I am currently working on my six battery balancers so I don't cook my AGM batteries when I charge them. I am using a very simple, old design by Lee Hart, a very experienced EV guy who is prominent on the EVDL (Electric Vehicle Discussion List).
I won't bother trying to explain how they work; I will just give the link that explains it very well:
Lee Hart's Minimalist Battery Regulators
I am using a 6.2V and 6.8V zener diode in each regulator so they start shunting at 13.0V.
Here is my finished product:
Once these are finished I can install my Kelly charger. Then I should see a dramatic efficiency increase.