Unless we're talking about these cheap overnight wall-chargers....

The main difference is that a charger got a cut off device built in.

for NiCd's and NiMh's, the cut off can be determined by time, voltage drop or temperature.

Timed charging: Problem is, how much was left in the pack, and how much can it take over all? Since the charger dont really know, a pack charged this way is usually overcharged or undercharged. In, let's say, an electrical tooth brush, it dont matter much. But since we're stressing our NiCd's / NiMh in our cars, it makes a huge difference.

Temperature cutoff: This is a very effective cut off method, no false peaking and no over charged packs. However, it requires that the ambient temperature dont differ too much (or the charger shold compensate for that) and, especially, it requires that the user remember to mount the temperature sensing device....

So in moct cases, we use delta voltage sensing. When the pack is fully charged, the voltage will drop. So when the charger sees this, it stop the charging.

These three methods can be combined.