A couple power supply theorys

[BrianG]

Answer to your two theories:

1) Inverter. This is also a switching power supply, but it boosts voltage. Simply wiring it backwards will not work.

2) Car batter charger. Well, I kinda wonder the same thing, but I don't think it would work well for higher currents because their duty cycle at their max rating is small - in other words; they can't run at the max capacity for very long before they overheat. So, you'd be limited to lower currents. Then, you'd also want to regulate the output, which means a regulator IC, several pass transistors, large heatsink and other components.