Batteries in parallel with different amps on positive and negative

[Cheap 4-life]

I am trying to figure out which of my four batteries in parallel has the lowest capacity without testing the exact capacity of each battery. My understanding is that the battery with the lowest capacity will also have the least amps being drawn from it. I got readings of the amps on each positive and negative on each of the batteries in parallel while discharging with a constant load.
Battery #1 and #2 both had real close to the same amp readings on their positive and negative wires. 6.7amps on positive and 6.8amps on negative.
This is where my confusion starts.
Battery #3 had 5.9amps on its positive wire and 6.7amps on its negative wire.
Even worse Battery #4 had 5.7amps on its positive and 7.2amps on its negative.

1- Why is there different amount of amps on the positive and negative. I thought they are supposed to always be the same?

Going by the logic that the battery that has the lowest amps being drawn from it is the battery with the lowest capacity, I would assume #4 with the 5.7amps is the lowest capacity. But the amps on that batteries negative is higher than any of the other batteries positive or negative amps..
I’m starting to think that the difference with the positive and negative amps on #3 and #4 is because those batteries are indeed lower capacity and somehow this makes the negative wire read different (higher) amps than their positive.

2- Maybe because battery #1 and #2 are somehow supplying power to the lower capacity #3 and #4 through the negative wire to keep the voltage of each of the 4 paralleled batteries the same?

3- thoughts as to why battery #3 and #4 have different amount of amps on positive and negative

4- which battery has the lowest capacity going by the amp readings given?

I have checked this multiple times while discharging with similar results.
These four batteries are each Chevy volt 60v 55ah in parallel. I don’t think the chemistry or battery type is relevant to the issue. All batteries are connected with the same gauge and length wire.

 

[Cheap 4-life]

I know a lot of people that are knowledgeable enough to help figure this out, build 18650 packs. Usually that means all the cells are paralleled with no way of checking the amps on the individual cells positive and negative. The difference in the positive and negative amperage that I’m seeing couldn’t be seen on most 18650 packs because each cell in parallel doesn’t have its own negative and positive wire. Each of my 4 batteries (that are paralleled together) have their own positive and negative wires (that I used for the amperage results) that go to a busbar. If I checked the amperage at the busbar (which would be similar to checking the amps in and out of a 1s 100p 18650 pack) then the amp difference on positive and negative that I’m seeing wouldn’t be able to be seen.
If four 18650 cells with varying capacities-internal resistance, were paralleled by using lengths of their own wires (instead of nickel strips with no room to check) so that the amperage on the positive and negative of each cell could be checked, then I assume the same results (different amps on the positive and negative) would be seen.

 

[ajw22]

1- Why was there different amount of amps on the positive and negative. I thought they are supposed to always be the same?

That simply can't be. Must be some sort of measuring error. Either there is some sort of device/cable that letting some current bypass the sensor, or the sensor is simply inaccurate. Perhaps you had balancing leads that were drawing current?
Or you had a fluctuating load that changed current draw as you were moving the sensor from one cable to the other?

Going by the logic that the battery that has the lowest amps being drawn from it is the battery with the lowest capacity, I would assume #4 with the 5.7amps is the lowest capacity.

Since all the batteries are of the same type, a very reasonable assumption. Though only as accurate as your current sensor is.
Firstly need to clear up the in/out current discrepancy to make sure you're working with accurate numbers.

 

[Cheap 4-life]

That simply can't be. Must be some sort of measuring error. Either there is some sort of device/cable that letting some current bypass the sensor, or the sensor is simply inaccurate. Perhaps you had balancing leads that were drawing current?
Or you had a fluctuating load that changed current draw as you were moving the sensor from one cable to the other?

It’s happening. Only on those two batteries. I have checked it many times. The clamp meter seems to be accurate enough for this purpose. There isn’t any balancing from the bms happening. It’s turned off. Load wasnt fluctuating.
I’m thinking battery 3 and 4 are the lower capacity and are seeing a circulating current (to keep voltage even) from the higher capacity batteries. This wouldn’t be able to be seen with 18650 packs from cell to cell. It could be seen from pack to pack only if the packs were paralleled, but most of the time the packs get series together.

 

[Cheap 4-life]

When charging Im seeing the opposite. Battery 3 and 4 have more amps on the positive than the negative by roughly the same amount of a difference. I’m thinking the reason is that 3 and 4 have a higher internal resistance (lower capacity) so they are getting circulating current from batteries 1 and 2 which are lower resistance (higher capacity) . Seems like the higher capacity batteries are trying to keep the voltage the same on the lower capacity batteries.
When charging the amps should be (and are) going into the battery-cell thru the positive and out the negative so if battery 1 and 2 are trying to add power to 3 and 4 to keep the voltage the same then that extra-balance amps would be on the positive wire, which is what I’m seeing.

Opposite when discharging, amps into the battery-cell should be (and are) flowing out the positive and into the negative. My amp meter showed more amps on battery 3 and 4s negative when discharging. So the lower resistance batteries 1 and 2 are supplying the higher resistance batteries 3 and 4 with extra-balance amps thru the negative to keep the voltage the same on all the batteries..

Apparently thats how cells-batteries in parallel behave as they are keeping the voltages of the cells-batteries equal. The larger the capacity differences, the more the amps will be different on the positive and negative of the battery-cell. Larger capacity differences means more amps are needed to balance the voltage of the batteries.

 

[Wolf]

@Cheap 4-life
Take a look at my study Behaviour of cells in parallel take 2 https://secondlifestorage.com/index...r-of-cells-in-parallel-take-2.8857/post-71290
I'm droping you into the section where things get interesting but nevertheless you should read the whole thread.
It doesn't matter whether it is one cell or a bunch of cells in parallel there is an interaction between cells. The largest interaction is between high and low IR calls but also high and low capacity.
In essence your (4) 60V batteries are in parallel and will behave very similar to the 4 cells in parallel that I ran through my tester.
As far as different amperage being supplied by the 4 batteries in parallel is understandable due to their capacity difference and possibly IR.
The reason why positive and negative amps don't match? On that I haven't a clue. Although there should be a time when all (4) Batteries are approaching full charge the current should be really close.

Wolf

 

[Cheap 4-life]

I have read all your threads. I’m sure that’s part of the reason why I could figure out the answer (for this post) to my own question.

You said “haven’t a clue” I explained why that part is happening. It’s the interaction of high and low resistance-capacity. That can be seen from the extra amps (coming from higher capacity lower resistance batteries) on the negative wire when discharging. It’s on the negative due to the direction the amps are flowing while discharging. The extra amps from the higher capacity batteries (to balance-bring up the volts of the lower capacity cells-batteries) can’t come in the positive while discharging because the amps are going out of the positive to the loads. So those balance amps from the higher capacity batteries-cells has to come in on the negative wire while discharging.

The opposite happens when charging as I explained because the amps are flowing in the other direction which makes the balance amps (from the higher capacity batteries) have to come in the positive wire instead.

Yes, I would think the same would happen with 18650s of varying capacity’s-resistances, if you tested the amps between each cell on their positive and negative.

 

[Cheap 4-life]

.

 

[Wolf]

I will need to do some experiments regarding that. I think I can visualize what you are saying in my minds eye but at this point in time I can't wrap my head around it although in some strange way it seems to make sense.

 

[Cheap 4-life]

Hard to agree 100% unless you see it yourself like I did. That’s usually how you and I end our conversations with you saying you need to do some experiments regarding that.

 

[Hanssing]

It’s happening. Only on those two batteries. I have checked it many times.

It's not. Period.

You either have a meassurement error, og a current bypass you are missing. For instance are the cells touching with their negative housing?

Sum of electrons in = sum of electrons out.

Try meassuring the total amperage in on positive and negative to pack this has to be equal.
Then the sum of individual cell-current has to equal the pack.
Then go from there and find your error.

 

[daromer]

I agree, pos and negative cant have different currents going in and out from a cell. Its impossible by physics.

 

[Redpacket]

Re the measured current differences, sounds like the clamp meter might be interacting with other cables?
Maybe try clamping to a cable, measure, then move it while still measuring on the same cable, move other cables away/closer, etc, see if the readings change?
If the meter has been dropped (ie damaged magnetic loop in jaws) or the jaws don't close fully it might be picking up other currents.

 

[Wolf]

Your mission if you you accept it is to give us all the readings as demonstrated in this diagram. Voltage drop on all the cables being the most important.

From there we may be able to figure out what's going on.
This message will self destruct in 42 seconds.


Wolf

 

[Cheap 4-life]

It's not. Period.

You either have a meassurement error, og a current bypass you are missing. For instance are the cells touching with their negative housing?

Sum of electrons in = sum of electrons out.

Try meassuring the total amperage in on positive and negative to pack this has to be equal.
Then the sum of individual cell-current has to equal the pack.
Then go from there and find your error.

Not possible for Chevy volt batteries to have cells touching negative housing. Not possible for current bybass either.
Electrons move in a different direction than amps.
For the parallel batteries to keep themselves the same voltage the lower resistance higher capacity batteries have to supply balance amps to the higher resistance lower capacity batteries. If the batteries are discharging to a load (thru the positive Ofcourse) then there’s no option but for the balance amps to go thru the negative. This is exactly what I’m seeing.

And like I said I see the exact opposite when charging. This in itself proves that it’s happening.

Yes the total amperage in and out of the entire pack (all batteries) when charging-discharging is equal. But the amperage differences I’m seeing are coming from lower resistance batteries to the higher resistance batteries thru the busbar. This can be seen as a describe on the individual batteries wires to the busbar

 

[Cheap 4-life]

I agree, pos and negative cant have different currents going in and out from a cell. Its impossible by physics.

How else would the different capacity batteries in parallel balance themselves by voltage when being discharged or charged

 

[Cheap 4-life]

Re the measured current differences, sounds like the clamp meter might be interacting with other cables?
Maybe try clamping to a cable, measure, then move it while still measuring on the same cable, move other cables away/closer, etc, see if the readings change?
If the meter has been dropped (ie damaged magnetic loop in jaws) or the jaws don't close fully it might be picking up other currents.

The other cables are far from the measured cables. I have checked the measurements at various places on the cable. Clamp meter isn’t damaged. Jaws are closed tightly

 

[Cheap 4-life]

Your mission if you you accept it is to give us all the readings as demonstrated in this diagram. Voltage drop on all the cables being the most important.
View attachment 24447
From there we may be able to figure out what's going on.
This message will self destruct in 42 seconds.

View attachment 24448
Wolf

For me, I see no reason to do all those tests. It’s cut and dry IMO. Obviously the voltages will be slightly different between the batteries when being charged-discharged. The higher capacity batteries must balance the volts of the lower capacity batteries . And the direction amps flow when discharging-changing are opposite. Balancing amps from the other battery can’t go into and out in the same direction at the same time.

 

[daromer]

-> How else would the different capacity batteries in parallel balance themselves by voltage when being discharged or charged
The positive and negative terminal of 1 battery can never have different currents going in and out. U = I * R. As long as you dont have losses and you dont have losses on fictive points.

if you push in lets say 1A of energy into a battery it MUST be the same equal on the other side for the equation to be true The only thing that can happen here is voltage loss over the wire or the battery itself but if the measurement points is on the same place its 0. Nothing else.

A clamp meter is not aqurate at all and is very much influenced byt surrounding equipment and can vary alot.

So if we interpretate what you say you have either measured wrong or have issues with the equipment.

Balancing between cells is not the same as having the negative and positive end of a cell having different current If you put 2 cells in parallel for balancing to same voltage it WILL ALWAYS have same current going in and out both of the ends. You can not have electrons vanish there

-> And like I said I see the exact opposite when charging. This in itself proves that it’s happening.
To me that proves that you have a flaw in your way of testing it.



Lets look at above. A and A here will always have the same amount of electrons flowing. And why doesnt the heat in the cell change that? Because those are meassured over the cell and not over the points A and A.

You could say that A and A are a resistor. Thats the way to meassure currents going in and out by checking voltage drop. So for this to be true both resistors are the same value lets say 1ohm? If they are different you have introduced errors in your wiring or cables.

And since the voltage drop or resistance in the cell doesnt affect the meassurement in A and A we get on both

U/r=I. And as you saw i said A and A. thats because they have same IR. Then we also know that the formula is same on boths ends

=
U/R = U/R

Hope that explains it somewhat. Even though its hard to write in simple terms to explain it. Ie think like this. Energy cant dissapear into thin air. It will only convert between different forms.

 

[Cheap 4-life]

Practical Considerations - Batteries | Batteries And Power Systems | Electronics Textbook

Read about Practical Considerations - Batteries (Batteries And Power Systems) in our free Electronics Textbook

www.allaboutcircuits.com

Here it explains circulating current between batteries in parallel. My batteries are not extremely different in capacity-resistance but the measurements I’ve shown are this circulating current