the car's 12v rail would connect straight to the supercap, not the lfp battery.

At the end of the day no lithium chemistry except perhaps lithium titanate can handle the heat under the hood. Heat is the #1 degradation factor of all lithium chemistries. People here on YT have posted results of putting a LiFePO4 battery under the hood and typically within a year the battery has degraded to 50% of its original capacity and one can only assume that with that level of capacity degradation the increase in internal resistance must be pretty substantial. The other issue that has recently been solved is a BMS that can withstand the starting currents of larger gasoline engines or pretty much any diesel engine in a vehicle. Now there are a few dozen options on the market but at the end of the day a mosfet based BMS is not ideal...using a contactor would be the correct way to go about it and would be a lot less likely to fail. You're looking at 200-400A needed for a few seconds to start a small gasoline engine (3.0L V6) and 400A-800A to start larger V6's or most V8's. The smallest diesels are probably going to land in the area of 400A-800A while any pickup truck is going to be in the 800A to 1200A range. Once you step it up to buses, semi trucks, heavy equipment used in construction you have hit a point where lithium titanate oxide (LTO) is going to be the best option as there will be very few options in the LiFePO4 chemistry that will provide the amount of power you need. YinLong 30Ah, 35Ah and 40Ah cells are quite popular but they are only 10C cells...perhaps 15C or so burst rating with a 20C pulse rating. So a 40Ah 6S1P YinLong pack might not start some of the larger diesel vehicles and often times people will go with 6S2P or 6S3P if they want the extra headroom. LTO performs fairly well in the cold compared to any other lithium chemistry which is a huge bonus as well. Cobalt based lithium and LiFePO4 should not be charged below freezing and perform terribly in conditions below 10C. Under -10C their performance is abysmal at best. Toshiba SCiB cells (2.9Ah, 10Ah, and 20Ah) and Lishen LTO (16Ah, 18Ah and from what I've heard has an even higher C rating the 26Ah) look to be the best on the market. I see a lot of datasheets giving numbers like 30C continuous and 50C burst for the Lishens and the SCiB's are 20C continuous and 40C burst with a 75C pulse rating which is likely a <1 Second rating. Toshiba does make some 20Ah SCiB's that are not as powerful as the other so make sure you are getting the high power ones if you go for the 20Ah. I have only seen the 10Ah cells marketed as 75C pulse (and I'm kind of assuming a real world 20C continuous/40C burst for 3-10 seconds) based on load tests on KZbin. Planano 24Ah LTO come pretty close to these two LTO cells. The Shengquan LTO cells are a bit small (6Ah) but are rated around the same C rate you see for the SCiB's....I think they also make a 10Ah cell but overall they are fairly expensive which is why they get mentioned last. IMO the Lishens and the SCiB's appear to be the best balance of power, price, and ease of assembly There is a video of a guy here on YT that made a Lishen LTO battery for his school bus conversion. I believe it was 5S or 6S and 3P? Might have been 4P. Can't remember exactly which cells he used but its a pretty good overview of how a DIY LTO battery can be a pretty simple drop in for the massive Lead Acid/AGMS that usually have to be put in parallel to start some of the bigger diesel engines. It cranks over his school bus without any hesistation whatsoever and will likely last him 10+ years before it begins to see any degradation that would make him consider replacing it....for about the same cost as 3 large 1100CCA lead acid batteries that would last...what 3-4 years at most? Using a BMS on these LTO's is somewhat pointless. LTO is so safe that its pretty much impossible for them to hurt you or start a fire so long as you don't breathe in any of the gases it vents (not even sure if those are dangerous but I know with LiFePO4 they are and I'm guessing its from the electrolyte turning into a gas at higher temps). You can watch some destructive tests of LTO here on YT and you'll see no matter what you do to it they just get a little hot and then the voltage drops to 0. The other thing about LTO is that you can take it down to 0 volts without destroying the battery. Overcharging it can cause damage but you have to significantly overcharge it so as long as your alternator isn't going to suddenly start outputting 17V instead of 14.5V you really have nothing to worry about.....and a BMS that can handle 2000A starting current will run you $200 minimum and will likely cause you more headaches than safety. If you use LTO you just need to look at the voltage curve of your particular cell and decide if 5S or 6S is right for you. The Lishens have a higher nominal voltage than the others 2.5V vs your standard 2.3V or 2.4V so technically you could get away with 5S but most people opt for 6S and don't charge up all the way since the very top of the curve contains very little capacity. Its not until you drop to 2.6V that you really see a lot of the capacity....for the Lishens I think it is about 2.6V to 2.2V contains a huge percentage of the energy so multiply that by 6 and you get 15.6V and 13.2V....maybe a tad too high but you could drop a tenth of a volt off each cell and bring that down to 15V even without losing much capacity....and keep in mind its a starter battery so as long as it has enough capacity to run some interior lights for 30 minutes when your engine isn't running it should be enough capacity to prevent you from accidentally draining it dead by leaving an interior dome light on for 5-10 minutes.

@@ericklein5097 How might you go about preventing an LTO battery from overloading the alternator? Eg being able to limit its current draw to 40 amps? I guess you could have big diodes going both ways, with the ones going into the battery having a current limiter of some sort.

The capacitor has a self discharge rate. If you aren't using it with another battery, it will drain quickly. Keep in mind that your car is also using some power.

Can you explain how you would do that and also what about charging the battery how will the affect it ?

I soldered 1 side of the power resistor to a 4mm2 wire, and covered this soldering with a heat shrink. Then I connected this 4mm2 wire to the + pole of the LIFEPO4 battery via a suitable clamp. Then you connect the other side of the power resistor to the + pole of the super capacitor. The - pole of the LIFEPO4 you connect to the - pole of the super capacitor. Then you connect the black -pole ground wire (1 pole of your car; to the negative - pole of your previous setup) the + pole of your car to the remaining + of your setup. When you are going to use the setup you must have it charged to about 13V and then you start the engine. After starting the engine and letting it run for 10 minutes the battery is charged. I used an 18Ah LIFEPO4 battery from the brand XZNY here. and built it into a battery cabinet from Turmera.

@ would the backflow of the current going back to the battery not overheat and stress the power resistor ? How do you choose an appropriate power resistor?

@ @ would the backflow of the current going back to the battery not overheat and stress the power resistor ? How do you choose an appropriate power resistor?

They've done this. Got something like a 10 mile range I think? But it was stupid fast charging...like a couple minutes. Downside is you need a SERIOUS DC charger. 400kW or something in that realm

I think he uses 6x3000F capacitors, total capacitance is 500F

I bought and installed them. Fabulous!!!!

16v is only the upper limit. The Super Caps will only Charge up to the same voltage of the battery and will deliver a Large amount of Current within seconds. The super caps will only give the Intial powerful kick and the battery will sustain.

yes

no you want the capacitor to be charged to the same voltage before connecting or the capacitor will try to charge itself and cause a big inrush current to come out of the battery.

@@richie4540 ​ ye I already experienced it firsthand, melted my tiny wire rip.

Lifepo4 sometimes have automatic heaters in them.

I'd have thought hot weather plus a hot engine bay would be a more likely issue. But yeah a low-temp heater would be worthwhile in certain areas I guess.

The capacitor won't use that much energy from the battery, so it should be fine, if you start your car once per week.

The capacitor bank will stabilize your batteries voltage.