Thanks for the thoughtful reply.
It’s taken me a while to figure out what’s going on when I interact with this poster in his current (and prior) incarnation.
I think the issue is he assumes when someone asks for a clarification (or pushes back on his assertion) that the person is wholly ignorant and requires an explanation from scratch. This is a cognitive quirk I have often seen in engineers. I think it is a drawback of university-as-votec.
Don't conflate what function a BMS performs with the health of the battery.
I had not mentioned state of health at all. This explanation followed:
a BMS helps protect the health and give us the longest lifetime with no fire).
He continues (and I wll not comment further):
A BMS does use voltage (temp does play a factor also). Just like the gas light in your car, at a certain level, it turns on (or, off in this case). There's a lot of argument about the best way to configure a BMS. THAT's the argument I was trying not to get dragged into. (Thanks for not doing that heheh) The health of a battery (often written SoH) is a much more difficult thing to ascertain. It involves a LOT of variables. It's difficult to pin down after it's been in use for some time though, since it's like being asked to navigate to a place with no map. How can you be sure where you are, or where you are going?? If you monitor the battery over its entire life, you can get a very good idea of the health at any point in time, since you've tracked it its entire life. The most reliable way during this time is simply to calculate how much current the battery has been discharged and charged with (technically, "Coulombs"... easy way to describe it is an Amp-Second... so there are 3600 Coloumbs in 1AH). Over time, there becomes a disparity between how much it has discharged and how much it will charge again. This begins to be evident. As a quicker method, we can measure the specific gravity of a flooded battery. We can also actually put a load on the battery and see how it performs. When you take your battery into a service station to have it properly tested, they SHOULD keep it overnight to perform several operations on it. Charge it until the charger says it is full (hopefully a calibrated charger), then let it rest for at least 24 hours to allow the surface charge to bleed off, then put it under a load and see how it performs. The further away from spec it tests, the more likely it is to "need replaced". But if you have taken your battery into the shop... they are going to tell you it needs replaced. I've taken known good batteries into pep boys and they have said it needed replaced. Turns out it was just a bad ground on the engine block. Hmmph. Do they work on commission I wonder?? Lithium needs a BMS to protect it since the tolerances are so tight, and we can't use internal resistance (or voltage) to tell if it's good or bad like we can with pb. Load testing and whole-life monitoring are the only real ways to know how their SoH, from what I know, anyway. There's a lot of work being put into Li right now, so any day some magical device may come out that I am not aware of though. The easy way to think of a BMS is just a "clipping" circuit. Voltage goes too high or too low, shut the voltage off. Like I said before, it's highly debatable on how to configure that, 10-90% or 20-80% are the two most common schools of thought. But in pb-land, there's just as much argument over certain things. Everyone in this sub swears that taking an FLA battery to even 49% SOC will kill it instantly... that's not the case, and damned far from the truth. The manufacturers provide a handy chart for each product they produce that shows the anticipated life (in charge/discharge cycles) based on DOD. A good rule of thumb is below 50% REALLY shortens the life of the battery, but it's not going to die instantly. Lithium OTOH will straight up get damaged beyond repair if you take it too far past the knees. That is why a BMS is require on them. +1 for proper semi-colon usage.