Compression - thoughts?

[cjmssmd]

One interesting thing was COMPRESSION .. since they are vertical -- the bottom battery is literally supporting 80 pounds ... and the TOP one zero ... you know though -- at this point in life -- i'm just not going to worry about it ... at 54 volts right now and 325 Ah -- thats over 17,550 Ah and I'm just going to let it be what it is ...

@ghostwriter66 I have read through your posts with interest - thanks for posting here!!

I am building a lawn mowing robot with a 22.8kWh pack consisting of 30 LF230s (purchasing from DOCAN - I agree they are good) in a 2P x 15S configuration. We go pretty gentle on the cells, charging at 0.12C, discharge at 0.15C (peak discharge 0.4C for up to 10 minutes). We require the the pack to last 7 years and 1,750 cycles. The pack is installed in a custom-designed aluminum and steel rover. Currently our cells are insulated from each other and from the chassis with FR4/garolite and held in place with ratchet straps.

My question to you is, what is your thinking on the benefits of compression? The EVE specs state 300 kgf compression to achieve 2,500 cycles. Are you compressing your cells? Are you aware that anyone is? The PCBWay DIY Pack uses threaded rods. This teardown of a Tesla pack (and this one) using prismatic LFPs looks like they have some foam between the cells but the frame itself sure doesn't look like it is beefy enough to provide 300kgf (per cell; 10 cells side-by-side means 3000kgf!). Compressing with threaded rods seems pretty imprecise. These people observed that with a rigid frame the cell swelling & shrinking causes differences in pressures between cells - which can be used to provide a much better SOC estimate that just cell voltages alone.

What do you think? Thanks in advance for your thoughts - and thanks again for all of your posts here.

Chris

Few pics of our rovers here https://photos.app.goo.gl/2pcyJTWpAhDcW3AP9

 

[off.the.grid]

I've given up trying to be too technical on the matter.
I've seen a few commercially made packs and most are just strapped together with bands or the enclosures just has a plate that bolts up firmly.
You are better off to secure in place firmly with bolts and a heavy plate at each end that does not flex.
In other words, I'm going with restriction over compression, if you tighten too much you will do more damage.

 

[OffGridForGood]

Poron.

 

[fatjay]

I've given up trying to be too technical on the matter.
I've seen a few commercially made packs and most are just strapped together with bands or the enclosures just has a plate that bolts up firmly.
You are better off to secure in place firmly with bolts and a heavy plate at each end that does not flex.
In other words, I'm going with restriction over compression, if you tighten too much you will do more damage.

Commerical packs don't have to last as long as I want them to. I want the max life, they want whatever their warranty is. Usually 10 years. Manufacturer cycle count claims up to 20 is possible. I'm shooting for 20.

I followed the manufacturer specs. At 30% SOC, the pack is greater than 3000 newtons. At 100% SOC, the pack is below 7000 newtons. I've seen a few other folks use springs to the same avail. Because I'm double wide, I didn't trust plywood not to flex. Its easier for single's. If you look at the pics post in show and tell, there's plenty of folks that have accomplished this through various designs.

 

[OffGridForGood]

Because I'm double wide,

why didn't you put four in the centre instead of two?

 

[fatjay]

I think threaded rods without springs OR foam is a mistake. Granted I haven't been doing this for very long, I've watched hundreds of videos and studied lots of other people's specs, and have learned a lot. Expansion is going to happen. Ensuring even expansion instead of bloated bellies is the best way to ensure longevity.

 

[fatjay]

why didn't you put four in the centre instead of two?

I adjusted the spec for spring compression to 1.4x of the outer rods. It's still within spec, I have a fair amount of wiggle room with the springs I chose.

 

[robbob2112]

Commerical packs don't have to last as long as I want them to. I want the max life, they want whatever their warranty is. Usually 10 years. Manufacturer cycle count claims up to 20 is possible. I'm shooting for 20.

I followed the manufacturer specs. At 30% SOC, the pack is greater than 3000 newtons. At 100% SOC, the pack is below 7000 newtons. I've seen a few other folks use springs to the same avail. Because I'm double wide, I didn't trust plywood not to flex. Its easier for single's. If you look at the pics post in show and tell, there's plenty of folks that have accomplished this through various designs.

Thse class T fuses are not intended to be structural. If you do use them like that hanging in air I would secure the wire so they can't move if they blow and come apart.

 

[OffGridForGood]

I adjusted the spec for spring compression to 1.4x of the outer rods. It's still within spec, I have a fair amount of wiggle room with the springs I chose.

the load in the centre of the plate is double the load at the ends of the plate. you need x2 force per spring with only two rods in the centre.

 

[fatjay]

Thse class T fuses are not intended to be structural. If you do use them like that hanging in air I would secure the wire so they can't move if they blow and come apart.

Yes, I'm not done. There is no stress on them due to bends in the wire, but they will be secured in the final version.

the load in the centre of the plate is double the load at the ends of the plate. you need x2 force per spring with only two rods in the centre.

You have to take rigidity of the aluminum plate into account. 2x the compression at 30% would be to much at 100%.

 

[OffGridForGood]

we only need to know if the two rows of cells transmit equal force, expand equally together or not.

consider a single stack of cells (rather than two side by side) do we agree the single row would need equal springs and rods at each end of the plate spaning a single row of cells ?

Now lets consider two rows of single cells close together side by side, and separate plates - do we agree that between the two rows will be twice as many rods/springs?

lets consider another case - double row of cells but no springs in the centre - the plate is uniformily loaded by the expanding cells, but deflects (bends) as it transfers load to the ends. In your case you have some compression in the centre, but it is lowwer than the loads applied, so some bending moments are applied to the plate - not sure if you can see any deflection or not - pretty thick plate.
I was just curious with all the work you did, why not four in the centre - maybe too tight to fit.

 

[fatjay]

That's what I thought at first, too. Double the force on the inner two. and the first run ended up being concave. Then I reduced it to 1x, and the second was convex.

So I sent it to one of my engineers and he came back with 1.4x and a brief explanation of load being transferred over materials. Think about it this way, what if you had no rods in the middle, would you have zero clamping force in the middle? No, because the external bolts still exist, and there's a degree of deflection to account for pressure in the the middle still being applied.

 

[robbob2112]

What are your endplates made from? Metal? Thickness? Source?

 

[fatjay]

What are your endplates made from? Metal? Thickness? Source?

3/8" 6061 aluminum plate from midwest steel. I finished them myself.

 

[CVCPJared]

Commerical packs don't have to last as long as I want them to. I want the max life, they want whatever their warranty is. Usually 10 years. Manufacturer cycle count claims up to 20 is possible. I'm shooting for 20.

I followed the manufacturer specs. At 30% SOC, the pack is greater than 3000 newtons. At 100% SOC, the pack is below 7000 newtons. I've seen a few other folks use springs to the same avail. Because I'm double wide, I didn't trust plywood not to flex. Its easier for single's. If you look at the pics post in show and tell, there's plenty of folks that have accomplished this through various designs.

Looks good! What kind of springs did you use and how did you calculate/measure the compression force at 30/100%?

 

[fatjay]

Looks good! What kind of springs did you use and how did you calculate/measure the compression force at 30/100%?

Lifter springs out of a 2015 corvette LT4 engine.

The cells may expand up to 3%, so I took the measurement of 8 cells end to end and took 3% of that, which is ~0.65". The range is 3000-7000 newtons, or 675 to 1575lbs. For 4 bolts that's 175lb at min and 375lb at max per bolt. So I tightened it to 175lb and marked it. The tightened the bolt an additional 0.65" and found that the compression pressure was just over 300lb, well below the max limit.

 

[cjmssmd]

Lifter springs out of a 2015 corvette LT4 engine.

The cells may expand up to 3%, so I took the measurement of 8 cells end to end and took 3% of that, which is ~0.65". The range is 3000-7000 newtons, or 675 to 1575lbs. For 4 bolts that's 175lb at min and 375lb at max per bolt. So I tightened it to 175lb and marked it. The tightened the bolt an additional 0.65" and found that the compression pressure was just over 300lb, well below the max limit.

Thanks for this reply. I agree that these LFP cells expand and contract during normal operation. They swell at medium state of charges (SOCs) and shrink at low and high SOCs during discharge. Storing at 0% and 60C for 7 days causes a lot of swelling due to gas generation (so, don't do this!!). In a rigid structure the pack pressure is highest at 30-40% SOC

 

[S Davis]

I used 160lb die springs to get just below the 12psi recommended.

 

[fatjay]

Why are daly bms's so massive? My JBD's are microscopic compared to them?

 

[S Davis]

Why are daly bms's so massive? My JBD's are microscopic compared to them?

These are 250amp models.

 

[fatjay]

These are 250amp models.

Ah, can you get 250a out of 4s? I thought that was to much.

 

[S Davis]

Ah, can you get 250a out of 4s? I thought that was too much.

If everything is sized properly, these are EVE LF280N rated for a 1c discharge so technically I can go up to 280amps. I run two in parallel with a 3000va Victron.

 

[miran90]

@ghostwriter66 I have read through your posts with interest - thanks for posting here!!

I am building a lawn mowing robot with a 22.8kWh pack consisting of 30 LF230s (purchasing from DOCAN - I agree they are good) in a 2P x 15S configuration. We go pretty gentle on the cells, charging at 0.12C, discharge at 0.15C (peak discharge 0.4C for up to 10 minutes). We require the the pack to last 7 years and 1,750 cycles. The pack is installed in a custom-designed aluminum and steel rover. Currently our cells are insulated from each other and from the chassis with FR4/garolite and held in place with ratchet straps.

My question to you is, what is your thinking on the benefits of compression? The EVE specs state 300 kgf compression to achieve 2,500 cycles. Are you compressing your cells? Are you aware that anyone is? The PCBWay DIY Pack uses threaded rods. This teardown of a Tesla pack (and this one) using prismatic LFPs looks like they have some foam between the cells but the frame itself sure doesn't look like it is beefy enough to provide 300kgf (per cell; 10 cells side-by-side means 3000kgf!). Compressing with threaded rods seems pretty imprecise. These people observed that with a rigid frame the cell swelling & shrinking causes differences in pressures between cells - which can be used to provide a much better SOC estimate that just cell voltages alone.

What do you think? Thanks in advance for your thoughts - and thanks again for all of your posts here.

Chris

Few pics of our rovers here https://photos.app.goo.gl/2pcyJTWpAhDcW3AP9

LFP do not like too slow charging since it will overcharge them disconnect at 3.65 should be and disconnect should be 0.05c for 230ah will be 12A and 0.014c at 3.45 will be 3.5A. charge them with .2c at least

 

[robbob2112]

3/8" 6061 aluminum plate from midwest steel. I finished them myself.

I just ordered 4 x 1/2 plates to make 2 batteries ...

What size are your threaded rods?

 

[miran90]

check this out actually i don't have a mean to measure the compression, I have the lower shelf with too much compression that it bent 4mm steel XD.