BMS low cutoff voltage rather low

[ferez21]

Hi,

I'm building a 10S4P pack for an escooter, and im going to use the BMS in the photo:


The BMS low voltage cutoff is rated at 2.8v, which sounds too low to me, how do I make the LVC higher (do i need to add another component)? And what is the recommended LVC?

I will mention that this pack will be connected in parallel to the original one, that already have BMS and a cutoff of around 3.3v i believe.
Thanks

 

[docw009]

If used in an ebike, your bike controller probably cuts off at 3.0 volt per cell. I've got some 10S packs where the cutoff is 2.5V.cell, but they trip my bike controllers when they sag under 30V. This happens when the nominal volts are down to 32-33V.

You could buy a (edit) BMS controller with more appropriate specs. Also, are there balance circuits on the other side of that card? They can take a very long time to do their job, but I would rather have them in my battery.

 

[ferez21]

All the controllers from China i look at have an LVC of below 3.0v, so i dont have any other alternative (for my budget)

 

[dogman dan]

2.8v not really that low, since you will cut off sooner when the pack is under load. And high enough to save cell damage if the drain is very low amps and there is no sag under load.

 

[fechter]

There is no way to change the LVC setting on a BMS like that unless you replace all the voltage detector chips.

2.8v is probably OK as you need some allowance for voltage sag under load. You should not normally be draining the pack to the point where the LVC is triggering on the BMS anyway. Your pack level cutoff in the controller should always be triggering first (or just watch the voltage manually).

 

[ferez21]

Is there any cheap component/circuit i can add just to limit the voltage of that pack?

 

[fechter]

Is there any cheap component/circuit i can add just to limit the voltage of that pack?

Sorry, I'm not really understanding the question. I see you have a scooter and want to run two 10s packs at the same time.

Normally you want the motor controller in the scooter to cut off before the BMS in either pack cuts off. It is possible to change the cutoff voltage in the scooter.
To change the cutoff on the BMS board is very difficult.

 

[ferez21]

The cutoff of the controller will affect the auxilliary pack as well?

 

[flippy]

the cutoff means the pack will be disconnected and you lose that pack putting the full load on the other near empty pack.

 

[Sunder]

Is there any cheap component/circuit i can add just to limit the voltage of that pack?

You can put a cheap low voltage alarm that will warn you at a set voltage but won't cut power. There is no simple way of modifying a non programmable bms to trip at another voltage.

There are BMSes that make "allowances" for low cell voltage when the pack is under load. The one on the vectrix for example, will show a low battery warning when the pack is at 125v on idle, but not if it even dips as low as 110v under load. Simpler BMSes just set the trip voltage lower. If you've ever had a look at a discharge graph, all voltages fall off a cliff near the end. So 3.3v to 2.7v is probably only 5% difference in capacity. Tripping either is shortening your battery life.

If your other pack has a 3.3v cut off, its probably ignoring any measurements under heavy load.

Either way, your two batteries must be kept at the same voltage. That means when the 3.3v BMS trips, all load will go to your new pack. The sudden increase in load will probably trip that too.

 

[docw009]

The cutoff of the controller will affect the auxilliary pack as well?

Yes, and we're all saying that since it is higher than either of the battery packs, that's what controls the LVC on your bike. On some controllers, you can use the LCD to adjust the controller LVC up/down a little bit.

The ebike controller box also can be set by component changes, but usually should be left alone.