Continuing from here: https://www.qashqaiforums.co.uk/viewtopic.php?f=29&t=8995
Warning...this is DIY experimental circa 2017/18. Two videos. The second (part 10) video is 1 year of testing in car.
The engine being started is 2 ltr and at temperatures below freezing.
The first video is 7th of about 12 episodes covering the development of Super Capacitors combined with a LIthium (LiFePo4) 12v 10Ah lightweight/small Battery. Compare that to your heavy AGM/EFB etc battery of 12v 70Ah. The small LiFe Battery here is not being directly used to turn the starter motor. Its the high energy stored in the Super Capacitors doing that job.
The electronics/wiring is a complication of the in-car monitoring (initial safety/performance) this guy has incorporated. Its not necessary normally, His cost to build here is about £150.
So this is a Hybrid arrangement that dramatically diminishes the weakness of big Batteries. Main problem for DIY is the quality of the Super Capacitors. Those here are about £9 each.
Reviewing a number of videos on this topic, the US folk sometimes mention that these systems are being integrated into some cars by makers. Nissan where are you?
Part 7 - 1 month in Car update (Test is with Hybrid in Boot of car...long cables to front)
Part 10.. After 1 year under test in Car (Hybrid modified and in Engine Compartment..main old Batt gone))