ok here is some more..
The "fully charged" voltage on a battery can often be considerably above the nominal voltage..........the "nominal voltage" is likely to be the voltage of the "plateau" that is seen in the discharge curve when it is discharging at some rate like 0.2C.
If a battery is fully charged and in a charger which is managing it, then that battery voltage will often be rather higher than nominal....even when it initially starts discharging.
Many batteries such as lithiums, have charge protection circuits in them, so you get a small amount of voltage drop across those components also, .. when the battery starts discharging that is.
If a lithium battery goes below a certain voltage, it is ruined and shouldn't ever be used again.
in laymans terms, pretend you are an electric current (or rather a bit of electric charge) resting in a battery......then you get up and start walking through the wires connected to the battery because somebody has made a circuit up..........the energy that you take walking through the swamp of the internal resistance will tire you, and you will be less spritely when you have got through the internal resistance............but this is wrong because as long as the current flows it is always going through the internal resistance of the battery, so its difficult to bring a layman analogy.