DIY starter LiFeP04 battery for car

[draghixa]

Hi all, I want to build a starter battery for my car. I took some reference on internet and find this LiFeP04 battery on Walmart

https://www.walmart.com/ip/Deep-Cyc...ergency-Start-1200CCA-Group-Size-31/458039754

which has a function called "Emergency Start". I believe it is for an one-off start should the battery is over discharged and shut off by the BMS. I would like to have your advice on how the circuit is designed for this function so I can add it to my DIY battery, thanks very much!

 

[HRTKD]

Most cars pull too many cranking amps for the common BMS devices that many of us are putting on our DIY LiFePO4 batteries.

I'm concerned that my trailer's Onan 5500w on-board generator will pull too many amps at startup for my DIY 560 Ah LiFePO4 battery bank, even if I bypassed the BMS. Starting a car would be a lot more intense.

 

[Luthj]

Typically start current is about 30-70A per liter of displacement. 70 being diesel, 30 being gasoline. Very cold engines need more. For Example my 3 L turbodiesel needs approximately 200 A for five seconds to start when cold. That includes approximately 40A for the glow plugs.

If you set up your BMS so that it disconnects with a five to 10% reserve margin. That will allow you to still start your vehicle even if the battery has reached the disconnect state. So when you push that emergency start button you are simply activated BMS from a slightly early disconnect to the absolute lowest disconnect setting long enough you to start the vehicle.

There is a brief spike in current while the starter motor spends up. This can be as much is 800 A. But it only lasts for 15 to 100 ms.

If we are talking less than 25 seconds of duration, Most LFP cells can likely sustain 2C Without any damage.

 

[JoeHam]

Remember that if you ever drive in below freezing temperatures you cannot charge the battery.

That is the weak point of LiFePO4 for this application IMHO.