TL431 would work better than a LED, but maximum current is 100mA, it would take a long time for it to drain anything significant off a 500F capacitor. As I said before, I don't know how much over-voltage it takes to permanently damage one of these, or what is the failure mode if it happens. Such a big capacitor is outside most peoples' experience, anyway. Most are astonished that capacitors that size are available, when I tell them what I'm about to do. When I went to electronics school many years ago, we were told that a one-farad capacitor would be as big as a house.
The battery is rated at 380CCA. And PCA (I'm not sure what that is) is 620A. It doesn't get that cold where I am, so I don't think there's any temperature issues. Weather here is pretty much like Southern California. That's why I live here, largely.
If the cycle life of this kind of battery is typically 3000 or so, then it seems to me it should last at least 10 years; with capacitor bank to help it, maybe a lot longer than that, if you don't try to run things much without the engine running.
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