The reason that you may see a higher voltage than the 4.8 volts spec. is: the voltage you are reading is a peak reading produced by a poorly filtered dc charger. The charger takes the 110 AC from the wall and steps it down to a lower voltage which is the rectified and filtered to produce the DC voltage. If you were to look at the output of the charger with a scope you would see the DC voltage and a series of bumps which is the ac peaks riding on it, these peaks is what your voltmeter is seeing. An if you are making the measurement without a load on the charger it will read higher because it is not loaded by the battery. If you put a load on the charger such as a lightbulb, resistoro or dead battery you will see the voltage drop, with a load which matches teh output current of the charger you will see the voltage at leaast close to the spec reading. Now this may or may not be close to the 4.8 volts depending on the quallty of the charger and its regulator setup, in most cases you will find it close but not exact.
When charging standard batteries close is good enough...

Li-pols, lithium ion and alike are more demanding thus the chargers are much more expensive.

If you have a good variable DC power supply with current limiting function this may also be used to charge your batteries but they are not cheap...