Took out the dead battery. As I've suspected, it was frozen. As in covered with frozen leaked out electrolyte. 5.64 volts at terminals. Dead as a dinosaur.
Acquired a "virgin" (dry-stored, filled upon purchase) AGM motorcycle battery of 12Ah from Varta. Compared to the defunct 10Ah is a bit heavier (~4kg weight) and potent enough for a 1.8 liter engine by itself, as factory recommended for 1800cc Suzuki Intruder. Got it thoroughly tested and warranty covered as an automotive-systems auxiliary battery, to avoid future hassle. Once filled up, left to settle and charged to specs it holds 13.02 volts.
This should bring the fully assembled hybrid to 6kg in weight. Cutting corners (literally
) means little advantage.
Once fitted, it may bring some minor advantage in mileage, as the supercaps charge very quickly and there is little work to do by the alternator. (Alternators are miserably inefficient, about 40% over most of the rpm band, so they draw pretty kilowatts from the engine while under electrical load.) This is not its purpose, the goal is removal of weight from outside the wheelbase.
Removed the balacing circuit boards and tested the supercaps again. They charge much quicker with a battery charger, hit about 14.25 volts, charger LED lights up and they don't seem to get more charge. Left them to settle down. Dropped to about 14.08 volts and individual voltages in the string are:
+ 2.40 2.27 2.29 2.43 2.21 2.48 -
Charged again to ~14.8 volts and tested the individual voltages in the string. Highest voltage cap closest to - terminal, 2.58 V, lowest 2.38 V (2 caps), the rest in the 2.4x range.
The ratios between individual voltages are similar to what I've got before, with balancing boards fitted. Proof the boards did nothing useful.