Old, All the chargers I've seen use only 2 diodes and a transformer that has a center tap. The output, if seen on an oscilliscope, are postiive 1/2 waves. It is still considered a full wave power supply, because there are 120 + waves per second.
Old go to http://en.wikipedia.org/wiki/Rectifier
About half way down is what I copied below. I wasn't able to copy the diagrams. As stated below, there are two ways to make full wave power supplies. The second one is what chargers use, 2 diodes and transformer with center tap.
A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and yields a higher mean output voltage. Two diodes and a center tapped transformer, or four diodes in a bridge configuration and any AC source (including a transformer without center tap), are needed. Single semiconductor diodes, double diodes with common cathode or common anode, and four-diode bridges, are manufactured as single components.
Graetz bridge rectifier: a full-wave rectifier using 4 diodes.For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (cathode-to-cathode or anode-to-anode, depending upon output polarity required) can form a full-wave rectifier. Twice as many turns are required on the transformer secondary to obtain the same output voltage than for a bridge rectifier, but the power rating is unchanged.
Full-wave rectifier using a center tap transformer and 2 diodes.