I decided to give this thing a shot. I like Redpoint's idea of using a lithium battery in parallel, but I would like to give this idea a go. I have somewhat changed the design a bit though.
First off, the voltage regulator will be a
300W dc-dc buck converter that I found on Amazon for $14. This will take the 17-21V from the lithium pack and convert it down to right around 14.5V for charging the lead acid battery. The voltage will be set to right above what the alternator charges the battery to.
I am also going to ditch the BMS in favor of individual cell chargers. I was able to purchase 5
12V 3A power supplies for $3 from bgmicro.com. These power supplies will put out 12V to a
5A dc-dc buck converter that is set to whatever cell voltage I want, probably 4.0-4.1V. This will balance each cell every time it is plugged in to charge. As long as it isn't discharged too far, it shouldn't be a problem.
The last piece major of the puzzle is the batteries. I still haven't figured this out yet.
Battery Hookup is out of the cells I really wanted to use, and they are also out of stock on the second choice as well. These were mainly my choices as their prices were very low. As I piece things together I'll keep an eye on what they have. If all else fails, I'll just build a pack from 18650 cells.