The capacitors are still within 0.01v of each other after a dozen or so charge/discharge cycles, so I haven't implemented a balance circuit yet.
Laserhacker references diode part #1N4001, which has a forward voltage drop of roughly 0.5v. The red led has a voltage drop of about 1.8v, so combined the circuit should limit each capacitor to 2.3v. Multiply that by 6 capacitors, and the circuit will slowly bleed off anything above 13.8v. Since the capacitors are rated for 2.7v, and automotive alternators typically charge to 14.5v, it seems a waste to bleed off the "extra" voltage.
I tested various LED colors for measured voltage drop, and will attempt to build a balance circuit that limits individual capacitor voltages to 2.6v instead of 2.3v.
Here are the results of the forward voltage drop for various diodes:
Diode - 0.52v
Red diffused LED - 1.8v
Red clear LED - 1.64v
Green diffused LED - 1.79v
Orange diffused LED - 1.61v
Yellow diffused LED - 1.7v
Yellow clear LED - 1.79v
If I place 2 diodes and 1 orange diffused LED in series, I get a 2.65 volt drop. When I get a chance, I will test various configurations for their performance at balancing the capacitors at various states of charge.
