balancing strategies as they relate to BMS

[ArthurEld]

Thanks for the great ideas guys!

 

[ArthurEld]

The busbars are an open question.

I was planning to use 1/8 x 3/4 110 copper bar
But for parallel I might use the free ones

 

[BiduleOhm]

If you want to use a BMS while parallel charging, just hook all the cell sensing wires together. The BMS isn’t smart enough to know if it’s hooked up to the same cell, and will disconnect at the high voltage setpoint.

Pretty sure it'll not work in 99 % of cases as the BMS need a voltage high enough to power itself.

 

[Craig]

If you want to use a BMS while parallel charging, just hook all the cell sensing wires together. The BMS isn’t smart enough to know if it’s hooked up to the same cell, and will disconnect at the high voltage setpoint.

How will the BMS get enough power to operate at 3.3 volts?

 

[Craig]

Pretty sure it'll not work in 99 % of cases as the BMS need a voltage high enough to power itself.

LOL you beat me to it

 

[Gazoo]

Couldn't the BMS be powered externally with a power supply?

 

[John Frum]

If you want to use a BMS while parallel charging, just hook all the cell sensing wires together. The BMS isn’t smart enough to know if it’s hooked up to the same cell, and will disconnect at the high voltage setpoint.

Can someone please test this?
It would be great if a BMS still works with all but one cell dead.

 

[ArthurEld]

Why are you sad about my busbars smoothJoey?

 

[MisterSandals]

If you connect all the balance wires together, doesn’t the BMS see 3.x v for cell 1 and 0 v for the rest?
My understanding was that the BMS expected an incremental cell voltage increase to subsequent leads.

And if that is correct, wouldn’t the BMS disconnect on any 0v cell?

 

[ArthurEld]

This will be even more confusing.
Can you see if their is continuity and/or voltage between dc_ok and ground when the charger is off and when it is on?
This drawing is simplified to show how to charge one cell and use the pack bms to programmatically disconnect the charge source.

cc_cv {
    charge {
        pos
        neg
   
    }
    dc_ok {
        pos
        neg
    }
}
bms {
    neg {
        upstream
        downstream
    }
    balance.0<-cell.0.pos
    balance.1->cell.0.neg
    balance.2->cell.1.neg
    balance.3->cell.2.neg
    balance.4->cell.3.neg
}
opto_coupler {
    upstream {
        pos->cell.0.pos
        neg<-bms.neg.downstream
    }
    downstream {
        pos->cc_cv.dc_ok.pos
        neg<-cc_cv.dc_ok.neg
    }
}
battery_configuration {
    cc_cv.charge.pos->+cell.0-|+cell.1-+cell.2-+cell.3->bms.neg.upstream
                              |->cc_cv.charge.neg
}

I get no continuity between ground and DC-OK with power on or off.
I get 3.8V for less than 1 sec between ground and DC-OK with power on.
I get slight voltage for less than 1 sec with power off.
My experience with electrical is minimal but I think I checked it correctly

 

[John Frum]

Why are you sad about my busbars smoothJoey?

Because the are rated for a amperage for 4s and above and usually not even that.
They are not rated for the voltage drop for this use case.

 

[John Frum]

I get no continuity between ground and DC-OK with power on or off.
I get 3.8V for less than 1 sec between ground and DC-OK with power on.
I get slight voltage for less than 1 sec with power off.
My experience with electrical is minimal but I think I checked it correctly

I was hoping we could use dc_ok as a control signal for the charger.
Does not look encouraging.

 

[ArthurEld]

Shorting pins S+ and S- turns the power off. I did it by accident poking around.

 

[John Frum]

Shorting pins S+ and S- turns the power off. I did it by accident poking around.

cool !
there ya go!

 

[toms]

Can someone please test this?
It would be great if a BMS still works with all but one cell dead.

I use a Batrium BMS.

 

[John Frum]

I use a Batrium BMS.

That's great.
Did you test the scenario?

 

[toms]

Yes, the Batrium will function with the cells in parallel. It has an external power supply (even when running the cells in series)

 

[Craig]

Yes, the Batrium will function with the cells in parallel. It has an external power supply (even when running the cells in series)

That's part of the reason a barium is like $600.00. Most even without an external power supply will not work this way.

 

[mrzed001]

Stupid idea (just thinking), but what about normal serial charging the cells (to 90-95%) with 2-5A ... using an active 5A balancer like this: https://heltec-bms.com/project/5a-capacitive-active-equalization-active-balancer/

The higher V cells will not charge because all its charging A is transferred to lower V cells.
And then charge last 5-10% to each cell separately ?

 

[John Frum]

Anyone see a potential short here? @BiduleOhm
I just don't see it.

battery_configuration {
    cc_cv.charge.pos->|+cell.0-|+cell.1-+cell.2-+cell.3->bms.neg.upstream
                      |+cell.1-|->cc_cv.charge.neg
}