It all depends on how much expansion you want to plan for. I assumed no more than 0.5 mm per cell. For your pack that would be 8 mm. 1/4" of foam is 6.35 mm thick. If you smash it flat it still has thickness, but even if it didn't it can't take 8 mm because it isn't even that thick.
Compress or not, flexible busbar or not
- Thread starter Cheap 4-life
- Start date
The voltage monitor leads? I soldered spade lugs to the bus bars. No, the leads are not the same length, but that is not important because they have very little current in them.
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
My pack will have 19 cell not 16..
Using one 1/4” piece of foam, You are saying having more cells in a pack will have more expansion force than 4 cells therefore increasing the psi within the pack if the cells are allowed to expand?
So 16 cells expanding .5mm each will have more force than 4cells expanding .5mm each?
If the one 1/4” foam is smashed more with 16 cells vs 4 cells then there has to be more force applied to the foam due to there being more cells.....
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
It’s my understanding that different length balance leads will cause the bms to see the voltages of the cells incorrectly.. many have reported this and some changed to same length wires and the cells all got closer in voltage...
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Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
Love the idea of the spades and soldering. A lot better than putting the balance leads under the nuts and messing with the torque
I see no way that the resistance of the voltage monitor leads is significant.
Do the math, please. 16 X 0.5mm = 8mm.
8mm = 0.315". That is more than 1/4"....
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
I definitely agree.. I’m not the one saying that more cells do not add more force vs fewer cells. 16 cells expand more than 4 cells if expansion is allowed (almost unpreventable) which adds more force which creates more psi. Yes that force can be lessened within the pack with different springs to allow more expansion and more foam to allow more expansion but that isn’t changing the fact that more cells=more,,. which can be seen if the same springs or same thickness foam is used for both packs.. I mean make the foam 1” so there’s enough room for 16 cells expansion and use that also in the 4 cell pack and you will see the 16 cells pack foam crushed more. Or use 4 1/4” pieces in both packs, same thing the 4 pieces will be crushed more in the 16cell pack because of more expansion force
More cells means more expansion, not more force. It is completely up to the springs or foam or rigid frame to provide the force.
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
Ok so take the springs and foam out of the equation. Make it just a rigid fixture.. tightened to 300kgf at low SOC, now the 16cells expand at high SOC and the 4 cells do the same.. which rigid fixture has more expansion against its walls?therefore causing more pressure/force within the fixture..
Did you just go back 15 pages? Go back to the quiz. The answer is that A, B, and C are all the same.
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
So with the springs and foam not used and just solid fixed rigid walls, there’s nothing to change the force. So how can’t the force/pressure within the rigid fixture increase with more cells...
there’s no more extra foam (meaning the extra 3 pieces) in the 16 cell to absorb the extra expansion..
there isn’t different springs, length or strength or whatever to absorb the extra expansion
there’s no more extra foam (meaning the extra 3 pieces) in the 16 cell to absorb the extra expansion..
there isn’t different springs, length or strength or whatever to absorb the extra expansion
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Look at it this way: assume each cell is 50 mm thick and it could expand 0.5 mm.
So for one cell, 0.5 / 50 X 100 = 1% change in thickness.
Now consider your pack of 19 cells. The pack is 19 X 50 mm = 950 mm thick and can expand 19 X 0.5 mm = 9.5 mm. The change in thickness is
9.5 / 950 X 100 = 1%.
How can you expect something different to happen when both change by 1% of their thickness? It is the same amount per cell.
So for one cell, 0.5 / 50 X 100 = 1% change in thickness.
Now consider your pack of 19 cells. The pack is 19 X 50 mm = 950 mm thick and can expand 19 X 0.5 mm = 9.5 mm. The change in thickness is
9.5 / 950 X 100 = 1%.
How can you expect something different to happen when both change by 1% of their thickness? It is the same amount per cell.
Yes, the walls constrain the pack and the force builds.
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
If that 9.5mm has no where to go (rigid fixed structure) then the force/pressure or whatever inside the pack would rise more than the much less mm for a 4 cell pack needs for expansion.. I mean the rigid walls tightened down at 30% SOC to the same pounds on both packs, now there’s no more extra room for the 16 cells to expand vs the 4 cells packs room. Neither have foam or springs to give extra room
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
This is all I was ever saying..... so 16 cells does cause more force to build up than 4 cells... which increases the pressure in the pack?
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
So then I’m back to using springs or foam,, can’t do a rigid fixture for 19 cells (like the new data sheet does with one cell) and have that much force building right? 19 cells causing a lot more force to build than 1 cell in a rigid fixture.. or am I still wrong?
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Still wrong. I just showed you that the relative expansion is the same no matter how many cells you have. How could the pressure be different?
Ampster
Renewable Energy Hobbyist
I have 48 cells in a 3P16S configuration and I did not use foam or springs, just threaded rod. I did separate each group of 3 parallel cells with thin plastic to reduce the chance of voltage leakage if the plastic covering the aluminum shell was compromised.
In the end it is just a matter of personal preference.
This statement concerns me as well. I don't know how I never saw it. In my defense, it is on the next page after the table, between the header and some Chinese. Every time I read the datasheet I missed this sentence.
The table makes it look like the pressure can be up to 9kN after cell damage. The text clearly says the opposite.
Somebody needs to ask EVE for clarification. I'll check with my taiwanese friend to see if I can get a free translator.
