turn into heat and dissipated into environment.
Why 1 cell is charged 490mAH, all the others is 490mAH additional charge? Because it is in series! Current is same in series. Why it relative state of charge is differ at the beginning? This is the result of some slightly different in each cell property example different in self discharge rate. It is unpractical or even impossible to manufacture all these cell to have zero deviation in their property. It contains some error let say +/-1% from cell to cell.
Ni-MH is very famous those day because it doesn't need special protection circuit. After it fully discharged, it internal resistance will become very large, and the machine will shut down due to the loading effect of the internal resistance, the cell will never fully drain. Even if Ni-MH is overcharged, this will not result in disaster as long as it temperature is within the permitted range.
Later, those Ni-MH was replaced by Li-ion, high energy density cell without memory effect. Li-ion cell in general, will not allow current passing through it after it is fully charged, it internal leakage is relatively small. That mean after the cell reached equal potential with the charger Voltage, no more current will pass through it. Further more, to avoid this and prolong battery life, manufacturer have designed protection circuit that will block the charging after the current reached a very low threshold for example 100mA. Therefore, li-ion battery cell can not be balanced by this way, it must includes complex circuitry in any application that required serious consistency toward relative state of charge of it battery cells.
In conclusion, Li-ion battery do not need 8 hours on the first charge, it serve no purpose. As long as the indicator show 100% or one of it cell fully charged, charger will stop charging the battery.
