I've had a 4.2 Ah hobby LiFePO4 battery and a 6s Supercapacitor setup in the Acura for quite some time, as the Pb-acid battery had become too weak. I relocated the battery into the glovebox using about 10 feet of 10 gauge wire, with the supercapacitor sitting where the original battery used to be. These are connected in parallel, with the battery keeping the caps topped up, and the caps delivering the majority of the cranking power.
In a rush to get the girls to swim class the other day, the car wouldn't start. Had to pop the hood open and use a portable jump pack to get the car started. I figured the battery was on the way out, but in hindsight I think I just hadn't fully plugged in the trickle charger (designed for use with LiFePO4).
With the inlaws in town this weekend, I decided I needed to complete my battery project, so they have reliable transportation.
The LiFePO4 32700 7Ah cells were purchased for $4 each including shipping from Aliexpress. The price was so low that I was initially skeptical and only bought a few to test ($40 tester also from Aliexpress). Every cell delivered well over the rating (about 7.4Ah), and seemed to have consistent quality.
I struggled to get good spot welds on the nickel tabs, mostly due to bad technique. Then my project sat for ages while I tried to figure out how to solder 10 gauge wire terminals. It sat for so long that one day I accidentally dropped car keys on the battery and sparks went flying everywhere. If you see burn marks on the battery, that's why.
I purchased a Pinecil portable soldering iron that goes up to 90 watts. My USB-C charger could only output 60 watts, but that was still enough to sloppily solder 10 gauge wire to the tabs.
I've got an XT90 connector to attach the battery in the glovebox of the car.
So far I'm only measuring a peak of 60 amps when cranking the car, but it requires about 290 peaks amps, meaning the supercapacitor is doing the bulk of the cranking. Perhaps I'm just not getting an accurate read. 60 seems low to me. I'll throw my voltmeter on there to see how low it sags when starting.
The balance circuit was $10, also from Aliexpress. These are all pre-inflation prices.
I bought enough to make 2 more batteries, so I'll hone my skills and make a replacement motorcycle battery. The neat thing is I can use the LiFePO4 trickle charger on these while the balance circuit takes care of keeping the cells even.
UPDATE:
I measured 174 cranking amps from the supercapacitors, and 60 from the battery. Battery voltage sagged to 9.64v while cranking.
That distribution of power flow seems about ideal to me. The length of 10 gauge wire must present enough resistance to limit amps from the battery.
I need to replace the 300 amp breaker with a 100 amp one. I'm not even sure a dead short would trip the 300 amp breaker.
Here's the hobby battery I replaced
Here's the 350 farad supercaps in a foam box connected where the battery used to live
