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DIY pack tripping bms

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georgia088

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I have been reading and trying to do the best I can to understand lifepo4 cells. However, I’m stumped. I have built another lifepo4 105ah pack with eve105 cells. 48v 16s. Data sheet:


These cells are rated for 1c continuous and 3c for 30 seconds. I have built a pack before with the same cells and really haven’t had issues with the other pack, but it is my dads pack and he only uses it sparingly. I built them for golf carts.

I got the cells put together and did a top balance by placing an adjustable voltage supply set to 57.6 with a current of 5 amps. I set my jkbms with 1 amp of active balancing settings to:

Ovp 3.6
Ovpr 3.55
Uvp 2.65
Uvpr 2.55


The pack charged cut on and off until the CC Went to nearby 0 and all cells were really close in voltage. I will try and post a picture of what they were at.

I used the bus bars that came with the cells which are 1mm thick. I also designed a PCB with large connecting traces on both sides to connect the cells in addition to the bus bars. I did this to help the bus bars share the current load between cell terminals, and I also added 1/8th inch traces from each positive terminal to a screw terminal where I connected my bms battery lead wires. I felt I could keep it a little cleaner and neater this way. I left all of the wires the full length that the manufacturer supplied. I will post a picture of my PCB as well. The cells come prethreaded with m4 screws. I didn’t like the m4 screws with the last pack I built so I decided to get m4 studs and lock nuts. I threaded the stud into the terminals with red locktite and let them sit for several hours. I then added my bus bars and the PCB on top of the bus bars and tightened the nuts down to 3.25Nm with a digital torque wrench. I would have gone tighter as I see the spec sheet calls for higher but I stripped one of the threads in a previous cell going to 3.5Nm

I really thought I was doing good by doing all of this. However, I got it all put together and in a golf cart. The golf cart is lifted with a rear seat and large tires, but it only has a 250 amp controller on it. It has a sepex motor. The cart will trip the the 2.65 cell under voltage alarm quickly after taking it off the charger with only me on it. 175lb. The app shows several cells dropping voltage lower than what I think they should at 100% capacity. The amp draw is relatively low as well.

I took a couple of screen videos of the app as I drove it and you can see the voltage sag. I will post a couple of links to the video as I don’t think the site will allow me to post them.

I’m at a loss. I can’t think of anything else I could do. These are brand new grade a (supposedly but how do you really know) cells. Could I have a bad batch? Am I doing something g wrong? The weather was around 45 degrees F but I wouldn’t think that would effect the cells that negatively. Could you please help guide me as to what I should try? I tried to give as much info as I possibly could! Thanks in advance!

https://youtube.com/shorts/-OveXJc9u...fr38yu10IMwm1d



Thanks!
 

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That’s quite the C rate for 105 cells. Lower quality cells can have huge voltage drops for high C demand, no BMS is going to fix that.

Is there any way you can drop the motor demand? To a max of 100a?
 
The bms is tripping. It is the low cell voltage alarm that is tripping. These are supposedly grade A eve105 cells. Just purchased.

I could limit the motor current draw to 100 amps, but the performance would greatly suffer. These cells are supposed to be capable of handling the demands. How low should the fully charged cell voltages sag while discharging at 1c? If you watch the video the current draw on the bms is rarely over 1c and only for a second or two. Is it normal for cells to dip that low during sudden inrush of 1c discharge?

Thanks.
 
Just focusing on the cell voltage difference of 500 mV in the video I believe there is a connection issue or the cells need to be balanced.

Yes I would limit the current to 100 amps or even less until this is sorted out.
 
No, the cell voltage should ot drop to 2.5 volts from 3.5 volts in just a few seconds.

Sounds like a connection issue.
Check the torque on your cell nuts.
Check the balance lead connections at the BMS.

But, I think the most likely cause is the connection to the cells.
As a test, can you remove the PCB and use ring terminals on the BMS leads?
 
400bird said:
No, the cell voltage should ot drop to 2.5 volts from 3.5 volts in just a few seconds.

Sounds like a connection issue.
Check the torque on your cell nuts.
Check the balance lead connections at the BMS.

But, I think the most likely cause is the connection to the cells.
As a test, can you remove the PCB and use ring terminals on the BMS leads?
Click to expand...
I did check and all nuts seemed to be tight and torques to 3.25 Nm. I could pull the PCB off and put ring terminals on it, but why would that be causing problems? Could the length of the wire of the bms lead in addition to the trace on the PCB be causing low voltage readings?
 
georgia088 said:
I did check and all nuts seemed to be tight and torques to 3.25 Nm. I could pull the PCB off and put ring terminals on it, but why would that be causing problems?
Click to expand...
The PCB stacked on top of the bus bars and cells are the only unusual component about your build. From what I've seen, it should work. I saw cell 9 and 10 go low, those are right in the middle of the pack if my counting and understanding of the layout is correct.
georgia088 said:
Could the length of the wire of the bms lead in addition to the trace on the PCB be causing low voltage readings?
Click to expand...
Probably not.
 
400bird said:
The PCB stacked on top of the bus bars and cells are the only unusual component about your build. From what I've seen, it should work. I saw cell 9 and 10 go low, those are right in the middle of the pack if my counting and understanding of the layout is correct.

Probably not.
Click to expand...
3,4,5,9,10,11 All drop to 2.9 or lower.

What would be a normal voltage drop be at just slightly over a 1c discharge (for only a second or two) and fully charged batteries?

Thanks again!
 
georgia088 said:
What would be a normal voltage drop be at just slightly over a 1c discharge (for only a second or two) and fully charged batteries?

Thanks again!
Click to expand...
How does the other batter perform in these conditions? If they all drop to 2.9 then fine but they have over a half volt differential between the cells. Connections might be fine at a couple amps but 100+ amps there is an issue that get big.

Did all aluminum get polished with fine sand paper just before torquing down?
 
time2roll said:
How does the other batter perform in these conditions? If they all drop to 2.9 then fine but they have over a half volt differential between the cells. Connections might be fine at a couple amps but 100+ amps there is an issue that get big.

Did all aluminum get polished with fine sand paper just before torquing down?
Click to expand...
No I did not polish the terminals. Is there some adhesive or something you can put between the bus bar and terminal to help keep the mate between the bus bar and terminal? It seems strange that so many would be getting a bad connection though.
 
Torque is the adhesion. Aluminum oxide is high in resistance. Cleaning and blocking the oxidation just before assembly can make a big difference.
 
time2roll said:
Torque is the adhesion. Aluminum oxide is high in resistance. Cleaning and blocking the oxidation just before assembly can make a big difference.
Click to expand...
What grit sand paper should be used to polish aluminum? Should I polish the aluminum and then put the noalox on under the bus bar before tightening or on top of the connection after tightening?

Thanks
 
Partimewages said:
After you have fully charged the cells what is the resting voltage of each cell with no load?
Click to expand...
I'm not sure I will have to check. I am considering using larger studs with course thread to try and retap these cells. The screws that come with these cells are m4 fine thread. I think I am going to try using #10-24 course thread. Should I use locktite or jbweld when I put studs in to help keep the studs from stripping?

Thanks
 
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