Cut-off voltage, BMS or motor ?

[Bjr49000]

Hi,
I have a BBS02b kit installed on my bike since a while. Recently I had problem with motor and display switching off abruptly.
I'm trying to understand where the problem comes from.
It seems to me that the problem is battery voltage drop too much under load.

I would like to know if the switch off security features is triggered by the BMS of my battery or by the 'Low Battery Protect' that you can set when programming your Bafang motor.

When my bike switch off, motor and screen go off. To power it on, I have to set the battery switch off and on to switch on the bike from the controller again.

Is there a difference in how motor voltage cut-off and battery voltage cut-off works ?
From what happen, can you tell if it is the BMS or the motor ?

Thanks for answers ! First post

 

[AHicks]

I need to ask for the reason you are running your battery voltage that low, where there are the issues you're wondering about. Generally, I think most of us will charge the battery much sooner, avoiding a practice that's known to be hard on batteries - running them on low voltage.

 

[TNC]

I'm curious about this too. I participated in a thread over on ES about battery and BMS units but haven't seen any responses to what I'm looking for. I'm thinking, understandably, that all BMS units are not equal. While the low voltage shutoff and high temp shutoff may be a more or less universal thing in most all BMS units, all that stuff in between seems to be a crapshoot as to how all BMS units operate. And even the low voltage shutoff may be relatively unknown without testing confirmation. I'm curious why all batteries with a BMS...therefore most...don't have an included data sheet that show the values and parameters controlled by the specific BMS. What's the low voltage number?...what's the high temp number?...what other features are controlled/offered by the BMS?...how can this specific BMS be turned back on once tripped, if possible?...etc. Apparently some BMS units detect a big, abnormal swing in voltage and trip, even though the voltage swing is within safe voltage limits.

Anyway, that's my greenhorn rant on ebike batteries and their BMS units...LOL! Maybe someone here can illuminate me. OP, in your case do you have a Bafang programming cable? Other than programming being one benefit I see with the cable, there is also the ability to unplug the display and plug in the program cable to see if you get response from the throttle...kind of a rudimentary diagnostic step that removes the display as a problem. Also, have you checked the battery itself by using a multimeter for voltage? Since your display goes dead too, it doesn't appear you have a brake or shift sensor killing the motor. If you hook up a $20 programming cable and run the bike on throttle, and it dies like before...I think you pretty much know something is going with the battery/BMS. If the bike continues to run on throttle then I'd look at the display being an issue. Still, battery power goes into the motor first. This could involve everything from a loose connection, partially broken wire, etc. I say "loose and partially" because apparently your restart of the battery switch causes everything to power back up. So while this is a possibility of a loose/weak connection, I'd put it lower on the list of elimination...but not dismiss it.

 

[TNC]

I need to ask for the reason you are running your battery voltage that low, where there are the issues you're wondering about. Generally, I think most of us will charge the battery much sooner, avoiding a practice that's known to be hard on batteries - running them on low voltage.

I was wondering the same. I never saw mention of what the OP was seeing as voltage value in his display when it is on or what a multimeter readout check revealed on the battery leads.

 

[Bjr49000]

I'm not trying to run my bike at lower voltage than it should.
My battery just drop voltage abruptly when drawing power, hence triggering the low voltage cut-off of the battery and that within the first kilometer of usage after a full charge.
Fully charged the display shows 53.6V. From that, by using the throttle a bit, it can goes below 49V within the first minute of usage.

I'm wondering wether I programmed the BBS02b voltage cut-of value even lower than battery self low-voltage cut-off.
I don't currently have any Windows computer to change that value and let battery BMS deals with low voltage.

I'm trying to know if there is a difference of behavior in motor low voltage cut-off and battery BMS cut-off.
For example : Is display still on when the motor low-voltage is reached ?
Knowing that would help me determine if the problem is from that setting.
Nonetheless, I know there might be a problem with this abrupt fall of voltage that I should inspect later.

 

[TNC]

No battery or BMS battery guru here, but the fact that you see your display voltage drop that dramatically that quickly would indicate to me a battery problem instead of a program or motor problem...assuming your display is correct. Despite where the BMS cutoff is or the controller program shutoff is, that's way too radical a drop that quickly from my experience with my BBSHD, battery, and display. I'm sure you're not going up a steep grade with a trailer full of cinder blocks when this is happening, right?

 

[Bitmugger]

To see your battery pack nose dive in minutes from ~54v to 49v suggests a problem with the cells to me. Is it by any chance a square shrink wrapped battery pack purchased from Aliexpress or eBay, etc? I am wondering if someone sold you a battery pack advertised as cheap and with good specs and it's turned out to be made with poor cells or recycled cells?

 

[Gionnirocket]

I'm not trying to run my bike at lower voltage than it should.
My battery just drop voltage abruptly when drawing power, hence triggering the low voltage cut-off of the battery and that within the first kilometer of usage after a full charge.
Fully charged the display shows 53.6V. From that, by using the throttle a bit, it can goes below 49V within the first minute of usage.

I'm wondering wether I programmed the BBS02b voltage cut-of value even lower than battery self low-voltage cut-off.
I don't currently have any Windows computer to change that value and let battery BMS deals with low voltage.

I'm trying to know if there is a difference of behavior in motor low voltage cut-off and battery BMS cut-off.
For example : Is display still on when the motor low-voltage is reached ?
Knowing that would help me determine if the problem is from that setting.
Nonetheless, I know there might be a problem with this abrupt fall of voltage that I should inspect later.

Seems like you have a very poor battery.
First full charge voltage should be 54.6v, so it's already not able to accept a full charge.
Then with the drop/sag that you are experiencing... it has to be made with either used, damaged or extremely poor quality cells.
When/where did you buy it? What is the battery rated capacity?
To further answer your question... If both LV were set equally, I believe that the BMS would shut down before the controller. The controller will shut everything down but I believe it lags behind the BMS to allow for brief sags.
I'll add.... There are 1000's of different BMS's, hence many different ways it could function as far as LV cutoff.

 

[Bjr49000]

I bought it on Aliexpress, with motor and display as a kit. I would say a year ago.
It's a 17Ah 48V battery in a Hailong 2 case.
Although, my battery worked great a month ago. There was definitely this volatile voltage drop also, I guess less than right now, but I could easily travelled 30km in a single charge, even recently.
Should I try to discharge it and charge it again, the same way you resuscitate battery with low voltage ?
If so, is there another way to discharge the battery rather than using the bike ?

 

[Gionnirocket]

I'd guess that you have a cell group that went bad... Or a bad BMS.
Discharge/charging with Li_Ion isn't a solution as with other chemistries.
If anything I would try a slow, low current charge... But I'm guessing you're charger is already only 2 amp.
Have you verified that your charger is working correctly? As stated earlier, full charge V is 54.6.
In all I'd be careful charging as a damaged battery is potentially unstable and dangerous.
I'd probably open up the battery and check each cell group voltage and that the BMS is outputting to all groups when charging.
If you are unfamiliar with this... Seek professional help.

 

[AHicks]

Another vote thinking some cells are not charging/battery problem.

 

[Bjr49000]

Yes I don't have low amp charger to do that..
If I try to charge it again many times and using it, and that doesn't help fixing the issue.
What should I do ?
Is it possible to open the case and check for cells ? Is it easy to do ? (I don't have welding machine, nor voltmeter but I know how to use it and could get one)

 

[harryS]

You probably have an unbalanced battery. One of the cells is partially charged while the rest are at full charge, and under load, it is the first cell to drop below the minimum voltage that was set for the battery BMS. This shuts off the battery, even if it is showing almost a full charge.

This graphic shows an unbalanced 36V battery, which has 10 groups, so full charge is 42V. If the red cell is off by just .50V, the battery only charges to 41.5V. As it discharges, the red cell will shut down first as it gets under 3v, even though the total voltage is quite healthy, nominally close to 36V.

Some batteries have a built-in balance function, so if you were to leave it under charge for a long time. it would try to charge that red cell. However, I believe most of the aliexpress packs don't have balance BMS.

You could get a voltmeter and measure the charger voltage, and then the battery voltage when the charger says it's done. Both should be 54.6V. If the battery is a few volts lower, it could be unbalanced. Then try leaving it on the charger, in a safe place in case of a battery fire, for a couple of days. See if the voltage after charging goes up. If it didn't, the battery cannot balance. Nothing more you can really do at this point.

It's risky for a novice to poke around with the insides of a Hailong. The objective is to measure the voltages of all 13 series groups. This will tell you the exact health of the battery, However, it's not feasible to change out bad cells. It's possible to manually rebalance individual cell groups, but you need a special charger.
.

 

[Gionnirocket]

Yes I don't have low amp charger to do that..
If I try to charge it again many times and using it, and that doesn't help fixing the issue.
What should I do ?
Is it possible to open the case and check for cells ? Is it easy to do ? (I don't have welding machine, nor voltmeter but I know how to use it and could get one)

If you do not own a soldering iron or Voltmeter... I would assume that an attempt to repair the battery is beyond something that you should try as it is not for beginners.
But as I said earlier, I'd probably open up the battery and check each cell group voltage and that the BMS is outputting to all groups when charging. But again you may want to seek professional help as causing a short circuit can cause a very dangerous situation.
It is possible that a faulty BMS is causing the problem, but if it is faulty cells I would recycle the entire battery and purchase a new one of better quality.