Help with voltage drop when discharging.

[joe009]

Hi sorry to make a whole thread aboutt this I just can't find an answer anywhere on the site as to where my problem likely is.

I am having some problems with my cells or with the BMS I am not sure which is the problem. I have 4 280ah eve cells with a 4s 100A Daly BMS, all ordered from Alibaba, I tested their voltages at first and all were within 0.01 volts of eachother. (I have no idea about the quality of the cells as this is only my second Lithium battery, I have not capacity tested them).

At first everything seemed to work fine the BMS balanced all the cells to within charging/discharging and had my 4 cells connected to the bms with my positive discharge cable+solar charge cable on the positive of cell 1. My Solar negative and negative discharge on the negative of the BMS. (happy to post a pic if that helps)

Now if I draw a current of like 10 amps+ the voltage of one of the cells goes very low from 3.35V down to 2.7V. But it is not always the same cell each time it can be a different cell that has the voltage drop (I think any of the 4 in fact from my observations). The overall voltage tends to go from like 13.3V all the way down to 11.9V while the discharge is happening which cuts out my inverter is happening, but on the individual cells it is always one cell that seems to be the main cause of this not all 4 dropping together. Another thing that happened once was the BMS randomly stopped working and wouldnt provide any current and I had to reset it by re doing all the cables, not sure if that could be linked?

The fact it is a different cell each time experiencing the voltage drop makes me think it is a BMS problem? Or maybe I have set up something wrong? Or are they just bad cells?


Any help much appreciated spent quite a bit of money now on batteries infuriating to still have problems haha. Right now I am thinking I should just buy another BMS and see if that solves the problem? Just don't want to commit more money without knowing what the issue is...

 

[FilterGuy]

The fact it is a different cell each time experiencing the voltage drop makes me think it is a BMS problem? Or maybe I have set up something wrong? Or are they just bad cells?

Hmmm... The fact that the Inverter cuts out indicates the voltage drop is real. That implies it is not the BMS.

Now if I draw a current of like 10 amps+ the voltage of one of the cells goes very low from 3.35V down to 2.7V.

3.35 is not empty, but it is certainly not full either.... Does the problem happen when the battery is fully charged?

 

[mikefitz]

Have you checked the cell volts with a meter?
I suspect you have connection issues between the buss bars and cells.

Mike

 

[FilterGuy]

happy to post a pic if that helps

please do!

 

[joe009]

please do!

Hi here are some photos if it helps you guys too see the issue? The bus bars are all screwed on quite tight although I did them by hand not with any tool to make them really really tight... Maybe there is another issue you can see with them?

The two positive cables are from from a 12v load like lights etc and the other one an inverter. I had both negatives going to the BMS negative in picture 6 but i switched the inverter negative cable over to the negative terminal on the first cell to see if bypassing the BMS helped the voltage drop problems.. But yea it didnt.

 

[joe009]

Have you checked the cell volts with a meter?
I suspect you have connection issues between the buss bars and cells.

Mike

Hi yes they are all sitting at 3.32 perfectley balanced but then when I turn a load on they drop to like 3.1/3.2 and one will go all the way down to 2.7ish. Then they all come back and sit at 3.32 again.

 

[joe009]

Hmmm... The fact that the Inverter cuts out indicates the voltage drop is real. That implies it is not the BMS.



3.35 is not empty, but it is certainly not full either.... Does the problem happen when the battery is fully charged?

Yep it cuts out with a low voltage alarm I think, once it hits like 11 something volts.

Yea I think it still has the same problem as Ive charged it to like 14 something volts once on solar set the float a bit high and still had the same issues.

 

[mikefitz]

The buss bars seem to be incorrectly assembled. The cable termination eye for the BMS leads ideally needs to be on top of the bar, not sandwiched between the bar and cell terminal.

Why are there two high current cables on the battery negative, should not all current go through the BMS?
The connection to the BMS negative output looks very 'casual'.

As I stated in my early post it sounds as if there are interconnection issues to the cells and the pictures seem to support this.

There does not seem to be a protection fuse or breaker near the positive battery terminal.

Mike

 

[Sojourner1]

@joe009 did you actually top balance the cells or did you just see that they were all near equal voltage and started using them?

Charging " to like 14 something volts once on solar set the float a bit high and still had the same issues." Isn't really telling us much, when the batteries were at 14.somthing were all the cells at equal or near equal volts to each other?

 

[FilterGuy]

The buss bars seem to be incorrectly assembled. The cable termination eye for the BMS leads ideally needs to be on top of the bar, not sandwiched between the bar and cell terminal.

Good Catch. The small ring lug under the busbar guarantees a poor contact between the bus bar and the Cell terminal pad. This has a high probability of problems. Don't bother with any other debugging till this is fixed.

BTW: There is a resource for this very topic:

Basics for putting lugs on posts

I saw some folks on the forum that were assembling lugs onto their cells incorrectly so I thought it might be useful to document how to do it. Click on the orange 'download' button in the upper right corner to get the document. As with all...

diysolarforum.com

 

[joe009]

The buss bars seem to be incorrectly assembled. The cable termination eye for the BMS leads ideally needs to be on top of the bar, not sandwiched between the bar and cell terminal.

Why are there two high current cables on the battery negative, should not all current go through the BMS?
The connection to the BMS negative output looks very 'casual'.

As I stated in my early post it sounds as if there are interconnection issues to the cells and the pictures seem to support this.

There does not seem to be a protection fuse or breaker near the positive battery terminal.

Mike

Hi ok thanks for the help, sorry for the slow replies been busy at work.

So for the buss bars I need to put all the BMS eyelets on top of the bar between it and the screw, for cell 1 with the main positive terminal the eyelet should be on top of any high current wires too?

So one of the high current cables is from the BMS which is meant to be there I think? The second one I recentley moved there thinking if I bypassed the bms maybe I wouldnt have the voltage drop issues but of course they continued. Yes the BMS negative is a bit casual, I was thinking I should screw a junction box into the wood and make all the connections really secure as right now they are not super tight, not sure fi that could cause the problem?

So I guess I will take it all apart and fix the issues you outlined. Is there anything else there that might be the cause of the cell voltage drops?

 

[joe009]

@joe009 did you actually top balance the cells or did you just see that they were all near equal voltage and started using them?

Charging " to like 14 something volts once on solar set the float a bit high and still had the same issues." Isn't really telling us much, when the batteries were at 14.somthing were all the cells at equal or near equal volts to each other?

No I don't think I did, I just saw they were near equal and started using... How would I go about top balancing them?

But yes when it hit 14 volts the cells were the same voltage and then dropped together later on and sat at the same voltage again. I think maybe I should charge it up to 14.6 volts or soemthing and see if they are all equal?

 

[joe009]

Good Catch. The small ring lug under the busbar guarantees a poor contact between the bus bar and the Cell terminal pad. This has a high probability of problems. Don't bother with any other debugging till this is fixed.

BTW: There is a resource for this very topic:

Basics for putting lugs on posts

I saw some folks on the forum that were assembling lugs onto their cells incorrectly so I thought it might be useful to document how to do it. Click on the orange 'download' button in the upper right corner to get the document. As with all...

diysolarforum.com

View attachment 59384

Ok great thanks I will fix this and stick to this guide! Will let you know if it works.

 

[Sojourner1]

No I don't think I did, I just saw they were near equal and started using... How would I go about top balancing them?

But yes when it hit 14 volts the cells were the same voltage and then dropped together later on and sat at the same voltage again. I think maybe I should charge it up to 14.6 volts or soemthing and see if they are all equal?

Top of this section as a sticky.
https://diysolarforum.com/threads/top-balancing-how-to.15839/

 

[time2roll]

Now if I draw a current of like 10 amps+ the voltage of one of the cells goes very low from 3.35V down to 2.7V. But it is not always the same cell each time it can be a different cell that has the voltage drop (I think any of the 4 in fact from my observations). The overall voltage tends to go from like 13.3V all the way down to 11.9V while the discharge is happening which cuts out my inverter is happening, but on the individual cells it is always one cell that seems to be the main cause of this not all 4 dropping together. Another thing that happened once was the BMS randomly stopped working and wouldnt provide any current and I had to reset it by re doing all the cables, not sure if that could be linked?

Especially if you have different cells dropping quickly I would be looking for a connection issue. Placement of the ring terminals on the bus could be an issue. Also the very small sensor wires are notorious for not getting a good crimp on the terminal.

The cells in series all drain and charge the same. If it was a bad cell it would be the same one drooping every time. I think you have a connection/measurement issue.

Also good to clean all items with very fine sand paper and use some no-ox just before assembly.

 

[Gazoo]

The bus bars are all screwed on quite tight although I did them by hand not with any tool to make them really really tight... Maybe there is another issue you can see with them?

So you didn't use any tool at all to torque the nuts? I don't think hand tightening is going to make for a good connection. After you have done what others have recommended, torque down the nuts a bit with a tool to ensure a good connection. It doesn't take much torque so be careful. I can't tell from the photo but those don't look like laser welded studs.

 

[Bud Martin]

So you didn't use any tool at all to torque the nuts? I don't think hand tightening is going to make for a good connection. After you have done what others have recommended, torque down the nuts a bit with a tool to ensure a good connection. It doesn't take much torque so be careful. I can't tell from the photo but those don't look like laser welded studs.

+1 about needing to use the tool to tighten the connection.
Watch this video about what effect of the tightening of the connection can have. start @ 15:00 mark is really good.

 

[FilterGuy]

@joe009 Any updates?

 

[joe009]

Hey yes so update think its working now! Seemed to be the connections as you all said.

First I tried yesterday took off the bms and all other connections just had the bus bars with nothing sitting on/under them and then tried to top balance and it shot up to 17v from 13.3v and made a buzzing sound (was about 10 amps from solar coming in). Quickly took the solar off and waited till voltage went back down to 13.3 again (all cells were balanced, think they were very unbalanced when it went to 17v however). Tried to draw some current with 3 amp coolbox and the voltage went down to 11v again cells really unbalanced.

Then today I tightened all the nuts with a socket set and then drew the 3 amp current and it only went down to 13.15 v. Solar same story hardly raised the voltage, and no scary sounds . So yea seemed to be that the connections were loose which was causing the problems. I think the reason I actually didnt tighten them originally was because when I watched a video on how to set up lifepo4 cells on youtube I thought the guy said don't tighten too much as they de thread really easily, so just did them by hand until they were hard to budge... But ovbiously still have to tighten them a bit more haha.

Will try and top balance now to 14.4 V without bms attached, then put it on after and it should be good to go right?

Thanks for all the advice!

 

[time2roll]

The BMS should be connected for that charge to 14.4 volts.