I took out a bestec powersupply out of an emachine, and used the 12v rail to power up my hobby charger.
I tried charging a 7.4v li-po pack at 1200mAh, and the voltage levels kept falling down below 10, where my charger would turn off. The fan audibly slowed down when the charger was charging, and the voltage on the 12v rail would not hold up until I decreased charging rate down to 80mAh.
The charger's efficiency, I'm sure, is above 70%. It's disappointing that the power supply won't hold up, even to this. It wasn't even powering a computer. I suspect I might be doing something wrong, but I can't imagine what. It worked fine when I plugged it into my corsair 400w, which was powering a computer.