Inspired by youtuber Lasersaber of laserhacker.com, I built this unit 3 years ago and have never looked back.
6x Maxwell 350 Farad Super Capacitors In series, these units produce just over 58 Farads at 16 volts. It is enough to start a 4 cylinder or a 6 cylinder car. But you only get one shot so if the engine doesn't start, you'll need to recharge it. This is why I added the small 7AH SLA battery. So if the car doesn't start with one go, I just wait a few seconds and the battery recharges the boostpack again for another try. I've done this 3 or 4 times in a row as a test and it worked fine, but didn't go further since I didn't want to hurt the small battery. I have no protection circuitry to protect the battery from going too low. Due to the low cost to replace it if necessary, ($12 on Amazon) I didn't feel the need to dig into that at this point. I've seen other recent videos of people adding all kinds of things like high wattage resistors between the battery and boostpack, charge controllers, etc. I just don't think it's necessary. I mean come on, this has been running for me for 3 years so far with no issues and none of the fancy circuitry.
I don't think that the average person would like this setup for the long haul because the capacitors, even with the addition of the small 7AH SLA battery, just doesn't have a lot of reserve power. I.E Leave the headlights on for more than a couple min and you're gonna need a jump start.
Consider searching for Lasersaber's Channel and watch his videos about this project.