Update: Schematic is posted here: https://www.youtube.com/watch?v=zBDtUxPdhz4
This was a hobby project I decided to do for fun after my car battery died in my V6 2006 Corolla. The door didn't shut all the way and the dome light killed my old battery. The replacement was $149 and I decided there had to be a better way to make a more durable battery that could last 10+ years with cheaper parts. So I built this digital smart battery as a home hobby project realizing some goals and finding new ones along the way. The battery uses a 7.5Ah lead acid batteries to feed the parasitic load of the car and keep the capacitors topped off. The battery is fed through a low voltage disconnect (cut off at 11V) and then through a 1ohm resistor to limit current draw (keeps the strain low on the battery from starting the car). This feeds to a set of x6 (see below increased to x12) Maxwell 350F super capacitors with balance board that can start my car up regardless of how cold temperatures get. I have soldered thick copper between the capacitors in series to transfer the high current during a start. The capacitors job is to take all the stress from starting away from the battery and onto the capacitors themselves since they do not wear out as fast as the battery (hope to get 10-20+ years out of them but we'll see ;). I have an emergency push button to quick "jump" to the capacitors if a device in the car is left on and the battery is disconnected by the protection circuit to protect it. After a test the push button charged the capacitors in about 15+ seconds. I also have a digital LED volt meter installed that will let me check the capacitor and battery voltages on the fly.
Now I have a home made smart car battery that can start my car in the coldest of weather. If something is left on in the car a couple of days the battery will not be harmed and still be able to start the car! It was a really fun project and cost me around $145 in parts (not cheap but was still a lot of fun building for me). I'll report more after a month of tests on how it's doing. :)
Build Update Note: I have finished adding an extra row of x6 Maxwell 350F capacitors to the smart battery for my V6 for a total of x12 350F capacitors. One row has just enough power for 1 start of the car without if there is no delay. However I've been thinking if something happens and the engin needs to crank a little longer than normal, adding extra capacitors would give me more backup crank time if needed (without having to wait for the capacitors to recharge from the battery over the resistor or pushing the bypass button for a faster recharge). So right now with tests I would say at a "minimum" (more capacitors is better depending on your needs) x6-x12(x12 recommended) 350F for a 4 cylinder, x12-x18 for a V6, and x18+ (or switch to 2000F or 3000F for a V8 or bigger). I also recommend NEVER use non-Maxwell capacitors for automotive. I have tested several (wasted my money) on Chinese/Korean capacitors and their discharge loss is WAY to high. Plus they can not put out the amps near as quickly as a Maxwell in almost all cases I have tested.
The schematic has been posted here: https://www.youtube.com/watch?v=zBDtUxPdhz4