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BMS required for 12.8v 4s3p headway pack?

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As I learn more and decide on the goals for my EV I keep changing my mind on batteries/motor/controller. This week I'm looking hard at headway cells because of the increased performance for the same price as a TS/CALB pack but with reduced range. My commute is only ~12miles each way and I can charge at work and even the headway pack should give me 50-80 miles at 80% which is probably more than enough, and it provides 2-3x the peak power of the TS/CALB pack. And for this project power is more important than range. The Headway pack is also lighter, so I could add more cells at a later time if I needed more range.

Since I will be ordering tons of cells and probably a few extra in case I have a bad cell or two, I thought about replacing the LA 12v battery with a pack of headway cells. Maybe a 4s3p type pack that would be "charged" by the dc/dc converter. Would this need a shunting bms, or simple voltage monitoring or nothing at all?

I will be building my own bms for the main pack that measures the voltage of each cell in real time and show me the high cell, low cell and average of all the cells. I'm sure it wouldn't be too hard to extend this idea to the "12v" battery but I'm wondering if it's required? The dc/dc converter would probably be set to about 14v which is 3.5v per cell (if they're equal of course)

Remember this pack would have minimal draw unless the dc/dc converter failed or can't keep up with the load. I won't have a power steering pump, I'm switching to a manual rack, so just the usual car stuff, vacuum pump, stereo will be on the 12V line. Having no 12v battery isn't an option so it's stock or something like a headway pack.
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I'm using Headways (8ah) for my lithium upgrade for the same reasons you are. I built a 4S2P pack of Headways to replace the existing 19ah lead house battery. I'm using an IOTA DC/DC convertor, set at 14 volts. And I'm not using any BMS. Last year I built a 24 volt 4S4P pack for my ebike. After riding 1500 miles last year, it looks great. Last week the cell voltages on the bike pack were all sitting at 3.2 volts after resting for siz months. So I believe it is safe enough to go without any BMS.

BTW, I bought 460 8ah Headways from Manzanita Micro and they are the best! All 460 cells initally charged to 3.7 volts, no clinkers. I found 6 cells where the screw holes and threads are a tiny bit out of square, but not enough to warrant asking for replacements.
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