Electronic – Why do we have to discharge the capacitor before testing it in an LCR Meter

Very large electrolytic capacitors are generally used at relatively low frequencies (below 1 kHz) to smooth fluctuating DC voltages such as in power supplies. At 100 kHz, the impedance of an 8200 uF capacitor is less then 1 milliohm. This is very difficult to measure accurately considering wire resistance and the effect of parasitic capacitance and inductance that are inherent in the construction of the capacitor and have significant impedances at 100 kHz. This is why the capacitance of electrolytic capacitors is usually specified at either 60 Hz, 120 Hz or 1000 Hz. It is also why electrolytic capacitors used as bypasses on high speed IC's are almost always placed in parallel with a much smaller capacitor that is more effective at high frequencies.

Normally I would let you go and look as this is not a hard question to solve, but as I am feeling generous here is how we get there:

From fundamentals, we know that \$Q =CV\$

If we take the derivative with respect to time (remembering that \$I = \frac {Q} {T}\$) we yield

\$i = C\frac {dv} {dt}\$

Rearranging, we find that \$\frac {i} {C} = \frac {dv} {dt}\$

Therefore charging a capacitor from a constant current yields a linear ramp (up to the compliance of the current source).

I will leave finding the solution in terms of time versus some voltage to you.