How does a contactor BMS work?

[MullerEnergy-Australia]

So I've done a fair bit of work on MOSFET BMS before and we even have our own version of the JBD BMS (review video linked below for those interested).

I've got a very good understanding of how a MOSEFT BMS operates, with opposing body diodes so that you can have charge disabled while having charge enabled (or vice versa).
But I don't think I've ever used a contactor BMS. So I was just wondering if anyone had an explanation of how they work when charging is supposed to be enabled and discharge is supposed to be disabled?

Here's my assumption (which might be completely wrong):
I'm assuming that it will basically allow you to draw a current with discharge disabled, but only for a short time and once it senses the discharge, it will open the contactor. And another guess of mine is that it monitors both the battery voltage and the voltage across the open terminals and then closes the contactor again once the terminal voltage is higher than the battery voltage.

Am I on the right track at all?

 

[justgary]

The contactor models of JBD BMSs still have charge and discharge FETS. The contactor provides a low-loss path that bypasses the FETS for the 99.9% of the time when everything is rosy. Basically, the BMS circuit is truncated to just the current sense resistor (and the contactor poles) when everything is OK. This allows high-current operation that might otherwise overheat the FETS.

My problem with these JBD models is that they have an idle timeout that will cause the BMS to open the contactor and then the CPU will sleep until the circuit senses enough current to wake it again. JBD does not provide any status of the contactor in the data stream, nor any way to control the contactor (e.g. turn off that "feature"). The result is that some of us experience issues when trying to use these models in parallel. One BMS will go to sleep and never wake up while the other one handles the full load.

JBD now has "parallel models" of BMS available, but in my opinion all they need to do is allow me to turn off the sleep feature. I have a hardware mod completed and ready to test, but this would be simple to do in software.

 

[yabert]

JBD now has "parallel models" of BMS available

Relay BMS?

 

[aussie desert]

I use a REC BMS which has separate contactors for charge and discharge. High voltage opens the charge contactor, low voltage opens the discharge contactor.

 

[hwy17]

Orion contactor BMS have no magic charge/discharge feature like a FET BMS.

You can design a system with separate charge and discharge circuits and separate contactors for each but if you want a common circuit design then the BMS has to interrupt everything to protect the battery.

I did do that but now I will be building all common bus batteries as I expand. Protection should be a rare event anyway if you configure your charge sources correctly, manage your discharge, and ensure the battery gets adequate opportunity to keep itself balanced.