That is the battery being charged. It is reverse polarity of the supply battery shown to the right. The charge battery negative terminal is to the top and shown connected to the positive from the coil and the positive from the supply battery.

Imhotep for this thread. Very interesting.

I have a bunch of "stuff" lying around, so I thought I'd put one of these together. I did have to pick up a supply battery. I got a 12V 7aH lead acid battery for my supply battery. For my charging battery, I took a dead battery out of a power mower that I have. It is a 12V 17aH lead acid battery that has been "dead" for over a year. It won't even keep the mower running it has so little charge. When I put the voltmeter on it, it had ~2V and negligible current output (in other words, put a load on it and the current went to 0A, the volts went to 0V). The supply battery measured at 12.88V.

On an interesting side note... You mentioned that one of your 3 would self start. At first, I had to fiddle with things, but at one point I put the lead for the charging battery (after the diode) back to the emitter and found that when I do that, it would self start. Currently, it has been running for maybe 1 1/2 hours (give or take) and the supply battery voltage is now at 12.73V and the charging ("dead") battery voltage is reading 6.99V.

A few things I've noticed:

1) The charging battery voltage seems to vary from reading to reading over the course of a few minutes, and the readings aren't always going up. The readings have been as high as 8.24V and fallen down into the 5V range. Still pretty impressive considering that this was a "dead" battery that I thought I'd have to replace this year because it wouldn't hold a charge from a conventional charger.

2) Varying the potentiometer value only changes the fan a minor bit. But, if the value goes over ~2k by very much (I had a 5k pot on hand, so I used it) or if the value drops down too much, the fan stops. I haven't measured the values (yet). There is a small increase in speed when reducing the value of the pot, but if you weren't "looking for it", you wouldn't notice. It does increase the drain on the supply battery, but doesn't seem to increase the charging rate to any noticeable degree. Also, the fan speed isn't a high rate. I'm not concerned with that, just mention it because of comments that folks have made about the high fan speed. It has a speed at which it wants to run ("resonate") and that is where it seems to be the "happiest". Sooooo, that's where I'm letting it run at.

3) When I picked up the supply battery, I also picked up the neon bulb. (I don't have that kind of thing "on hand") They only had red... no amber, but I didn't think that would make a difference. Also, you don't mention it, but they said Radio Shack said they only carry these little neon bulbs in 120V... I was looking for a 12V bulb... The 120V bulb looked like what you have in the video (except red ) so, that's what I got. (all that explanation just to mention that...) on the neon bulb... Nothing. No light, no nothing... I do have some LEDs laying around and noticed that putting one across where the neon bulb is causes a huge drain on the circuit (and in short order will stop the fan). Using a smaller LED, it doesn't stop the fan, but will also cause a drain. Either will light when put in parallel with the charging battery, but the larger one is pretty dim. From what I've read, I kind of expected things to run better with additional loading, but I haven't seen that at all. After the LED tests, the charging battery read at 6.55V (down from the 6.99V previous reading).

I realize that using the 12V 17aH "dead" battery as my very first charging battery was probably ambitious, but things seem to be working... even if not quite as "efficiently" as others have seen.

Thanks again... Maybe I'll pick up some of those cheap NewEgg fans. Can't beat $1.29/fan. I have the rest of the parts (except for neon bulbs ) to build at least 5-6 more of these... It doesn't take that long either. (It's just that these eyes are getting too old to see those fine coil wires )

Thoughts, comments, suggestions?