Modded PC power supply for charging

[BrianG]

First of all, you want to figure in a typical charger efficiency of ~80%, and then I like to add another 15% as a safety margin. So, that 250w charger will require around 370w. Assuming the PS can deliver a stable 12v, it needs to be rated for at least 31A.

When shopping for supplies, make sure that they don't have multiple 12v rails. What you want is a single high current 12v rail.