[freddyzdead]

I guess we've drifted a bit off the original topic; me, I've only just arrived, and am all set to go as soon as the bits I've ordered arrive. I've bought 6 of the Chinese 500F capacitors and a 7.6 Ah LiFePo4 battery, which is internally arranged already to charge from a system designed for lead-acid. It cost me $AU99, which is more than I wanted to pay, but I don't have to modify it, it should just work. 380CCA, I think that is more than enough for my little 659cc 4-cylinder engine. I could have got a smaller one for $20 less, but I'm trying to do a proper job with this, so, what the hey.

I am quite concerned about balancing for the capacitors. Although the idea of using LEDs for this purpose shows some inventive thinking, the result is much less than ideal. The conduction knee for LEDs is not very steep, and they start conducting way before you need them to. If you pad them with diodes, the top end goes out of range. When they are really needed to perform, it seems that they cannot drain away the excess voltage quickly enough. I'd probably try zeners, they have a sharper knee. Whether balancing is even necessary in this application is another point. In my experience, obtaining components that are all from the same batch usually means they are pretty well matched. But these capacitors are specified at +/- 20%. If you happened to have one on the high side and one on the low side, you could be in trouble. I don't really know what happens if you over-voltage one of these, but I assume you get a dielectric breakdown, which could, I suppose, develop into a full-blown short. That wouldn't be good. Somewhere back in this thread, I recall someone mentioning something about voltage drops increasing the nearer you get to the negative terminal. What? As I recall, Kirchoff's law says the current is the same in all parts of a series circuit. So that can't happen. Ok, so anyway, I went ahead and ordered a balancing board so I can stop worrying about it.

Originally, I was thinking of trying a 4s arrangement of Li-ion cells (I've got lots of 18650s lying around), realising that they won't fully charge, but maybe that's not such a bad thing; you won't get their full capacity, but you should get more cycles. I would put them in the passenger cabin to save them having to put up with the nasty engine compartment environment. The car would be relying completely on the capacitors to start, and I have no idea what kind of cycle life I'd get from the Li-ion cells. But the battery I bought makes all that obsolete. I'm hoping I can install these things and forget about it for the life of the car. I'll let you know what happens when everything arrives.