From the practical side, unless you are dealing with a high draw light like HKJ says, most button top cells are protected, while as a general rule flat top cells are not. A protected cell is one that has had a small printed circuit board added that will 'shut off' the cell if it is subjected to a too large charging voltage, too large of a current draw (usually around three amps), or if the cell gets to hot, which usually signifies internal problems. Some protection circuits will also shut off if the voltage from the cell drops to low, to protect it from damage. Usually the specifications for the cell will specify the cutoff current and low voltage cutoff point. And yes, all this protection comes at a price, since no circuit does its duty without an energy input. But the overhead of the protection circuit is small enough that for all practical purposes it doesn't exist, and whatever overhead the circuit adds is considered minor in relation to the protection its existence gives to the cell.
Oh, and most modern chargers are capable of detecting a tripped protection circuit and resetting it as part of the charging process.