Joined Nov 24, 2022
10
Hi!. I’ve got a question about this parallel group. It’s possible that at the cells inside the green circle current flows in the direction of the red lines?. If not, why? Thanks!

#### BobTPH

Joined Jun 5, 2013
8,659
No, it is not possible. Why do you think it would?

Joined Nov 24, 2022
10
No, it is not possible. Why do you think it would?
Because if the current can flow towards the cells at the right position which are also the negative terminals, why couldn't the current flow to other cell with also the same negative terminal?. That's something that i don't undertand. It something related with the amount of nickel strips that i need to put in the battery pack.

#### crutschow

Joined Mar 14, 2008
34,019
It looks like those cells are in parallel-series, so if all the cells have reasonably matched voltages then equal current will flow out of all the plus terminals and into the minus terminals, as determined by the battery load.

#### drjohsmith

Joined Dec 13, 2021
850
Because if the current can flow towards the cells at the right position which are also the negative terminals, why couldn't the current flow to other cell with also the same negative terminal?. That's something that i don't undertand. It something related with the amount of nickel strips that i need to put in the battery pack.
Can I check , your making this pack ?
Do you have safety circuit on each cell ?

So current flows with the voltage potential , from high voltage to low voltage .
If you have two cells in parallel , if they are both the same voltage , they will provide the same current.
But if one is lower voltage than the other , then current will flow from the higher voltage one , to the load , and the other cell, till the voltages equalise.
If all is well, thats fine .
But
Cells hold a lot of explosive energy.
What happens when one of the parallel cells goes short circuit ?
All the other cells try to discharge into the short.
Result , flying in fire metal .

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#### Audioguru again

Joined Oct 21, 2019
6,606
The 6 cells on the left side have their - terminals (silver ends) welded together and are also welded together with the + terminals (black ends) of 6 cells on the right side.
The bottom terminals of the cells also have 6 cells on each side welded together and the load is connected to each side for double the voltage of one cell because the groups of cells are in series.

Why are you thinking about "direction of current flow" When there is "conventional direction of current flow" and there is "electrons direction of current flow" which are opposites and you did not say which direction you are asking about?
Your red arrows show "electrons" current direction if the groups of cells have a load.

Who cares about the direction of current flow? Simply connect the + of the battery to the + of the load and connect the - of the battery to the - of the load.

#### MrChips

Joined Oct 2, 2009
30,456
The photograph shows six cells bonded to together at the -ve terminals and together bonded to another six cells at the +ve terminals. Therefore, the entire battery pack is configured in a series-parallel configuration.

We don't really care which way the current flows. What we know is that conventional current will flow from a point of higher potential to a point of lower potential.

If two cells that are bonded together in parallel have different voltages then one cell will be charging the weaker cell. In this case there will be a reverse flow of current into the weaker cell.

#### WBahn

Joined Mar 31, 2012
29,846
Hi!. I’ve got a question about this parallel group. It’s possible that at the cells inside the green circle current flows in the direction of the red lines?. If not, why? Thanks!

View attachment 281338
I think the TS is simply asking whether there is a preferred direction of current flow in the bridge between two cells connected in parallel and is essentially stating that symmetry argues that if there is any current at all, that it could be in either direction.

This is a valid observation, the conclusion being that IF the situation is symmetric, then because the current could flow in either direction, the actual current must, in fact, be identically zero.

But, as others have pointed out, the situation is not always (if ever) symmetric. Whether the current flows one direction or the other depends on the details of the asymmetry. If one cell has a higher voltage than it's neighbor, current will flow in the direction needed to equalize the voltage. There is also voltage drops along the bridging straps and that will result in some current flowing in bridges that, ideally, would otherwise have no voltage drop across them.

#### MrChips

Joined Oct 2, 2009
30,456
The other thing to observe is that some cells are bonded to multiple straps.
In theory and practice it is only necessary to have a single strap to each cell.
Since these are all very low resistance straps, close to 0Ω, in theory there is zero potential difference between bonds. You can consider the entire welded strap as a single ground plane with zero resistance.
Thus there is zero current flowing between cells, - to -, or + to +.
The majority of the current is flowing between the cells connected in series, i.e. +ends to -ends.

Joined Nov 24, 2022
10
I think the TS is simply asking whether there is a preferred direction of current flow in the bridge between two cells connected in parallel and is essentially stating that symmetry argues that if there is any current at all, that it could be in either direction.

This is a valid observation, the conclusion being that IF the situation is symmetric, then because the current could flow in either direction, the actual current must, in fact, be identically zero.

But, as others have pointed out, the situation is not always (if ever) symmetric. Whether the current flows one direction or the other depends on the details of the asymmetry. If one cell has a higher voltage than it's neighbor, current will flow in the direction needed to equalize the voltage. There is also voltage drops along the bridging straps and that will result in some current flowing in bridges that, ideally, would otherwise have no voltage drop across them.
I think i understand what you are saying. Current could flow in both directions between the first cells at the left side which are connected in parelel because it could be a possible path in order to achieve the positive terminal. But because the same current will flow in one or another direction this both currents cancell each other and no current will flow between this parallel conection. I think that's what you're sayin. Correct me if i'm wrong. If i'm not wrong you answered my question.

#### WBahn

Joined Mar 31, 2012
29,846
I think i understand what you are saying. Current could flow in both directions between the first cells at the left side which are connected in parelel because it could be a possible path in order to achieve the positive terminal. But because the same current will flow in one or another direction this both currents cancell each other and no current will flow between this parallel conection. I think that's what you're sayin. Correct me if i'm wrong. If i'm not wrong you answered my question.
It's not a matter of current flowing in both directions and canceling out. Imagine two identical barrels of water that have a pipe connecting them near the bottom. What direction will water flow in that pipe? If the two barrels are filled to the same level, no current will flow. But if one barrel has a bit more water than the other, water will flow from the more-full barrel to the less-full barrel until they are even. Now imagine that there are several barrels connected this way similar to the parallel cells in your battery pack. As water is pulled from the edge barrels, the water drops in those barrels, causing water to flow into them from the barrels they are connected to. This process continues in such a way that water is flowing out of all of the barrels more or less uniformly. But it won't be perfectly uniform, so some barrels will flow a bit more than some of their neighbors, which will prompt some water to flow in the connecting pipes that symmetry would otherwise say should have no effect. It's the same with the cells in this battery.

Joined Nov 24, 2022
10
I think i understand what you are saying. Current could flow in both directions between the first cells at the left side which are connected in parelel because it could be a possible path in order to achieve the positive terminal. But because the same current will flow in one or another direction this both currents cancell each other and no current will flow between this parallel conection. I think that's what you're sayin. Correct me if i'm wrong. If i'm not wrong you answered my question.
It's not a matter of current flowing in both directions and canceling out. Imagine two identical barrels of water that have a pipe connecting them near the bottom. What direction will water flow in that pipe? If the two barrels are filled to the same level, no current will flow. But if one barrel has a bit more water than the other, water will flow from the more-full barrel to the less-full barrel until they are even. Now imagine that there are several barrels connected this way similar to the parallel cells in your battery pack. As water is pulled from the edge barrels, the water drops in those barrels, causing water to flow into them from the barrels they are connected to. This process continues in such a way that water is flowing out of all of the barrels more or less uniformly. But it won't be perfectly uniform, so some barrels will flow a bit more than some of their neighbors, which will prompt some water to flow in the connecting pipes that symmetry would otherwise say should have no effect. It's the same with the cells in this battery.
All righ, now we're talking haha. Thanks, you explained it pretty good.

#### MrChips

Joined Oct 2, 2009
30,456
This is a hypothetical question.

Imagine two water tanks feeding water into a pond.
There is an interconnecting branch pipe that lies below the surface of the water in the pond.
Which way does water flow in the branch pipe?

Joined Nov 24, 2022
10
This is a hypothetical question.

Imagine two water tanks feeding water into a pond.
There is an interconnecting branch pipe that lies below the surface of the water in the pond.
Which way does water flow in the branch pipe?
View attachment 281411
I'm not quite sure because i think water from the pond is also exerting presure upwards.

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