A site member spotted the passive voltage bypass on my batteries and asked me about them. I put together some images and a write up on how to build them and decided to post it here in case anyone else wanted to build them.
One of the challenges when batteries are charged in series is that the entire pack may became "out of balance". Even if you have a voltage converter and draw power from the entire pack, small differences in battery chemistry can make individual batteries have different charge/discharge profiles.
Once the pack starts to get out of balance, the batteries that reach voltage first will start to boil while the others will be undercharged due to reduced current available to them or due to the charger prematurely reaching its set voltage and turning off.
These devices basically sit there without doing anything most of the time, but when you charge the pack and the voltage across an individual battery reaches 10.2v (2.55vpc), the zeners start to conduct and they bypass some current around the battery so that the other batteries can continue to charge.
I saw the original concept in the EV car forums from a guy named Lee Hart, I did not like either of their implementations so I made some changes that I think makes the devices more reliable.
Here is a link with more information:
http://www.evdl.org/pages/hartregs.html
Diagram (Picture #1):
The part numbers are from Mouser electronics. The second circuit is to test the actual zener voltage of the diodes as they have vary.
Test Circuit (Picture #2):
I used this simple test circuit to measure the actual zener voltage of each diode and pair them together in order to obtain 10.2v. The particular zener in the picture (bottom left) tested at 5.18v, so this unit would be paired with a 5.02v unit.