Compressing at what SOC?

[Mountainlion]

I finished my compression rack today for 4 x 280 Ah cells.

compression will be achieved with 4 threaded rods between two aluminum plates (5 mm)

Will use the pack in a travel trailer. I want to reduce the movment between the cells.

Do you compress when cells are 10% SOC , 50% SOC or 100% SOC?

If I do it at 100% SOC there will not be be much compression when at 10% SOC?

 

[svetz]

Just wrote a wiki page on that: https://diysolarforum.com/ewr-carta/compression/
But, it hasn't been approved by the community yet so your best bet is to follow the links embedded in it order to decide.

 

[Mountainlion]

Thanks for answering. I was reading lot of threads yesterday about compression.....

Did not give me much answers. How much torque should I apply on 4 bolts compressing (4 EVE 280Ah) to achieve 12PSI?

 

[Topper]

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

 

[Mountainlion]

Thanks. What is the answer. My rods are M6 stainless steel?

 

[Topper]

M6 course or fine thread?

 

[Mountainlion]

Well that is a good question. Accordingly to invoice it is course.

 

[Topper]

So the spec from EVE was 300 KG force which rounds off to 660lbs. Battery face is approx 6.85"x 7.874" = 53.94 sq inches
660lbs/53.94sqin=12.23 lbs per sq inch
Divide 660 by 4 bolts that's 165 lbs Axial (clamping) force per bolt.
ASSUMING you are a course thread that should equate to roughly 8 INCH pounds torque per bolt. Realistically, that's a snug twist of the wrist on a regular nut driver for the average build mechanic. .

 

[Mountainlion]

Thanks Topper! That confirms what I felt made sense. A snug fit with the wrist.

 

[svetz]

Is there any way to measure the plate pressure as the SoC/temp changes?

I doubt the threaded rods will expand/contract at the same rate as the batteries and it sounds like going over 18 psi is bad.

 

[Mountainlion]

I’m sure pressure will change with SOC and temp. No doubt. Read the threads about springs and load cells. There is a bunch of smart people that is trying to solve that problem on this forum. I spent one evening reading those compression threads.

Trying to achieve ”perfect” is in my world meaningless and to expensive. I already made the choice not to go “perfect” when I bought cheap EVE 280Ah cells. If I wanted “perfect“ there is better cells.

I also very much doubt that there is much scientific data behind the recommendations from the manufacturer. They probably winged it a little bit.....

My reason to compressing is to reduce movement between cells as they will be used in an RV. If I get more cycles even better. But unfortunately I do not have as many cycles as the batteries. So I’m O.K. if cycles do not improve as long as I do not crush the cells.

 

[APhoton]

So the spec from EVE was 300 KG force which rounds off to 660lbs. Battery face is approx 6.85"x 7.874" = 53.94 sq inches
660lbs/53.94sqin=12.23 lbs per sq inch
Divide 660 by 4 bolts that's 165 lbs Axial (clamping) force per bolt.
ASSUMING you are a course thread that should equate to roughly 8 INCH pounds torque per bolt. Realistically, that's a snug twist of the wrist on a regular nut driver for the average build mechanic. .

T = K·F·d(1 - l/100); 165 lbs force using a 1/4" threaded rod would be 8.25 in-lbs or as you said 8 in-lbs rounded to the whole number. Thanks for the information. I'm going to use 2 pieces of 3/4 plywood with 4 1/4" thread rods to compress my cells. I will have my cells at 50-55% SOC, whatever that means-I guess 3.3v/cell?? Do I have this right??

 

[Topper]

SOC is most accurately measured by Ahr as the voltage curve is extremely flat. I clamped mine at 140Ahrs using EVE 280's.

 

[APhoton]

SOC is most accurately measured by Ahr as the voltage curve is extremely flat. I clamped mine at 140Ahrs using EVE 280's.

I hate to ask this question but my multimeter is 10Amax. How do you measure amp hours

 

[Topper]

AiLi 500A Battery Monitor High Low Voltage and Low Capacity Programmable Alarm Compatible with 10-120V Lithium Sealed Gel Flooded Batteries Capacity up to 9999 Ah : Amazon.ca: Electronics

AiLi 500A Battery Monitor High Low Voltage and Low Capacity Programmable Alarm Compatible with 10-120V Lithium Sealed Gel Flooded Batteries Capacity up to 9999 Ah : Amazon.ca: Electronics

www.amazon.ca

Or most smart bms will keep track as well.

 

[Manu]

Hello, and sorry for hijacking this post but this is the one I understood the most!

I'm building a 1p16s EVE LF280K pack. The pack will be physically divided by 2.
I plan to use 6 rods (6mm) with M6 bolts.

If I understand correctly and if I don't make mistakes about unit conversions or translation:

I need to apply a total of 660lbs x 2 = 1320 lbs to keep it "compressed".
For 6 bolts: 1320/6 = 220 pounds at each bolt

With the bolt calculator linked above:

• 330 pounds (lbs) axial clamp force
• bolt diameter = 0.23622 in (6mm)
• torque should be set at: 10.40 in lbs (1.175 Nm) convert unit used (in lbs to Nm)

Do I stand correct?

 

[Topper]

The only variable i don't see in your calculations. Thread pitch. You'll want to use an axial clamp force calculator. Fine vs course thread makes a huge difference on your torque values.

 

[Manu]

The only variable i don't see in your calculations. Thread pitch. You'll want to use an axial clamp force calculator. Fine vs course thread makes a huge difference on your torque values.

Do you have any idea how do I calculate that?
Or any better ways to find the right torque value applied for each bolts to match the recommandations?

 

[brum]

If you are compressing just to make the pack more rigid for transportation purposes put a silicone heat sink pad between the cells. This will provide insulation (electrical). It will compensate for the cell expansion/contraction so you don't end up with too much pressure on the cells. And it will provide stability of the pack (try to slip a cell over the silicon pad and you'll see why).

 

[Ampster]

I cannot tell what material the boards are at the end. Depending on their flexibility, it might create more even pressure to put thread rods in the middle between the two rows. I have a two row pack and used a total of six thread rods, with two at each side and two in the middle. I also covered the rods with tubing to reduce risk of electrical conduction between cell cases.