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Conclusion after 9 months: some cold water poured over the initial enthusiasm. The AGM battery did not withstand the test of time.
Supercaps did always fire the needed amps to start the engine in cold weather. But the AGM battery slowly lost the ability to hold a charge, and usually did not replenish them on time in hot weather. It lacked enough reserve capacity for this. 12Ah were not enough.
The alternator could give 120 amps instantly when needed. Alternators can give 70% of their rated amps in continuous use, that is 84 amps. At 2000 rpm, where alternators are most efficient.
Fuel pump drew 3-4A, ignition coils up to 30A at maximum rpm, radiator fans 15-17A on first speed and up to 35A on maximum speed, dash fan maybe 10A. That means a continuous draw over 60A when crawling in traffic and up to 79A with fans on maximum speed and dash fan running. Too little remained to replenish the battery and supercaps. So the battery slowly degraded in performance, day by day, and in the end it barely held any charge at all.
If the same supercaps were lashed to a bigger battery, in the 40-44Ah range, they worked just fine. But the 44Ah battery was enough to start the engine and hold a current reserve without any caps at all. So I've left the 44Ah battery alone. It still saves weight (10 kgs instead of the 16-18 kgs of the 60Ah factory battery).
For the supercap-battery combo to give satisfactory service over months or years, it needs either a stronger lead-acid battery, in the 30-40Ah range -to keep a current reserve- or a small LiFePO4 battery - to keep a current reserve by the fact it can use all its rated capacity, unlike lead.
I still have to try the same combo on my VW Bug, where there is no parasitic draw (no electronics) and the only electric consumers during normal running are a single ignition coil, an aftermarket electronic ignition and a radio. There is may give satisfactory performance and still save over 10 kgs.
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