Find the voltage rating of the capacitor. On large capacitors, it is printed clearly on the body of the capacitor, such as "25 V," for example. Smaller capacitors may or may not have the voltage rating printed on its body. If no voltage is indicated, check with the manufacturer for the capacitor's specifications.
Observe the polarity of the capacitor. High voltage capacitors typically have a thick line or arrow with a minus (-) sign printed on it indicating the cathode (negative) lead of the capacitor.
Insert the two leads with alligator clips into the positive and negative lead input jacks of a 25-plus volt power supply. Clip the negative lead of the power supply to the negative (cathode) lead of the capacitor. Clip the positive lead of the power supply to the remaining lead of the capacitor.
Turn the voltage knob of the power supply to its lowest setting before turning the power supply on.
Turn on the power supply, and slowly increase the voltage no higher than 25 volts. Increasing the voltage delivered to the capacitor beyond its rating will damage the capacitor and possibly cause an explosion. The charging of the capacitor is practically instantaneous.
Disconnect the leads and turn off the power supply. The capacitor is now fully charged and ready for use.