Overkill BMS vs multimeter
- Thread starter JJJJ
- Start date
ericfx1984
Solar Enthusiast
- Joined
- Oct 10, 2021
- Messages
- 765
Depends... one thing we do in electronics is establish a standard... Or at least something to compare it to...
An easy way to do this is to compare three or more voltmeters in their readings
Well it is possible that the BMS is incorrect at this point it is equally possible that your volt meter is incorrect... And if it proves that you have a cheap Chinese voltmeter I would say that the chances are even higher that your voltmeter is incorrect
An easy way to do this is to compare three or more voltmeters in their readings
Well it is possible that the BMS is incorrect at this point it is equally possible that your volt meter is incorrect... And if it proves that you have a cheap Chinese voltmeter I would say that the chances are even higher that your voltmeter is incorrect
ericfx1984
Solar Enthusiast
- Joined
- Oct 10, 2021
- Messages
- 765
Okay well I've had my coffee since I last answered so here's my thoughts
One thing that I noticed especially in lower voltage systems, minus 12 volt.. under heavy loads my inverter will read significantly lower then my battery Bank
Or to put it another way if I take a voltage reading directly at the inverter's inputs, the voltage will be lower than the voltage reading taken directly off the battery Bank furthermore the inverters internal reading shows slightly lower than what I have read myself
I think what you may be looking at is the fact that they're not pulling voltage readings at exactly the same point, or possibly the same way? I do know voltage readings are going to be different on different points just because of the nature of voltage sag
Hopefully as I get a little more coffee in me my answer will become a little bit more intelligible
ericfx1984
Solar Enthusiast
- Joined
- Oct 10, 2021
- Messages
- 765
good point as well. not to mention the cable might be under sized
There is one cell I am watching closely. The multimeter on the battery terminals and the positive and negative metal ring terminals (cell 8) both read 3.22 v. The BMS reading of 2.707 when the load test is turned on. If I turn the load off, the BMS and multimeter readings match. What happens over time is that cell 8 drops and stops the bms.
Yesterday I checked and reattached the positive and negative leads on cell 8. The readings on the BMS did not change.
Not sure if this is related. On three occasions, the 180w tester stated the test was done. The first time, I hit start again and it ran for over 98 amps. It then stopped twice more at about 2 amps. After the third time I restarted as before. When I checked the values of the BMS the last time, all of a sudden they were balanced and closely matched the multimeter settings.
Later the values adjusted back to the 8 cell low as before. If I stop the test the values go back to expected.
I am beginning to suspect a faulty positive lead from Cell 8. Initially the values are as expected when the test begins. If somehow the crimp connection is not optimum then micro drops in current may be leading to improper values in the BMS values.
I crimped the connection again to make sure it was secure.
I did isolate this cell and checked the voltage across the bms leads at the umbilical connectors and the voltage reading is as expected. However, the bms seems to be reading the wrong voltage. Might this be a faulty crimp that does not show up under load? Or a faulty bms?
Thoughts?
Yesterday I checked and reattached the positive and negative leads on cell 8. The readings on the BMS did not change.
Not sure if this is related. On three occasions, the 180w tester stated the test was done. The first time, I hit start again and it ran for over 98 amps. It then stopped twice more at about 2 amps. After the third time I restarted as before. When I checked the values of the BMS the last time, all of a sudden they were balanced and closely matched the multimeter settings.
Later the values adjusted back to the 8 cell low as before. If I stop the test the values go back to expected.
I am beginning to suspect a faulty positive lead from Cell 8. Initially the values are as expected when the test begins. If somehow the crimp connection is not optimum then micro drops in current may be leading to improper values in the BMS values.
I crimped the connection again to make sure it was secure.
I did isolate this cell and checked the voltage across the bms leads at the umbilical connectors and the voltage reading is as expected. However, the bms seems to be reading the wrong voltage. Might this be a faulty crimp that does not show up under load? Or a faulty bms?
Thoughts?
Last edited:
Classic sense lead failure. Most likely a broken conductor with only a single strand making contact.
Replace the sense lead on the bms…
Bob B
Emperor Of Solar
- Joined
- Sep 21, 2019
- Messages
- 8,955
If you are measuring at the battery terminal .... the problem could also be the bus bar connection to the battery terminal .... and that WILL be affected by adding a load.
Put the meter right on top of the sense lead terminal and compare to BMS.
Put the meter right on top of the sense lead terminal and compare to BMS.
I checked the crimp connection and it looks secure. I also checked at the sense lead terminal and it matches the battery voltage. If the sense wire is damaged that will take more. The sense wire is part of an integrated harness from overkill.
I rotated a couple of bus bars. The next test is to rotate a couple of batteries to check battery vs sense lead
I rotated a couple of bus bars. The next test is to rotate a couple of batteries to check battery vs sense lead
Ok. I figured this was a lead problem. Then I rotated the cell to position 6 and resumed the test. Turns out it is the cell.
What I can not figure out is that once I start the test it drops down to 2.5 per the bms and shuts down the bms. If I then isolate the cell it reads 3.19 volts
What I can not figure out is that once I start the test it drops down to 2.5 per the bms and shuts down the bms. If I then isolate the cell it reads 3.19 volts
If the cell is bad, it will rapidly drop voltage under load, and recover when load is removed.
You are saying that under load, a voltmeter shows the same voltage as the bms when measured at the terminals...
I think it is time you posted some pictures of the build. We might see something you aren’t noticing.
Ok. Made a simple change that made all the difference. I stopped the test last night and gave the cells (and my brain as well) a good rest.
This morning I added these BMS leads under the bus bar where it contacts the battery. This is for the 10mm battery where I needed to dremmel the bus bar to fit. The thinking is that the surface contact may have been compromised in the downstream battery. This was confirmed during my initial test this morning.
So far it has been running 90 minutes and (knock on wood) all battery levels are stable in relationship to each other.
I am so impressed by this that I am going to trial copper washers between the studs and bus bars. I understand that these are different metals but will not be pulling high amps.
Last edited:
The order needs to be, battery terminal, buss bar, sense lead, nut. You absolutely DO NOT want anything between the buss bar and the battery terminal.
It sounds like the battery terminal needs cleaned and was making poor contact with the buss bar. This would cause a voltage drop on the output of the whole battery, and also the sense lead, while voltage measured at the terminal would be fine.
Clean the terminals and the buss bars with very fine sandpaper. Also, you mentioned this battery has a different sized stud? If the terminal is not the same height, and the buss bar not perfectly flat against it, that will also cause this problem. If that is the case, you might need to bend the buss bar slightly so it is flat on the terminal.
Edit: just reading that you dremeled a buss bar. If the edges of the hole have any burs, that will cause this problem. Clean up the holes you modified with a file and sandpaper.
I did find the slightest burr on the bus bar. Some 2000 grit sandpaper and alcohol wipe down handled that. I scotch guarded the terminal bases and also wiped down with alcohol.
The 10 mm battery is slightly taller. The bus bar was striking the downstream terminal base at a slight angle causing a smaller connection between the bus bar and the terminal base.
Not sure how much "bend" I have left in the bus bar. Things are pretty tight at this time. Trying to see if there might be something I can place under the battery to handle the height difference.
The 10 mm battery is slightly taller. The bus bar was striking the downstream terminal base at a slight angle causing a smaller connection between the bus bar and the terminal base.
Not sure how much "bend" I have left in the bus bar. Things are pretty tight at this time. Trying to see if there might be something I can place under the battery to handle the height difference.
