Bms with and without voltage protection

OK so I bought 4 different battery management systems as in the picture. I want to know which components would be responsible for protecting the batteries from discharging below the minimum voltage.

I know the green board with the red X under it (3rd left to right) does not have the protection circuit because the manufacturer told me so.

I think the blue board with the blue checkmark (2nd left to right) does have it because I've had it connected in my circuit for a while and they've never gone under.

The one in the bag should have it and the ones on the far right (3x) might not.

Could someone tell me if and which of the components are present on the boards are responsible for that protection? I'm guessing it would be opamp, right?



Should be using a comparator, not an opamp.

I would surmise that any BMS device which does not monitor each individual cell of a battery bank cannot possibly provide undervoltage (or over-charge) protection to a lithium battery system. Does all 4 devices shown offer individual cell connections.

Yes they do have individual connections. My doubt cones because I wired the batteries and connected them to an mcu and let it run because I wanted to see how long that battery would last. But it ran the batteries down to 1.2 and 1.3 which renders them useless and it's what I thought a bms should protect against.

Indeed they should.

Any system that does not control the minimum discharge voltage isn't worth having. However, even those designed to monitor for minimum cell voltage will still demand some current from the cells being monitored so if left connected for extended periods will inevitably discharge the cells below their minimum safe level.

You also need to ensure the BMS you have is specified is suitable for the chemistry of the Lithium cells in use. Differing lithium cell chemistries produce different voltages for both full charge and minimum charge.

To get a specific answer to your questions we need to know the manufacture and model number of the BMS units and the chemistry of the lithium cells.

OK but you're saying that even knowing the li chemistry and manufacturer specs, any cells left for too long will eventually discharge completely?

If that's the case then I need another way around this issue because I will be leaving this out in the field and I expect that at some point they might dischsrge completely. I mean I will have them on a solar panel but if there are 3 days or 7 days of clouds, it might happen.

Then it comes down to capacity. Get a battery system large enough to last for several days.
It might help if you advised exactly what you wish to do rather than simply referring to an X-Y project.