Modded PC power supply for charging

[Serum]

I took am aluminum case 15 or 20W resistor, which i mounted inside the case, i don't remember what value i took, it think it was a 22 Ohm, which stayed cool enough in the airflow of the PS. (6.5W wasted energy, small price to pay for a cheap PS.. :) It's not really needed to get the formula, eversince it's about 12 - 13V that's used.

You can determine if you need a resistor by measuring the voltage. if the voltage differs much from unloaded/loaded, a resistor might be needed. In some cases the charger doesn't want to start if the voltage is too low.