Bafang system display boot issue

Joined
Aug 6, 2022
Messages
6
I have two virtually identical ebikes. On one of them, the red bike, when I turn on the battery switch then press the button to boot the display the display flashes some version of its opening screen then it shuts down; goes blank. Further attempts pressing the button elicits no response. Then if I power off the battery and power it on again the display starts in response to the button press as it should. The bike then will work fine. This has been very reliable behavior. I moved the battery to the second bike (yellow) (tried once): the second bike showed the same bad behavior. The second bike with its usual battery works fine.

I do not know how long the bike needs to sit with the battery switch off for the issue to repeat itself--overnight is sufficient, however.

I measured the unloaded terminal voltage of both batteries (On the bench). I get 54.2 to 54.7V. I put them on a 'scope and see almost no difference between them, although since the yellow bike gets a little more use its voltage is decreasing faster than the red. But we are still above 50V--not using them much lately.

My next test is to check the battery voltage when first the battery switch is turned on when on the bike. I now have a cable to permit me to do that.

Here are the details of the red bike, taken from the labels on the components. Clearly this is a Bafang system, installed in 2020. We used the bikes locally for about 1 year, then used them little due to a house move. No bad behavior at that time. We just started using them again about a month ago. The outfit that installed the systems has not been any help. Most recently I asked for wiring diagrams and details--no response. That is why I am hoping you people here can offer suggestions.

DISPLAY: C965 A0248 BFL.0 V5.0 2003300470
BATTERY:48V 12 Ah 29E 202006010085
MOTOR: BBS02B 48V 750W 2007040545
CONTROLLER: CR R209.750.SN; C571-30-512; 2007021391

Thanks,
-Denis Heidtmann
 
After overnight w/ the battery sw off, I would expect that the problem would repeat. Time for the test:
I unplugged the connector at the battery and inserted my test lead consisting of a 6 inch extension jumper with connections for a Fluke 87 voltmeter. That took about 5 minutes. With the battery switch still off the voltmeter read 1.6V; very slowly decreasing. I then switched on the battery. The voltage jumped to 52.9V. Then I pressed the button on the display and the display booted--no failure!

Is this perversion of nature? Is an overnight wait not sufficient? Did my disconnecting and reconnecting the battery cause the failure not to occur? Did the voltmeter change things? Was there a time element involved?

My next plan is to insert the test jumper and the voltmeter at the end of the use of the bike today and run the test again tomorrow. The goal is to see what the battery voltage does when the battery is switched on and a failure occurs. I read some comments somewhere that the battery management system could be shutting down the battery for some reason. I would hope that this test would determine if that is in fact the cause of this issue.

Anybody have suggestions?

-Denis
 
Progress. It acts like there is a capacitor in the system that when discharged draws enough initial current to trigger the BMS to shut down when the battery is switched on. However, when this happens it leaves the capacitor with some charge (38V at maximum). Hence, shutting off the battery resets the BMS, then switching the battery on again the BMS does not shut down since the capacitor has some charge.

If anyone is wanting to know the details of the tests I did to bring me to this conclusion I can provide. But the important question at this point is: what can be done? Live with it? Does the BMS have adjustments? Display settings need changing? Can the battery be repaired? It has little use on it, so replacement seems extravagant.

I note that there have been no responses to these postings. Is anybody even reading them?

-Denis
 
The controller has capacitors (fairly large ones, often enough, that can cause a pretty high inrush current), but this doesn't cause a problem with a normally-functioning system.

However, if the battery has a problem, it can.

If a cell group in the battery has a high resistance (getting old, defective, group interconnection problem, etc), then during the instant it has to supply very high current (much higher than normal) it could drop in voltage enough for the BMS to shut down to try to protect it. It would also drop in voltage under normal use, more than the other cells, but probably not enough to cause a problem (except when it is closer to empty and under a high load).

You'd have to open the battery and monitor voltage across the cells (preferably at the BMS sense/balance connector) to tell if this is happening, unless your BMS has BT to talk to an app on a phone, etc.

Or, if the BMS has a current-sensing function that allows it to shutdown in the event of overcurrent, it might be "over reacting" to the inrush current to the controller at power on. This isn't usually something that can start happening later, but can be a problem from the start, if this is a new BMS (or entirely new battery). If it's not new, but the behavior is, it's probably not caused by this.
 
Thanks. Your first explanation makes sense. I did some analysis and concluded the capacitance in question is 1200 uF.
These batteries sat idle for a number of months, so that could be the cause of a weak cell or two.

Do you suppose if we continue to use this battery will it get better or worse? I have never opened a battery and would need to improve my knowledge before attempting it. If I did could I fix it?

I guess I need to go to other parts of ES to educate myself.

Thanks for you help.

-Denis
 
I think I could repair the battery but likely will not. I will look for a place to do the repair. In the mean time I will continue to use it.

This brings me back to my question about the likely future for this battery unrepaired: Is it likely to improve through use and charging? My present plan is to use it 'till it gets down to some voltage indicative of 25% or so, then charge it. See how it behaves then.

BTW, I measured the waveform when switched on after a day of no use. The initial dV/dt was 1 MV/sec. With a 1200uF capacitor that predicts 1200A initial current. If 1200A are what is needed to shut the battery down, I expect to get some reasonable use out of it.

Thanks for your advice.

-Denis
 
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