Hi ahcraton
You simply asked about power dissipation of a battery charger . but what kind of battery charger ? for what value of power range ? for what kind of application ? these are very important for judgement . because many of the chargers are based on flyback converter or self oscillating flybackc onverter or royer oscillator or transformer coupled damped oscillator .....
Sometimes based on thyristors and IGBTs ! and sometimes based on H bridge . it depends on your application . if you mention them exactly then we can easily judge about dissipation .
Best Wishes
Goldsmith
Many of the mobile chargers are based on HV oscillators which are without negative feedback and they only produce an AC wave which is HF and then a rectifier and then applying that to the mobile and then mobile will use it's supply IC to make it useable .
some of them are based on self oscillating flyback converter . some of them are based on royer oscillator . and some of them are fully drove flyback converters .
the efficiency depends on the active element which has been used as a switch or as a linear device .
loss will given by : VSwitch*Iswitch *d.c