4S LiFePO4 as car battery without BMS?

[Piotrsko]

I believe, but could be wrong, that the C rating is actually how much heat the battery can tolerate and not go into thermal runaway. Short bursts might be possible with a temp soak period to average back. My toy plane lipos have often gone over their C Rating for a couple of seconds during takeoff.

[Stubby79]

"C" refers to the capacity (AH) of the cell.

so 10C means it can put out 10x the capacity, in amps. (10ah @ 10c = 100 amps)

They're usually rated at continuous and, separately, at burst ratings. Running at burst ratings for too long will cause them to overheat.

[19bonestock88]

Originally Posted by Stubby79

"C" refers to the capacity (AH) of the cell.

so 10C means it can put out 10x the capacity, in amps. (10ah @ 10c = 100 amps)

They're usually rated at continuous and, separately, at burst ratings. Running at burst ratings for too long will cause them to overheat.

The cells are 15Ah and rated at 10C continuous and 15C burst. I think it’ll be fine for starting the car. I’m more concerned with charging in cold weather. I’m planning to insulate the pack and mount it in the cabin, but I fear that on the coldest evenings the pack might become too cool to charge

[Stubby79]

Battery warmer(plug-in) and a decent temp switch in the middle of the pack? If you're plugging it in to charge it anyway...the issue will be when you go and start it out in the field, below freezing, and the alt kicks in...

[19bonestock88]

The plan isn’t to plug in to charge, but to leave it connected to the car the whole time. Putting the battery inside the cabin of the car and insulating the pack will do a little to help but I fear that if the car sat a few days in severe cold the pack would get cold enough to suffer damage from charging

[redpoint5]

I've been looking at this to replace car batteries.

https://www.amazon.com/dp/B07X7MD2JK...v_ov_lig_dp_it

It says max burst of 80A, but 200A for 1 second should be fine, or I can pair it with supercaps.

[serialk11r]

The real reason you need high C-rate cells for a car is so the alternator doesn't wreck the battery when charging. In warm weather at least, you can get away with a pretty puny battery for starting a car, and the battery won't be damaged from high current for 1 second.

If you want to use LFP, you need to do two of these three:
1. Keep it topped up
2. Avoid cold weather
3. Don't be tempted by "3x Pb equivalent", get a bigger pack (I would suggest 20Ah at the very minimum)

A proper BMS for this purpose is actually quite expensive since it needs to be capable of switching several hundred amps. If you avoid letting the battery drain, you can get away with only cell balancing. Otherwise, you need low temperature and low voltage cutoff with a very large MOSFET bank, and it's not cheap.

[RedDevil]

Lithium is finally going to make its way into production cars to replace lead-acid 12V batteries...

https://www.youtube.com/watch?v=YAtLTLiqNwg&t=2298

Skip to 38:19.
It did that at first, now it starts from the beginning? Weird.

[samwichse]

The Kia Niro has a lithium 12v battery as well. Since 2017 I think. IIRC if you drain your 12v system, there's a button you can push that says "12v reset" that dumps power from the HV pack into the 12v pack and you're good to get going again. Like a built-in booster pack .

[Stubby79]

Funny...I was thinking that very thing as I sat with the engine off in my insight, waiting for the light to change...need to put in a switch to manually turn on the DC-DC converter, in case I ever drain my 12v battery too far!