The 8.900 uF cap is working great for my points bike as the draw is intermittent.
Using a cap in parallel with a battery is more about surge capability, as the cap will discharge into a load before the battery as it has less internal resistance. It prevents voltage sags, a useful trait.
With a cap only the voltage regulation will flail about more than with a battery. My casquette mounted analog meter windmills between 12V - 18V on cap only.
I'm not understanding the value of putting a capacitor in parallel with a usable battery? As I figure it, your 8900uF cap, if it starts at 13V and discharges down to 8VDC, is releasing less than 1/2 of a Joule of energy. 0.50 watt-seconds at 12volts is the equivalent of delivering 40 milliamps for a second, or 1 amp for 1/24 of a second. It's not much usable power. The cap, even if it were 50,000 uF, is a tiny amount of energy capacity, relative to the battery.
The battery, on the other hand, is like a massive capacitor. Discharged to half of its theoretical capacity, a 12V 7Ah battery would have delivered something like 151,000 Joules of energy. At 12Volts, that would be like a 2000 Farad capacitor. I don't see the benefit of putting a .009 Farad cap in parallel with that massively larger capacitance?
Maybe it does help keep the regulator calm, which is the point of having the cap there as a "battery eliminator". How much did the regulator bounce without the capacitor present? Didn't you suspect the regulator of misbehaving, on account of it overcharging the battery in the first place? I recall you discussing this, but don't recall the details. Can you point me to the thread(s), where adding the cap was recommended?
The C5 electronics like a relatively constant voltage source. If the charging system when kicking it over pumps it up faster than the electronics bleed it down, it might be a winner.
If your big cap allows for a start up with your electronic ignition, you could use a 20A plug in auto-style fuse for starting isolation, then just plug it back in once it was running.
My idea is quite a bit different. I'm thinking to start the bike using a few alkaline cells, like 4 AAs, through one of the cheap and tiny, but remarkably efficient DC/DC BOOST converters that are ubiquitous these days. This gadget would attach to my SAE battery tender port, and needs to generate enough energy to activate the fuel pump, power relay, ECU, and ignition. It can deliver that energy at it's leisure, over a minute or more. The capacitor is there to act as a 12V accumulator to start the bike, then keep the alternator / rect / reg happy once the engine is running. Once the bike is running the battery pack with the DC/DC Boost can be disconnected and go into my pocket. I think to play this game, the battery would need to be disconnected. Otherwise it would suck up all the power I can hope to deliver with a pitiful handful of dry cells.
The gadget is a kind of, as Dave Letterman used to say, a "stupid human trick", but, if my battery goes south, I much prefer to purchase my replacement over the internet, and in the meantime, I can ride, albeit carefully, probably turning off my headlight when the engine idles, or perhaps while revving my engine like the Harley guys in that South Park episode, "The F Word".
https://www.youtube.com/watch?v=ipDmsxQVxIMIf it's a wacky idea, it's probably best to hear that now. I know you guys won't spare the truth to save my feelings from being hurt.