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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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But let's say you made your own buck charger, like jack. Let's say that the igbt fails on for some weird reason, which is their failure mode. You could end up with a situation that would dramatically overcharge. The constant current would fail. I think it's nice to know that if you use the wrong bolt or something, that the LiFePO4 cells that never blow up can actually blow up. Not following specs == dynamite is useful to know.
There's an app for that!:D Seriously there's devices called Voltage Relay's that monitor your pack and can be wired to interrupt the charger if it over volts. I installed one made by ABB. Click it then you can read the specs. It's adjustable up to 330V I believe. Mine is wired so that a contactor is energized until the voltage reaches a set point, whatever you want it to max at then it kills power to the contactor. Problem solved. The only drawback to this relay is the voltage is not very precise, you have to tweak it a few times to get it to cutoff where you want it. Mine shuts off now at just over 171V.

IMO, if you're going to invest the equivalent of the price of a small car in batteries, it's a no brainer to have a backup such as this IN CASE of charger failure to shut down and continues to ramp up in voltage. It's CHEAP insurance. I've been running it on mine for a month or so before I upgraded to lead. I wanted to get it working before I installed the new pack. Works like a champ.
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