Best practice for 300kgf ‘fixture’ 280Ah cells
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Looks like you have found a way to stay in the sweet spot!! That is awesome!
But I can't quite tell what the graph is showing (image is too small for me to read). Is y axis pounds or kg and x axis time? If so am I correctly interpreting that you have two big plateaus at roughly 9psi and 14psi?
Your values (roughly) plotted against the A123 pressure/cycle life curve:
If I understand the idea correctly, I believe HaldorEE is using one ‘strength’ if Belleville washer to handle most of the expected expansion range and then when all of those have bottomed out (gone flat), he has some ‘stronger’ Belleville washers in each stack to handle any additional expansion at increased pressure...
Correct. Y = kg, X = time.
The plateaus are because I just stopped there. This was recorded using my load cell, and a bolt, washers and nuts. Those two values would represent 0% SOC and 100% SOC. The actual pressure is going to be constantly varying from one end to the other of that range.
And I biases the curve slightly down (my center position is 11.5 PSI). I would rather be just a bit on the lighter side, than to be squeezing too hard.
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Not any more. Because of the reduced range of motion required, I am able to use just the stronger washers. I was going to create a progressive spring if needed, but if I don't have to, they why bother.
McMaster-Carr
McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
I think this is smart for two reasons.
(1) Err on the side of caution
(2) Most packs will be sitting or cycling in the top half or top two thirds of the SOC range most of the time. So locating optimal pressure in the upper half of the SOC range seems ideal, since that is where your pack will spend the most time.
Bob B
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ThanksMcMaster-Carr
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Solarfun4jim
Solar seduced :-)
Out of curiosity, using a torque wrench, i'd love to know what torque it would take in Nm to release that nut, while you had it under 11.5PSI
Average movement is what I am interested it (average of all four springs)
Bob B
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He is not quite thru the full range yet .... We'll see where he ends up.
It is less than the 20 in-lb my 1/4" drive torque wrench is able to measure.Out of curiosity, using a torque wrench, i'd love to know what torque it would take in Nm to release that nut, while you had it under 11.5PSI
That is really not an acceptable way to determine how much pressure you are putting on your cells. I am using springs and use the a defined preload compression distance on the springs to set the force. That is the only practical way I know of to do this. I am using one of these to measure this force.
LWMH1 Load Washer Load Cell with Mounting Holes - Thru-Hole Load Cell
LWMH1 Load Washer Load Cell with Mounting Holes- Thru-Hole Load Cell is designed for use in force applications. This product is constructed from aluminium and is available in capacities ranging from 0.2 kN to 10 kN (45 to 2.25K lbf).
www.interfaceforce.com
I just tried it again. This time without the springs (just a nut over washers). I set my torque wrench for 20 in-lb and repeated torqueing and loosening the nut. Each time the torque wrench clicked.
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![PXL_20210126_041941673[1].jpg](https://diysolarforum.com/data/attachments/34/34638-e7b48bd65a0d0eda2ef16e352ce895a9.jpg "PXL_20210126_041941673[1].jpg")
I think that despite my torque wrench claiming it works down to 20 in-lb IT LIES!!!!
I repeated the test using 40 in-lb and got this. Which is amazingly close to the desired compression force.
That is also the appropriate force we are supposed to torque the bus screws. My head is spinning, what does it mean?
Note to self, don't use my torque wrench at 20 in-lb.
I repeated the test using 40 in-lb and got this. Which is amazingly close to the desired compression force.
That is also the appropriate force we are supposed to torque the bus screws. My head is spinning, what does it mean?
Note to self, don't use my torque wrench at 20 in-lb.
Caution. The above torque/clamping force result is highly conditional (albeit a lot more repeatable than I expected).
- I am using 5/16" x 18 zinc plated carbon steel bolt and nut.
- The only compliance in my test setup is the load cell. If you remove the load cell and just tightened a nut on a bolt the same way you would be likely to get different force. Clamping plywood? No idea.
- My threads are dry, but I have run a nut up and down the bolt many times.
Solarfun4jim
Solar seduced :-)
That is handy info, thanks
I don't know how handy or believable that is. Look at my caution post.
In another thread I implied that the torque we're looking at is so small that the margin for error is large, meaning that A) confidence levels are questionable and B) it doesn't take much to be off. Of course, I was talking about the torque on the terminals, but now I'm thinking the same thoughts may be appropriate for clamping.
Get the compression snug enough that you can lift the battery by the threaded rod with no cells falling out. If a cell falls out, it's too loose.
Get the compression snug enough that you can lift the battery by the threaded rod with no cells falling out. If a cell falls out, it's too loose.
Solarfun4jim
Solar seduced :-)
I have no way of working in Psi. My torque wrench goes down to 1Nm and supposedly accurate to 4%(iirc) (it is the 'click kind' and if you go really slow, the click is clear and positive every time).
So i was thinking(as an approximation) 300kgf = 29.4Nm divided over 6 rods = 4.9Nm. Now the washers i have on order are 1mm thick, but total height of 1.4mm(giving a deflection of 0.4 mm. The cells expand/contract approx 0.5mm each, so over 8cells in line = 4mm=10washers.
So 10 washers tightened flat to 4.9Nm when at 100% soc, would hopefully be in the ballpark(I'd probably drop back to 4.5Nm for margin of safety). However, presuming at 30%soc, the washers would have 'released' back to close on zero Nm being applied??? Thus i was wondering, and soon to experiment once i recieve my washers, if i doubled them up, could i still manage to torque them to 4.5Nm at 100%SOC and still be left with a reasonable compression at 30% soc. Early days in my thinking, but experimentation should be illuminating.
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