Kerry Wong wrote a post on his blog showing how he modified the 12V rail of a TL494 based ATX power supply to 14.6V for 4S LiFePO4 battery charging:
To charge the 110Ah battery bank I built, I need a power supply that can provide at least 10A at 14.6V. Since I have many old ATX power supplies lying around and the 12V rails of these power supplies are more than capable of providing 10A, I decided to modify one such power supply for using as a 4S LiFePO4 battery charger.
More details at his blog here.
Check out the video after the break.
This shows how to disable the OVP circuit and increase the output voltage. This does not make a battery charger, you need a constant current rather than foldback current limit, it should turn off when the battery is full, and 14.6V will kill a 4S LiFePO4. 13.8V is much better, 14.2V at most, and 14.4V at what the OVP should be set to.