Common port BMS cutoff?

[TRS300]

Hi all, Just finished my first battery 13s 15ah 60A. I'm using a DALY common port BMS and having looked at it I'm trying to understand how this BMS shuts off the charger when it reaches full SOC? Since the charger is connected to the batter discharge lines I don't see anywhere in the circuit that could disconnect the charger once it's fully charged. Or is that actually managed by the charger?

 

[amberwolf]

fi it's a common port BMS, then the charge and discharge fets are wired in series, so turning off charge fets turns off current going in.

Turning off discharge fets turns off current going out.

Turning both off turns off all current.


If it's not a common port BMS it cannot prevent current going into the discharge port, or out of the charge port.

 

[aae34]

It likely has separate fets for charging&discharging.

 

[amberwolf]

It likely has separate fets for charging&discharging.

There will always be that, because they have to be wired in different directions to block the current flow appropriately.

The difference is whether they are wired in series on the same external connection (usually P-, with no C- present), or separately on two different external connnections (P- and C-).

 

[Epithemeus]

I don't see anywhere in the circuit that could disconnect the charger once it's fully charged. Or is that actually managed by the charger?

As Amberwolf said, it can shut off charge current. But it should be your charger stopping charging in normal operation with the BMS blocking charge as an extra safety meqsure only if something has gone wrong. Like the charger not working properly, or a single cell group being overcharged.

 

[TRS300]

I just put this together and the positive from the charger is connected directly to the battery. Please see the wiring diagram I posted. I can't see where there would be positive fets anywhere in the circuit.
BTW: The charger did shut itself off. If it didn't I think the bms opens the main power fet on the negative power lead. It's the only way I think this can work. If you look at the diagram you'll see what I mean. The positive power lead bypasses the BMS and is connected directly to the battery.
I assumed this is how all common port BMSs are wired... Maybe not?

 

[Epithemeus]

You don't need to cut off positive specifically. Shutting off only one of either the + or - side is sufficient to stop current flowing. In this case they chose to shut off the negative side. I have the same BMS, I have seen it shut off charging current before.

 

[TRS300]

You don't need to cut off positive specifically. Shutting off only one of either the + or - side is sufficient to stop current flowing. In this case they chose to shut off the negative side. I have the same BMS, I have seen it shut off charging current before.

Thank you...

 

[amberwolf]

I can't see where there would be positive fets anywhere in the circuit.

There aren't, in any typical BMS. They simplify the whole electronics system by only using low-side control on battery negative.

Contactor-based BMS can easily use a two-pole contactor that breaks both sides, but FET-based ones don't (none I've ever worked with or seen so far, at least).