1) You can measure the RC time constant.
T=R*C
Where: T is the time in seconds required to charge to 63%
of the battery voltage, and
R is resistance in ohms, and
C is capacitance in farads.
Note that if you measure resistance in megohms, then the capacitance
will come out in microfarads.
Re-arrange the formula so that
C=T/R
charge your cap through a resistor, and note how long it takes the
voltage to build from 0 volts to 63% of the battery voltage.
Use the formula C=T/R to determine the capacitance.
The circuit might look like this:
Code:
----------- Switch ------ Resistor --------------------
| | |
Battery capacitor voltmeter
| | |
-------------------------------------------------------
Note that the meter used should have as high an impedance as possible
to avoid loading down the circuit. In most cases you would probably be
using a 10 Megohm DMM. You can increase the effective input impedance of
the meter by using an operational amplifier connected as a Buffer
in front of the meter. You can use an LF356 for this purpose.
Start with the switch open, and the voltmeter showing 0 volts (short out
the cap for a few seconds to discharge it. Then close the switch, start
your stopwatch, and wait till the voltmeter hits battery voltage x 0.63.
Sadly, this only works well for large capacitors like electrolytics,
because you can do the timing with a watch. But for small capacitors
such as are used in tesla coils, you need to use a storage oscilloscope
or electronic timer/counter to measure the time 'cause it will be in
milliseconds or even microseconds.
