Looks good! Thanks for the numbers. Don't pop a hernia moving that thing.
The rods are used to pull the plates against the sides of the cells. There is not a concern with torque if using ny-lock nuts. It is a matter of tightening the nut to pull the plates in firmly against the plastic cased cells so the cells do not expand.
1/4-20 dry, zinc plated rod/bolt, will produce about 75lbs of clamping force with 16.5 in-lbs of torque. This is pretty rough of course, lots of variables. It is quite easy to exceed that tightening with a 10" wrench for example. I posted a link earlier with a basic calculator for clamping load.
Given that the cells size, 600lb distributed across the cells surfaces is not even 10PSI. It may seem like a lot at first glance, but you can apply that much pressure with 2 fingers if you wanted to.
If you are using metal plates to clamp aluminum cased cells, you need some type of rubber or plastic layer. The cells are not perfectly square, with the edges being slightly proud when a compressive load is applied. So a 1/6" plastic or rubber sheet between the cell and clamping plate will provide some give and isolate the metal.
Am I understanding this correctly?
I’ll admit that 12psi is less pressure than I was expecting, on the other hand, how to translate psi to inch-pounds on a torque wrench is unclear to me...
Yes.
@Luthj already answered that just before your post; he posted a link to a calculator on the previous page: https://www.engineersedge.com/calculators/torque_calc.htm
You can also do all the math by hand if you prefer of course
Does every bolt need to apply 12psi of pressure (300Kg of force) or is that 300Kg of force divided by the number of bolts?
If using 4 bolts means each should only apply 75Kg of force rather than 300Kg, that would translate to a bit over 8 inch-lbs of torque (practically nothing).
I can understand stacking my battery vertically and putting a container full of 660 pints (82.5 gallons) of water on top of the stack and that seems like a lot of compression force (and too much trouble).
I’m having difficulty understanding how that 300Kg of force translates to the seemingly-small levels of torque we are talking about - seems like a lot less compression force...
Don't worry about pressure. Just work with clamp load divided by the number of bolts.
It's the total force, for example if you have 6 bolts then each bolt needs to apply 50 kgf only.
You underestimate the clamping force a bolt can have, that's pretty common actually. [/b{Even a small bolt can have a few metric tons of clamping force at max torque.
4 bolts would need ~8 in-pounds of torque which isn’t much. The the torque wrench that I have reaches 600 ft--inches in minimum increments of 25 ft-inches, so looks like I’ll need to get a more sensitive torque wrench...
I suppose you’re right. Time to find a more sensitive torque wrench...
Which brings up a few other questions:
-Is overtorqueing by 50% or 100% a problem? We know 300Kga of force is needed to get the full 3500 cycles, but will going (modestly) above that cause damage / a problem?
-I was going to go with Aluminum ends both for strength as well as heat dissipation, but ABS plastic may be plenty strong enough (and much cheaper).
Don't get into the weeds here. I would go to 20 in-lbs and call it good. That's basically "snug" with a short wrench. The important thing is that you don't accidentally put 50in-lb+ by mistake.
Your not going to damage the cells with a few hundred extra pounds of clamping force. These cells are made with a nearly solid spiral wound laminate. It is quite strong in compression.
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