A power supply converts ac current (the type of current that comes out of your wall or extension cord) to DC current (the type of current that comes out of your full size car battery or any battery for that matter). The benefit of a separate power supply is the wattage they make. A 1200 watt power supply can charge batteries a lot faster than say a HiTech x4 ac / dc charger that only pumps out 50 watts per channel. My opinion is that a high performing power supply should be built into the charger. I don't want to be carrying around two separate items. I'm assuming batteries need to be charged by dc current.