I finished building the 6x 400 farad capacitor bank and added the balance diodes. Not knowing what voltage drop I would want for the balance circuit, I performed a test with the following results:
Volts mA Config
2.70 8.55 1 diode 1 red LED
2.65 6.74 1 diode 1 red LED
2.60 5.20 1 diode 1 red LED
2.55 3.54 1 diode 1 red LED
2.50 2.36 1 diode 1 red LED
2.45 1.53 1 diode 1 red LED
2.40 0.88 1 diode 1 red LED
2.35 0.50 1 diode 1 red LED
2.30 0.27 1 diode 1 red LED
2.25 0.14 1 diode 1 red LED
2.20 0.067 1 diode 1 red LED
2.15 0.033 1 diode 1 red LED
2.10 0.017 1 diode 1 red LED
2.05 0.009 1 diode 1 red LED
2.00 0.005 1 diode 1 red LED
2.70 0.100 2 diode 1 red LED
2.65 0.068 2 diode 1 red LED
2.60 0.044 2 diode 1 red LED
2.55 0.029 2 diode 1 red LED
2.50 0.019 2 diode 1 red LED
2.45 0.013 2 diode 1 red LED
2.40 0.008 2 diode 1 red LED
2.35 0.005 2 diode 1 red LED
2.30 0.004 2 diode 1 red LED
2.25 0.002 2 diode 1 red LED
2.20 0.002 2 diode 1 red LED
2.15 0.001 2 diode 1 red LED
2.10 0.001 2 diode 1 red LED
2.05 0.001 2 diode 1 red LED
2.00 0.000 2 diode 1 red LED
I decided to go with just a single diode plus an LED for the balance circuit since 2 diodes increased the fabrication effort and parts count while not providing enough current flow through the LEDs to see the relative state of charge.
After 3 days of no charging, the LEDs are still trimming the voltage while dimly illuminating.
My plan was to replace the supposedly weak battery in the ATV, but it turns out the problem was not with the battery. I opened up the starter motor and found it covered in oil.
I cleaned everything up, including the grooves between the commutator and put it back together. With a dozen tries of the starter and original battery, nothing more than the solenoid clicking happened until finally not even the clicking was heard. I measured the voltage of the battery and found it flat dead at 0v.
I figured I just needed more juice to turn the starter over, so I installed my capacitor bank and gave it a shot. The solenoid clicked and a small puff of smoke came from the the vicinity of the capacitor and solenoid.
I'm pretty sure I assembled the starter motor incorrectly with a short to ground, and that may have in turn flattened the battery and fried the solenoid. I didn't have time to troubleshoot any further, so I still need to figure out what's broken. A solenoid is $10, and a starter rebuild kit is $23, so I might just get both and be done with it.
I gave my dad the battery out of Lafawnda (the Honda motorcycle), to put into the ATV for the purpose of running the onboard computer and powering the electric shifting. This gave me the perfect excuse to put the capacitor bank into the bike. It started right up, went 7 miles to work, and then brought me back home without issue.
I'm fairly confident I can run the capacitor bank alone in my bike as a permanent solution, but I have yet to test in stop and go conditions where my fan kicks on. This could change things since the bike doesn't charge below about 1500rpm (or does it just charge at a voltage below 12v?).
Replacing the battery with a capacitor bank has reduced my bike weight by about 10lbs, and I no longer have to keep it on a Battery Tender over the winter.
Since my capacitor bank is in permanent use, I purchased a replacement from
Ebay. I still need to buy a LiFePo4 battery to test how well it works at replacing a full-sized car battery. My hesitation is that LiFePo4 batteries are not to be charged when they are below freezing, or instant and permanent damage will occur. I'd want some way to keep them warm or restrict charging when below freezing.