Hmmmmm....

I found that my cap also charges much faster at low frequency 115hz as opposed to even 180hz. I was charging an uncooperative 9.6v NiCad and decided to put a large non-polarized 600uf 250v cap in parallel and break the connection to the negative of the battery leaving only the cap connected.

At high frequency 200 hz, the cap took 3 or 4 seconds to charge and the neon didn't even light up. At 115hz, the cap charged as soon as the battery was disconnected and the neon light right up. The connection to the battery sparked pretty good when reconnecting.

At first the NiCad would only charge to about 5volts but after shocking it with the cap blast a few times, it has decided to be more cooperative and is now up to 7.5volts and charging.