BMS Low Voltage Cutout

[robwalley]

I am just beginning to tweak settings on a BBS02 controller via the Bafang ConfigTool software and USB/Serial cable and it occurred to me that there are probably two different low voltage cutouts. One I have set for the controller and another on the battery BMS. I have a 14.5a 36V which the BMS LVC is probably around 31V but I don't really know.

Anyone know if it's worth setting the controller higher or lower than the BMS and are there different symptons to each cutoff, i.e. BMS cuts off and no power at all; Controller cuts off but display errors maybe??

 

[danielrlee]

BMS LVC (low voltage cutout) should only be relied upon for protecting the battery from irreversible damage caused by over discharging. Controller LVC should be set at a higher voltage - IMO, around 3.5-3.6V per cell (series string) for 18650 format cells, since available energy below this voltage is almost zero.

 

[Woosh]

I am just beginning to tweak settings on a BBS02 controller via the Bafang ConfigTool software and USB/Serial cable and it occurred to me that there are probably two different low voltage cutouts. One I have set for the controller and another on the battery BMS. I have a 14.5a 36V which the BMS LVC is probably around 31V but I don't really know.

Anyone know if it's worth setting the controller higher or lower than the BMS and are there different symptons to each cutoff, i.e. BMS cuts off and no power at all; Controller cuts off but display errors maybe??

It's about 29V.You should set LVC on the controller to 31V.

 

[robwalley]

It's about 29V.You should set LVC on the controller to 31V.

Sounds like a plan, but is the result the same? i.e. power cut's out or does the display stay on with Low voltage warning. Not and issue with the new colour display but just curious. Also 3.5v per cell as per as per other post seems a tad high to me.

 

[danielrlee]

Sounds like a plan, but is the result the same? i.e. power cut's out or does the display stay on with Low voltage warning. Not and issue with the new colour display but just curious. Also 3.5v per cell as per as per other post seems a tad high to me.

3.5V does appear to be a bit high if just looking at the numbers, but voltage to energy capacity is not a linear relationship the whole way through the discharge curve. Below 3.5V is only around 5% of remaining energy contained in the cell. More importantly, IR (internal resistance) raises substantially below this point and therefore so does the amount of heat generated by the cell when continuing to discharge much lower.

 

[Fordulike]

Sounds like a plan, but is the result the same? i.e. power cut's out or does the display stay on with Low voltage warning. Not and issue with the new colour display but just curious. Also 3.5v per cell as per as per other post seems a tad high to me.

Battery BMS LVC will cut power fully, until the battery is reset. This will obviously turn off the Bafang totally.

The Bafang LVC soft cuts the power. It feels like a stuttering sensation in the motor unit. The display will stay on, but power is drastically reduced.

 

[robwalley]

3.5V does appear to be a bit high if just looking at the numbers, but voltage to energy capacity is not a linear relationship the whole way through the discharge curve. Below 3.5V is only around 5% of remaining energy contained in the cell. More importantly, IR (internal resistance) raises substantially below this point and therefore so does the amount of heat generated by the cell when continuing to discharge much lower.

What you say does seem to tie up with 18650 cell discharge curves I've been looking at and in practice this is the reality as you will be charging before this event rather than leaving it run to 31v as you can with the colour display, but useful to have the reserve just in case even if you can only pull a few watts for a short period.

 

[danielrlee]

What you say does seem to tie up with 18650 cell discharge curves I've been looking at and in practice this is the reality as you will be charging before this event rather than leaving it run to 31v as you can with the colour display, but useful to have the reserve just in case even if you can only pull a few watts for a short period.

That's about the size of it. There really isn't a 'correct' answer, just a balance between capacity per charge cycle and total pack life.

Something that should be avoided though is a high discharge rate of a deeply discharged pack. The perfect solution to this issue is a controller that tapers the discharge current when it hits LVC, similarly to how a Cycle Analyst v3 does in order to avoid a sudden cutout and remain above (or at) LVC.

 

[Fordulike]

I reckon bottom line, it's much better to set the Bafang LVC to shut the power before the battery BMS. It'll be kinder to your battery.

 

[robwalley]

Now set to 31v. However now I have the colour display I can at least see what is going on and charge at around 35-36v and if still out and about I can be very light on the assist and avoid the throttle.

 

[anotherkiwi]

What you say does seem to tie up with 18650 cell discharge curves I've been looking at and in practice this is the reality as you will be charging before this event rather than leaving it run to 31v as you can with the colour display, but useful to have the reserve just in case even if you can only pull a few watts for a short period.

My LVC set to 31 V but there is really nothing left below 33.5 V. I just switch to assist level 0 and keep the LCD and the lights on. Often the battery will come back up to about 34-35 V which is just enough for a short burst at assistance level 1 for the last hill before home...

 

[robwalley]

First outing to test low voltage cutouts on battery today and a few things to note.

1. Cutouts appear to be only if the voltage goes below LVC(31v) for a few seconds or more, certainly doesn't happen instantly, read below 28.4v for a second on a steep hill and didn't cut out (Although this appears to be below the BMS as well - nothing happened)

2. 31V is too high (measurements/readout may also be inaccurate) - will change to 30v and test. Still plenty of power at 33.5V

3. Just to preach to the converted, hill's kill batteries. After LVC cutout on steepish hill, still load of miles of flat touring left in the tank.