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If it's over 10uF, put a 9v battery and a 1 Megohm resistor across it. Remove the battery and time how long it takes to drop to 37% of initial voltage. Time in seconds = capacity in uF.
"A typical test for ESR involves passing an AC current of around 100 millivolts AC at 100 kilohertz through the capacitor. The test signal can be sine, square, or pulse waves. The low voltage prevents damage or interference with associated circuits for in circuit tests. It also limits the reverse voltage applied to electrolytic caps. At 100 kilohertz, the AC impedance is nearly zero for larger caps. When measuring caps below 10 uF, the impedance due to the capacitance should be accounted for."
100kHz is the standard test be beware that larger capacitors will become inductive at lower frequencies than that. A typical 10,000:mu:F capacitor will resonate at under 5kHz.
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