[hayden55]

I would consider something like the lithium battery cell format (18650, 21700, 20650, 20700) with something of a rating of 30A+ of max burst discharge. The starting of most vehicles and things is more of a momentary max so you just need to be able to survive this without overheating your cells and aim for a low c of discharge. Typically, you will be able to charge at a much higher c than you can discharge (which is good). A lot of people don’t like 18650 because they remember the old cells which were only rated at 6A max discharge… meanwhile at the time Nimh cells of the similar format were rated at 60A max discharge. So, if you throw a couple of 3.6V cells in series you’ll hit ~14.5V nominal and this “S” will be able to handle 30A per “S”. Then you can parallel a couple more “P” ‘s to get your max amperage capacity and your total usable capacity you will need.
Typically, deep cycle car batteries are rated for 80% dod with a 400-cycle life, while lithium cells at will have a 92% dod capacity with a 500-750 life cycle. For reference of what can be achieved if you stay between 40-70% depth of discharge the Panasonic cells in the Tesla are rated for 40,000 cycles. Of course, with car batteries if you don’t sit and drain your car battery with the engine off they will last much longer so life cycles depend on C draw and dod. I would look more into building one, but I’m in college and can’t afford to fix something that isn’t broke. So, say if your car has a 200a starter you would take 7 P’s in a 4s config. Then just see if you’re capacity is large enough and add from there. Also, you would need to protect from overheat, overvolt, and over discharge which isn’t super hard with what is available now. At least the overcharge won't happen since the pack will be large, and the overdischarge feature is already a feature on the Prius unless you override it. You could even integrate a battery warmer and a cooler. Still going to be like a 300$ battery, but at least it will weigh nothing and be super cool. You could also sub in lithium iron cells in the aluminum box format from EV power. They are cheaper for capacity but less power dense. So, I would say super capacitors aren’t necessary unless you want to use them.