if the transformer in a wall charger steps down the voltage from my wall to 120V does this mean i could use it in the reverse direction as a step up transformer? I want to combine 4 car batteries making 48V and plenty of amperage, but what can i use to get it up to 120V for my 1/2 hp motor?
Not you can't really because transformers need AC (alternating current) to work. So your charger has got a rectifier in it and possibly some kind of a regulator circuit. You could use 10 car or motor bike batteries in series to get 120 VDC but PLEASE BE CAREFUL. You could get a nasty shock at 120 volts.
Truthfully you'd be better off setting up the batteries in series (for higher amperage) and buying a decent 12V power converter. Your two biggest problems will be if the batteries can handle the start up power surge generated by that motor and the power loss from converting the dc to ac in the converter.
