BMS sense wire voltage pulsing from 46.1v to 51.2v

[mattjrogers]

Hello, first post here, been lurking awhile.

So anyone ever checked the voltages at their BMS sense wires and found one that fluctuated back and forth between two voltages every second? That is my issue and it is shutting down my battery.

I have a BMS and battery issue that I could use some help with.

Generic LifePo4 battery pack 48v 40AH. 16s aluminum case prismatic cells. All cells individually test to 3.3v. The pack has been sitting for 2 months or so, no real change in cell voltages over that time. I am in the process of splitting the pack to mount on either side of my bike (Big Dummy). The battery was working fin before I took it apart and not after hooking everything back together as a split pack, I am getting some weird voltage readings.
I am posting a link to the pictures in case I don't get the image upload correct. https://goo.gl/photos/pfhnH8H4yJdck3WY9

First picture of the split pack. I plan on redoing the BMS sense wires at some point, they are just crappy soldered to some other wires at this point - how they arrived from China. But I checked each wire connection and they are all solid and have the correct voltage and continuity, except one - problem described below.


The next picture shows the BMS. There is an XT60 with the black negative from the other battery pack and a red wire which is the serial connecting wire between the two battery packs. I get exactly half the pack voltage from these wires when checking them against each other (26.2v) at this time (pack is partially charged). I also get full voltage between B- and the Positive battery output 52-53v. I get 52v from B- to one of the Mosfet terminals.


Here is where it gets weird. I hooked the battery up to the charger to fully charge it and when I took the charger off, I noticed that my Turnigy watt meter went dead. I checked the voltage manually at the battery output (XT90) and first found a voltage of 13.1v. I checked a lot of connections, tightened some, checked again and eventually got to the point where I was getting 1.7v. So I checked the sense wires and their voltages to see if one was dead and the BMS was shutting down the battery. I get normal voltages as I go down the wires 3.3v-6.6v-9.9v-etc until the last wire, when checked carefully it changes voltage back and forth from 46.1v to 52.1v at a rate of every second, constant pattern.
View attachment 1
I checked the board for burnt connections or components and don't see anything other than some left over sticky foam that was protecting the board. I don't exactly know what components the two rows of rectangular black boxes are that sit above the rows of resistors coming from the sense wires, but the one that I am dealing with has varying voltages across its terminals. One terminal switches back and forth from 46-51v, the other stays constant at 52v. The voltage at the top of the top row of components is 13.1v - the number that I was getting at the output at one point.

I am very mechanically inclined, but let electronic technology get a head of me. I would like to learn how to diagnose and fix this issue, so please give me some tips and hints of what to check. I will try my hand at replacing a component, if needs be.

Just let me know what info I am missing to help and I will get it right over to you.

Thanks,
Matt

 

[mattjrogers]

Update
I played around some more and not much changed, but I always thought it was weird that this BMS came with only 15 wires. I have heard that a good BMS should have 17 wires for a 16s pack. So I added the last sense wire and now my output voltage is back to 13.1v. The weird fluctuating voltage on the other sense wire is gone and all are steady voltage. Even when I take off the XT60 connector between the packs, the output voltage stay above 0, and it dropped to 12.1v.

So the pack ran for a summer without problems as a single pack, and with 15 sense wires. So I can't figure out why that would be the fault, but the only last thing I can think to try is to re-wire the sense wires. The last sense wire is on the positive terminal of cell 16 that has the large B+ wire, then #15 is on the same cell negative terminal, then every cell negative terminal. Perhaps I should move the sense wires over one terminal so that they are on the B- terminal and same cell positive and then on every cell positive. Basically, since I only have 16 balance wire positions, it either starts at one one other the other of the pack. The only issue with this is that the balance wires are soldered onto the series wire terminals and I have to unsolder then and move them all over one battery terminal.

Can anyone comment about BMS wiring when you only have 16 wires (or 15 in the weird Chinese case)?

Matt