A lot of the problems are down to the charger itself.

'Fast' chargers will 'cook' the battery if over-used, but some chargers are 'intelligent'.

'Intelligent' chargers will fast charge, and change to a 'trickle' charge when the battery reaches optimum volts.

Some 'intelligent' chargers also have a 'discharge' feature, which can be used to 'cycle' the battery and remove the 'memory'.

As for complete discharging, some NiCads will not recharge once totally discharged, although you can often find the cell that is faulty, and give it a short 'boost' charge of about double the voltage for a few seconds, after which it will accept a charge normally - until it is run 'flat' again.

The phenomenon of 'reverse' charging is the problem - once a cell reaches zero volts, it will start to reverse-charge if the rest of the cells in the battery have not also reached zero volts. This is why a failing battery seems to suddenly lose its capacity - one cell is sapping the energy from the rest.

'Reverse charging' means exactly that - the terminals will, for a short time, be reversed: '+' becomes '-' and '-' becomes '+'. This is because cells in a battery are connected '+' to '-' in series. Instead of stopping at zero, the cell continues into a negative state, and will start to get hot, although they are designed to 'fail safe' rather than cause a fire.