My first pack of prismatic cell will arrive next week. I think I will use Panduit Dura-Ty to compress the cells with plywood of the same size of the cells at both ends. I have used them before for telecommunications work and they are very strong.
The sides & tops are considerably narrower and stronger than the weak "faces". The pressure would have to be severe to have any effect on those surfaces. To reach that level, the cells would have to pushed to, or beyond their capacities and in Energy Storage that really is not a factor unless using really small capacity cells. ESS type systems use 100AH cells or larger, not 20AH or 30AH.Are you not affraid that the cells will swell on the side or top?
On my 2nd set of 8x 280 Ah Eve cells; and considering this compression option; I notice the batch I am working on does not have real even planes on the sides. I also notice that when sliding them together, and then running my hand across those sides; I can feel some very slight wavey sides of battery. I am pretty sure my 1st set is similar via hand rubbing across external #1 and #8 side cells. I put my first set of 8 280 Ah cell together with double sided tape between each cell thinking that would be better for keeping em cooler. As I consider compression of my battery bank ... My question is: Is it normal for the side planes of battery not to be a perfect flat plane ???? what do you folks who apply compression notice about that ??? And what is the advantage of compression over air space between them ???recommend that when compressing the cells, that it be done with them at a lower charge 50% is likely the optimal median... When cells are full 3.55V+ they do tend to bloat a bit (normal). This is more pronounced with the Aluminium Cased "blue" cells.
Is 15 in-lb on 4 bolts over the surface of the cell equivalent to 12psi?Do you have a compression force spec for the cells? If not, ballpark 15 in-lb. Which is a not much.
None of the aluminum cells I've handled were ever perfectly flat. The metal is not that thick, you can squeeze the cells with your fingers and you'll see what I mean.does not have real even planes on the sides.
Got it - thanks.This calculator here will give you a rough idea. 10 in-lb yields 200lb of clamping force. 4x200=800lb, which is a bit high. If you are using nylock nuts you can measure the free drag, but its around 5 in-lb. With nylock nuts that would be 15 in-lb total, 10 effective (approximately, just measure your nuts). With 4x 1/4-20 thread, you would want somewhere between 5-8 in-lb (effective, add locking fastener drag).
Bolt Torque, Axial Clamp Force, Bolt Diameter Calculator
This fastener bolt torque calculator will Estimate the unknown torque, diameter, axial force applied and coefficient of turning friction for the given conditions.www.engineersedge.com
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Yeah, tell me about it.Pitch matters, but at these low of torques its not significant.
With 4 x 1/4" fasteners, I would go 8 in-lb as stated above. Its not a lot of torque.
Thanks. Once I decide clamping to 12psi is definitely the way I want to go, I’ll pick up one of those.
Very nice! Have you performed any tests yet?Here is a prototype with bent steel plates and rods. Rubber pads at the ends, and eventually coil springs. If it tests well we might put together a convenient hardware kit.
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Yeah, no need for precision instrumentation.It hasn't been tested yet. That being said, once the springs are installed, the clamping force will be within 10% of the theoretical for any SOC. So I see no reason to go full instrumentation. This isn't rocket science. The primary purpose of this fixture is to secure the pack for rough mobile applications. Longevity is a secondary goal.
When do you anticipate starting first charge/discharge cycles on the cells in the fixture?
Can you please post pictures and details over here after you get a completed version?I can't say. This is being build by an associate of mine (across the country from me) using some parts we collaborated on.
As far as bus bars go, I don't recommend the solid type for mobile applications.
Clamping load is going to be determined by spring compression. It will be set at 100% SOC currently (to 12psi plus/minus 1psi). As the cell shrinks during discharge this will drop to no lower than 8psi. I suspect it will be between 9-13psi under all conditions, as the supplied values for expansion of the cells are fairly conservative.
Can you please post pictures and details over here after you get a completed version?
Pack / Cell compression Optimized By Using Springs.
I am opening this thread to split it off from the general cell compression thread. That way those looking for info don't have to sort thru a thread that has many paths for pack compression. If you are new to this idea, it stemmed from an EVE cell specification that indicated a significant...diysolarforum.com