"On the wall charger it says the output is 4.8V so based on that I would assume it can't fully charge the battery. However, I checked the voltage of the battery with the radio system on and it reads 6.53V. How is that possible?"

NiCd have a nominal voltage of 1.2v per cell and a a fully charged voltage of abiout 1.4v per cell. A fully charged 5 cell pack has a nominal voltage of 6.0V and a fully charged voltage of 7.0V. The wall wart is rated for it's RMS voltage or average voltage, the peak voltage will be higher. Assuming a RMS of 4.8 volts, the peak voltage of the wall wart will be about 6.7V. This means in theory, assuming no losses, it could charge a battery up to 6.7V. Fudge the numbers a bit, account for losses, etc., and your packs get charged to the 6.53 volts you mentioned. While the 4.8v wall wart will work on 6V packs, 6.53V is not fully charged yet (but close) so it is probably better to use charger that outputs 6V RMS at current rating that is 1/10 the capacity of your battery.

-Cheers