@modrasek;

Capacitors work on what is know as Q quantity of charge, litterally quantity of electrons.

One can look at the height of pulse as being created by a stack of electrons piled "end on end"
while the width of the pulse is created by a volume of electrons. So a pulse on the oscilloscope
can be looked on as an integration (summation) of the height of the pulse times the width of the
pulse.

A small capacitor will fill up fast, so a quantity of electrons will make it go to high voltage faster.
A larger (more uF) capacitor will go to a given voltage more slowly and require more quantity
of electron charge to get there.

Your motor winding has an inductance. It tends to reject (block) short (narrow) spikes. But
when you use those short spikes to charge a capacitor the voltage slowly rises to meet
an objective voltage. The DC voltage on the capacitor then runs the motor with DC for a
short time period which the inductor conducts more vigorously, but in the end then drains off.

=>In effect the capacitor has performed the function of down converting the high voltage spikes to
much lower voltage, but the quantity of electrons and therefore the energy, has been conserved
by the capacitor.

The impedance of the capacitor is a better match for the motor's impedance then the short
spikes circuit was. so you are using the charging cap as impedance matching.

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photo flash caps are exactly the same as any capacitor except the high working
voltage (~300Vdc) and moderate capacitance (~250uF) allows them to store a
relatively high ( and relatively more hazardous) amount of electrical charge.
They are really the second rung on the power (rather than signal) capacitor scale.

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Nicad batteries, of course, are infamous for the "memory effect" which happens when the cell's
of the battery are not discharged evenly. Voltage on some cells are still high while others
are low and there is no good way to "reach through"  the charged cells to get to the
discharged ones.  The spike voltage charging, I don't think is recommended for NiCad's,
and I believe LmNiH batteries are touchy to charge too. In acid/lead batteries you can
reach through with a "polishing charge". That leaves alkalines, which I don't know too
much about but would be worth a try.

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Eventually, I think you should convert to small gel-cell acid leads that would be a better match
for the chemistry and pulse charging and a better match for your relay's power requirement
which is I suspect to large for the the 9Volt size batteries. I have not been able to locate
a good source for the 9Volt sized acid leads.


:S:MSCoffman