BMS lead disconnected causing some unbalanced cells??

[SoCalbiking]

Hello forum i am new but always lurked with all the great info here.

I just started working on a Zev Z 7100 scooter and the main issue with it is the Battery. So I started with checking voltage on each of the 28 - 3.2v 40ah Lifepo4 battery cells for irregularities. I found one with a concerning 1v or so less then its group, and a few others had about 0.1v difference. I further ripped apart the bike to find the main BMS and trace back its wiring. Discovered an UNCONNECTED 5pin ribbon lead which i assume is the main balancing cable for this BMS.
How should I go about reconnecting?

Thinking I will replace the one bad cell, and charge/discharge the other unbalanced cells to the correct voltage before pluging back in the BMS Lead?
Thanks to all the experts here on the forum!

 

[amberwolf]

Discovered an UNCONNECTED 5pin ribbon lead which i assume is the main balancing cable for this BMS.

Those are individual balancing and monitoring units on each cell, with data interconnects between them, The last one may be either a lead used for diagnosis/testing/factory access, or it might be the main data cable to the master unit. But it isn't a "balance cable" as the word is normally used, in the sense that it is not there to directly monitor individual cell voltages via separate wires in the cable.

The master unit is the metal box that says "Lithium Battery Protection Unit". It should have an empty plug on the end of the black-jacketed multiwire cable (right side in your pic, farthest from the LEDs) that this cable will go into. Don't plug it in just yet, but look and find out first.

There is a high likelihood that the previous owner of the scooter unplugged that cable so that the main LBPU (BMS) could not see that the super-low cell was low, and shutdown the scooter preventing it from being ridden (instead of fixing the battery). Most likely it wasnt' super-low then, but subsequent damage and lack of the main LBPU's control/activation of balancing after disconnecting it caused the cells to drift farther and farther apart, leading to the present state.

A bit of poking around ES found this link
http://www.environmentfriendlystore.com/zev-tips-troubleshooting.html
which has a link to a pdf
http://www.environmentfriendlystore.com/pdf/ZEV-Electrical-Wiring-Troubleshooting.pdf
that might be useful.
View attachment ZEV-Electrical-Wiring-Troubleshooting.pdf

Thinking I will replace the one bad cell, and charge/discharge the other unbalanced cells to the correct voltage before pluging back in the BMS Lead?

That's a good first step. Once it's been used a while, you can recheck balance on the cells *before recharging*, and make sure you don't have any other weak ones that are going significantly lower than the others after a ride.

 

[SoCalbiking]

Thanks for the quick expert reponse. Yeah forgot to mention each battery has its own monitoring board. I also thought that the "unplugged" ribbon cable i found might have been for diagnosis purposes but then I saw it was perfect length to plug into the empty female connection of the first battery's board (very right of photo #2) After a bit of looking at images of half assembled bikes like this I found out it shows this exact wire connected to the first battery I highly doubt the previous owner unplugged it on purpose since he isn't very technical and stopped riding it a while ago because of battery issue . I assume it came unplugged accidentally and the cells eventually became unbalanced to where it would trip the battery management system under load and or get out of working voltage for the controller.. Thats a great link you sent me it has a good wiring manual. Also confirms my suspicion on unplugged connector. "This string ends in a connector to the black wire coming out of the BMS box" Tomorrow i will replace the bad cell, equal out any unbalanced cell, double check connection terminals, plug in unconnected ribbon cable, and finally recheck cell balance after a bit of use before any recharging Thanks again for your knowledge and the link you're a life saver

 

[amberwolf]

I also thought that the "unplugged" ribbon cable i found might have been for diagnosis purposes but then I saw it was perfect length to plug into the empty female connection of the first battery's board (very right of photo #2)

FWIW, I was just about to go edit my post, because I finally noticed in the blurry picture background that shows the unplugged cable that it *is* the cable going to the main BMS box, and thus would then have to go to whatever empty plug was on one of the battery boards. I had not yet noticed there was one in the pics already...but going back and looking at it, you can see where it is lifted up away from the board, which can happen from someone yanking that cable out of it (instead of using a tool / scribe to disconnect it so it wouldnt' damage wires and/or connector and/or board).

I highly doubt the previous owner unplugged it on purpose since he isn't very technical and stopped riding it a while ago because of battery issue .

Then my guess is taht someone "helped" them originally when the battery issue started, and unplugged it for them. Possibly after giving them the cost of wahtever replacement parts would be needed, and the owner said "no, is there anything cheaper?" It is a tale that has occured in various forms here on the forum on various e-vehicles numerous times: disconnect the BMS that's protecting the rider from destroying the rest of the expensive battery, so the rider can keep riding without the vehicle shutting off. The rider will not generally even admit they did this, or had it done, when later selling it.... It's happened before with other scooters, bikes. etc.

Remote possibility that it could also have been unplugged on purpose during a service at a dealer/etc for wahtever reason, and then forgotten to be plugged back in, but that is a *lot* less likely than the other scenario.

I assume it came unplugged accidentally and the cells eventually became unbalanced to where it would trip the battery management system under load and or get out of working voltage for the controller..

That's the thing--the cells CAN"T trip the BMS without that cable. It is teh communication line from the modules to the BMS master box, adn the modules can't do anything but monitor and ask the BMS what to do..if it doesnt' respond, they can't do anything (not even balance, based on the results you're seeing). That's why people disconnect stuff like that...because it *was* shutting it down and they didn't like that but didn't want to pay for fixing it, etc. :/

If the controller has it's own LVC then *it* might shutdown, but the BMS can't do it.