I have finally got round to designing the balancing circuitry for my ultracapacitor banks, which have a total voltage of 15v when fully charged. The 2600F capacitors have a max working voltage of 2.5v each, so to ensure reliable operation, balancing is required to make sure that each capacitor is charged fully.
The circuit above is a simple shunt regulator, which uses a 2.2v zener diode to regulate the voltage across the capacitor.
A 10W 1Ω resistor is connected to the BALLAST header, while the capacitor is connected across the INPUT. Once the voltage on the capacitor reaches 2.6v, the MOSFET begins to conduct, the 1Ω resistor limiting current to ~2.6A.
Each capacitor in the series string requires one of these connected across it.
Below is a link to the Eagle project archive for this. Includes schematic, board & gerber files.