Batteries in parallel with different amps on positive and negative

Cheap 4-life

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Ahhh.. you noticed the same as I was writing... Now also measure the current on each balance wire. Of course keeping the clamp direction consistent
Yes, while doing the video I noticed when flipping the clamp the reading changed.. in this pic I quick wrote down the amp readings with the clamp flipped both ways (showing a positive and negative amps) on each wire, even on the main positive and negative going to the busbar. The main positive and negative is at the top. Excuse the sloppiness, I was trying to write it fast so the load didn’t change.
 

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Cheap 4-life

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“The balance leads of the 4 batteries are all connected in parallel??? That's going to cause current flow to dynamically redistribute in all sorts of hard to predict ways!”

I’m using one bms for all the batteries. I’ve been using these batteries for a couple of years and I have never balanced them. They are as balanced as the day a got them. EV cells are matched by internal resistance very well in the individual batteries. Max of 25mv difference between the cells. For this reason I decided to use one bms so I had LVD and HVD on the cell level.
 
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ajw22

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I’m using one bms for all the batteries. I’ve been using these batteries for a couple of years and I have never balanced them.
Yes, yes, that's fine functionally in normal(*1) operation.

The issue is that one (x4) of those balance leads is connected to main negative.
Another(x4) is connected to main positive.
So now you have some current bypassing the main positive cable and main negative cable, thus your clamp meter is not measuring all the current coming out / back into the battery.
 

Cheap 4-life

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Yes, yes, that's fine functionally in normal(*1) operation.

The issue is that one (x4) of those balance leads is connected to main negative.
Another(x4) is connected to main positive.
So now you have some current bypassing the main positive cable and main negative cable, thus your clamp meter is not measuring all the current coming out / back into the battery.
I do have all 4 negative balance leads connected together and going to the main negative of each battery. The rest of the balance leads are going to each cells positive. There isn’t a main positive. The bms uses relays, not mosfets.
 
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ajw22

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I don't know how to explain it any plainer. Just measure the current on all your balance leads. You will find your "missing" current passing through one/some of them.

Or disconnect all of your balance lead connectors, and measure the currents on all your batteries again. You should no longer read any difference between Amps on the Pos and Neg terminals.
 

Cheap 4-life

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I don't know how to explain it any plainer. Just measure the current on all your balance leads. You will find your "missing" current passing through one/some of them.

Or disconnect all of your balance lead connectors, and measure the currents on all your batteries again. You should no longer read any difference between Amps on the Pos and Neg terminals.
Will disconnect the balance lead connectors and recheck the amps. Thanks for the idea. I guess it’s ok to leave one batteries balance lead connector connected so the bms relay still allows discharge
 
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Redpacket

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(Assumption #1: total battery pack current = constant during test )
(Assumption #2: no other connections eg balancing to battery during test)
In your diagram just above, yes, each battery can have different currents like you say.
But that is between batteries.
We are saying that for any one battery the current in its plus terminal ##MUST## equal the current out its negative terminal.
Eg you can have (measuring at the circled points in daromer's pic above):
Batt#1: 2A in plus, 2A out neg
Batt#2: 3A in plus, 3A out neg
Batt#3: 2.5A in plus, 2.5A out neg
Batt#4: 4A in plus, 4A out neg
etc
(Due to IR's, slight chemistry difference, different ages, etc)

What physics doesn't allow is something like:
Batt#1: 3A in plus, 2A out neg
Batt#2: 2A in plus, 3A out neg
Batt#3: 2A in plus, 2.5A out neg
Batt#4: 4.5A in plus, 3.5A out neg
It's a bit like saying a straight piece of wire can have different current flow at one end vs the other (can't happen).
If your clamp meter shows this, then there's something wrong with the measurement (eg instrument or technique or environment).

It really does sound like there are currents flowing in the balancing system not getting accounted for.
 
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Wolf

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Amazing what you can find out when you see the actual construction of the Battery. As they say the plot thickens and when all the evidence is presented. Occam's razor.
1617192071661.png
@ajw22 good eye and you beat me to it. I was reviewing the video and the red spaghetti was making my head explode. I wasn't quite sure what magic was happening below the white separator where all the red wires disappear into.

There is the answer. Clamp meters are probable reasonably accurate and will show the same once the "unknown" is removed from the circuit.

The laws of physics have been preserved and we can now sleep well at night again.

Wolf
 
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Cheap 4-life

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Amazing what you can find out when you see the actual construction of the Battery. As they say the plot thickens and when all the evidence is presented. Occam's razor.
View attachment 24485
@ajw22 good eye and you beat me to it. I was reviewing the video and the red spaghetti was making my head explode. I wasn't quite sure what magic was happening below the white separator where all the red wires disappear into.

There is the answer. Clamp meters are probable reasonably accurate and will show the same once the "unknown" is removed from the circuit.

The laws of physics and have been preserved and we can now sleep well at night again.

Wolf
All those red wires (68 of them) also made my head explode when I was rigging them up. I need to add some more zip ties to tidy it up.
I don’t understand how disconnecting them will change the reading on the clamp meter. I’m fairly certain that before adding a bms to the system that amp readings from each battery was the same as they are now. I will check that. Not today tho. My full Chevy volt pack will be here today, so I’ll be taking it apart.
The thing that really throws me for a loop is the clamp meter reading differently when it’s flipped. When the meter is reading negative on each wire, the totals of each battery equal (almost) the amps on the busbars main pos neg. problem with that is the negative value (clamp meter flipped to read negative) on battery 4 negative is 7.5amps (in the pic) so that would be saying that battery 4 is higher capacity than the rest because the rest have lower amp reading on their wires. Battery 4 is the oldest by a few years and with higher mileage so it would be the least capacity. When the clamp is flipped to read positive on battery fours negative, it then reads the lowest amps out of the 4 batteries. Which makes more sense due to it being the least capacity. But if using (adding) all the positive amp readings from each batteries negative (instead of negative readings) it doesn’t equal the amps going out the busbars main wires
 
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Cheap 4-life

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(Assumption #1: total battery pack current = constant during test )
(Assumption #2: no other connections eg balancing to battery during test)
In your diagram just above, yes, each battery can have different currents like you say.
But that is between batteries.
We are saying that for any one battery the current in its plus terminal ##MUST## equal the current out its negative terminal.
Eg you can have (measuring at the circled points in daromer's pic above):
Batt#1: 2A in plus, 2A out neg
Batt#2: 3A in plus, 3A out neg
Batt#3: 2.5A in plus, 2.5A out neg
Batt#4: 4A in plus, 4A out neg
etc
(Due to IR's, slight chemistry difference, different ages, etc)

What physics doesn't allow is something like:
Batt#1: 3A in plus, 2A out neg
Batt#2: 2A in plus, 3A out neg
Batt#3: 2A in plus, 2.5A out neg
Batt#4: 4.5A in plus, 3.5A out neg
It's a bit like saying a straight piece of wire can have different current flow at one end vs the other (can't happen).
If your clamp meter shows this, then there's something wrong with the measurement (eg instrument or technique or environment).

It really does sound like there are currents flowing in the balancing system not getting accounted for.
The load was a heat gun and some TVs. Should have been fairly constant.
No balancing was occurring during the test, it’s turned off.

I stated in the first post that I thought amps coming in and out of the battery should be the same. That’s why I was questioning what I was seeing. Turns out the clamp meter is giving different amps when it’s flipped. So now the amps in-out of each battery are similar enough. Above I tried to explain to Wolf about the remaining issue, although it was hard to word it
 

Cheap 4-life

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The balance leads are balancing each battery on the cell level simply because each cell# of each battery is connected to the same cell# of the other batteries. The batteries themselves are also balancing each other because they are all connected in parallel.

Ajw22 is that what you think the remaining issue is caused by? I’m referring to battery 4 having the highest amp readings and when clamps flipped also has the lowest
 

Wolf

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OK doing a bit of research on clamp meters there is definitely a polarity requirement for DC circuits as the direction of the magnetic field is different when the clamp meter is put on one way or the other. The internal circuitry of the meter will interpret the reading as either positive or negative on how the current is flowing. I am running a capacity test on pack #12 of my 3rd battery at a steady 30A regen so I did a bit of an experiment.
20210331_213856.jpg
First off there is a + and - indicator on the clamps or a flow arrow which should be pointing toward the load.
20210331_213416.jpg20210331_213425.jpg1617243071959.png

20210331_213946.jpg20210331_214100.jpg20210331_214020.jpg20210331_214128.jpg

There is an approximate .5A difference between the meter readings when you either read the current in a positive flow or a negative flow.
With my meter the positive reading is always a bit lower not by much but it is there. On the ≈30A readings you will see a - sign.
That being said either readings whether positive reading on the positive or negative cable, or a negative reading on the positive or negative cable, the measurements are so close they might as well be the same. That was a mouthful.

Wolf
 

Cheap 4-life

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The negative direction readings on my meter is also higher than the positive direction readings. I had up to a 2amp difference. Some of that might be my cheapo meter.
Still confuses me as to why the lowest capacity battery shows a negative reading that is the highest amps out of any direction on any wire on any battery. But the positive reading on the lowest capacity batteries same negative terminal also shows the lowest amp out of any direction on any battery. It can be seen on my sloppy pic above. Maybe as ajw22 said it has something to do with the way the bms leads are connected to each battery. I’ll test it soon but I’d still like to know more on the logic behind that suggestion
 
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ajw22

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Maybe as ajw22 said it has something to do with the way the bms leads are connected to each battery.
Since the BMS leads of all batteries are interconnected, you essentially have one big 16S4P battery (with four pairs of terminals), with very flimsy bus bars (ie. just thin balancing wires). At least electrically speaking, you do not have 4 separate batteries.

I only drew 2 batteries, but it looks something like this. The green circles are where you're measuring the current with your clamp meter.
1617280519229.png


When charging or discharging, most of the current (blue arrows) will use the thick cables, but some current (yellow arrows) can traverse "sideways" along any of the balancing wires to another battery to continue its path. Thus producing the seeming mismatch of current on the Neg and Pos terminals.
How much current flows "sideways" is very hard to predict. Certainly depends on the IR of each of the cells, and will likely change dynamically with the SoC and current draw.
1617280528066.png
 

Cheap 4-life

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Appreciate the explanation.
The longest yellow arrow, how can current travel that way thru the bms leads? Cell 1 and 2 of battery 2 are not connected to cell 15 and 16 of battery 2 thru the bms leads.
 

Cheap 4-life

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Ok so what your saying is that balance current from one battery to another thru the bms leads is causing the different readings on the meter.
I was saying that was happening thru the busbars and then assumed that it was balancing out thru the bms leads into the other batteries. I see now why a couple of you were saying it’s not possible to have different amps on positive and negative. It wasn’t known that I’m using one bms for all 4 batteries.
I will remove the bms from all batteries except one and retest the amps.
 
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Redpacket

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You're onto it.
I'd agree it is the balancing wires linking across between batteries causing the variation in your readings.

Assuming you use the meter consistently the same way around. To be sure, have you tried repeating the same measurement several times in a row to see if you're getting consistent results? Eg remove & replace clamp on same cable, same way around, note it, repeat, etc?
That meter does sound fussy....
 

Cheap 4-life

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My bms Is only connected to the battery via the bms leads. Other than that it is correct. Separate relays are powered by low voltage from the bms.
While typing this I’m realizing that maybe the power the relays are using could cause amp discrepancies. Maybe I should use a separate power supply for the bms so it doesn’t use the bms leads
 

Cheap 4-life

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You're onto it.
I'd agree it is the balancing wires linking across between batteries causing the variation in your readings.

Assuming you use the meter consistently the same way around. To be sure, have you tried repeating the same measurement several times in a row to see if you're getting consistent results? Eg remove & replace clamp on same cable, same way around, note it, repeat, etc?
That meter does sound fussy....
Yes I’ve checked it many times after writing it down with the same results
 
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