Xuba Electronics: DEAL - 280AH LiFePo4 cells. Purchase & Review

[Jejochen]

I asked Amy about the fixing. They didnt know the details and reached out to eve, but also did not get a reply.

 

[Gazoo]

I would disagree. The clearance between the roll (or accordion stack) and the case is negligible.

Then the only way would be to apply compression to the center of the case since the aluminum is thin and the edges of the case is much thicker....right?

 

[ArthurEld]

I thought of that some time ago. If I still had access to Poly banding equipment with the plastic corner protection, I would use it in two places. Ideal would be the type that uses heat to fuse the strap together.

Why not use ratchet straps. It would look ok if the ratchet part was hidden.

 

[Luthj]

Then the only way would be to apply compression to the center of the case since the aluminum is thin and the edges of the case is much thicker....right?

Ideally a stiff flat plate would work. With just the edges, like with a strap, would leave the end cells with much less compression.

The case walls appear to be the same thickness throughout.

 

[Sgt Raven]

Why not use ratchet straps. It would look ok if the ratchet part was hidden.

To bulky. Plus I'd need to find corner protectors. I could use a pocket screw jig and plywood to built a container. Clamp the pieces in place and screw it together.

 

[Gazoo]

Ideally a stiff flat plate would work. With just the edges, like with a strap, would leave the end cells with much less compression.

The case walls appear to be the same thickness throughout.

So there is nothing wrong with the way I plan to compress using thick plywood and threaded rods? I am not going to use straps.

EDIT: I said edges were thicker in my previous post. Sorry. I meant the sides (width) are thicker.

 

[ArthurEld]

Just like everything else, I keep looking at what others are doing. I will make up my mind at the last minute when it's time to get the parts together.
My batteries are a month away.

 

[Gazoo]

Just like everything else, I keep looking at what others are doing. I will make up my mind at the last minute when it's time to get the parts together.
My batteries are a month away.

Yeah same here except for the cell compression...I believe I have that one figured out. As far as everything else......

 

[ArthurEld]

I've seen some threaded rod ideas that looked pretty good. I'd like to have a way to hold 4 together for moving but that I don't have to remove when I put 16 together.

 

[Luthj]

So there is nothing wrong with the way I plan to compress using thick plywood and threaded rods? I am not going to use straps.

Perfectly acceptable in my option.

 

[Luthj]

Doing a bit of rough math.

Assuming the following
4 cell stack

• 0.5mm expansion per cell, so 2mm stack growth during charging from 30% SOC
• clamped with a rigid end plate (thick aluminum etc)
• 72mm nominal cell thickness for a total stack thickness of 11.3"
• clamped with 4 1/4" steel threaded rods

When charging, assuming the cells expand 0.5mm, the rods will need to elastically stretch 2mm, or 0.078". Since each rods acts like a spring we can calculate the clamp load assuming zero clamp load at 30% SOC.

Stress area of 1/4 rod is 0.031 in2. Steels elastic modulus is 29,000,000psi. Calculating for Krod is (0.031*29,000,000)/11.3 = 79,557lbs/in.

So 4 * 79,557 * 0.078 = 24,821lbs.

Of course this assumes that the cells MUST expand 0.5mm each during charging, which I have not verified, and I doubt they will if constrained with a "300kgf fixture".

But it does pose the question if its best to clamp cells at low SOC. I would personally just clamp when fully charged, which is typically the cell state after a top balance. Then verify cell security after a discharge.

 

[Gazoo]

But it does pose the question if its best to clamp cells at low SOC. I would personally just clamp when fully charged, which is typically the cell state after a top balance. Then verify cell security after a discharge.

Someone did that and found his pack was loose after discharge. I don't know how many have noticed expanded cells after top balancing. But yeah I understand your point and it is another advantage of using threaded rods as they can be easily adjusted.

 

[Ampster]

I think a plate or piece of plywood would apply pressure to the middle of the cell more effectively. That is what have done.

 

[Luthj]

One option is to use a firm closed cell foam rubber (say 1/4" thick) under one or both end plates. They are available in 10-20psi for 25% compression.

 

[Sharky722]

Gents could I get your opinions please.
My cells arrived and I arranged in parallel and charged/top balanced up to 3.65v and down to .5amp using a 30v10a psu.

I numbered all the cells 1 through to 8. Using the 150w load tester @20a I began capacity testing all the cells down to 2.5v cut off. They all came home at around 282-4ah with the exception of cell number 7 it came in at 275ah.

The cells were charged/top balanced again then arranged in 2p4s configuration , bms (jbd) connected this time for testing. The 150w load tester reconnected @10a and the test begins.

Towards the end of the test, series #1 which contains cells #8 and #7 starts to drift away from the rest of the series around 320mV and cuts off the bms @2.5v as per attached pic.
The whole pack came home @564ah on bms shut down.

Could this be an indication of a crook cell (#7)? Would it be worth separating this cell and “exercising” it a few times to see if it gets a bit better?

I’m not to fussed about it because I can just adjust the bottom cut off up a bit and sacrifice a few ah because I’ll never use 100% of the ah anyway.

During the sales video Amy sent there was a cell "caught my eye" as being a little off from the rest of them, I wonder if this is that cell.
What are your thoughts lads?

 

[Sgt Raven]

Gents could I get your opinions please.
My cells arrived and I arranged in parallel and charged/top balanced up to 3.65v and down to .5amp using a 30v10a psu.

I numbered all the cells 1 through to 8. Using the 150w load tester @20a I began capacity testing all the cells down to 2.5v cut off. They all came home at around 282-4ah with the exception of cell number 7 it came in at 275ah.

The cells were charged/top balanced again then arranged in 2p4s configuration , bms (jbd) connected this time for testing. The 150w load tester reconnected @10a and the test begins.

Towards the end of the test, series #1 which contains cells #8 and #7 starts to drift away from the rest of the series around 320mV and cuts off the bms @2.5v as per attached pic.
The whole pack came home @564ah on bms shut down.

Could this be an indication of a crook cell (#7)? Would it be worth separating this cell and “exercising” it a few times to see if it gets a bit better?

I’m not to fussed about it because I can just adjust the bottom cut off up a bit and sacrifice a few ah because I’ll never use 100% of the ah anyway.

During the sales video Amy sent there was a cell "caught my eye" as being a little off from the rest of them, I wonder if this is that cell.
What are your thoughts lads?

I have my BMS set to cut off at 2.875V and 3.5V per cell.

 

[Luthj]

Your capacity seems fine. There will always be one cell the hits bottom first.

 

[Gazoo]

During the sales video Amy sent there was a cell "caught my eye" as being a little off from the rest of them, I wonder if this is that cell.

What was different? The voltage, internal resistance, or both?

Could this be an indication of a crook cell (#7)? Would it be worth separating this cell and “exercising” it a few times to see if it gets a bit better?

I would exercise it in the pack like it is. No need to separate it.

I’m not to fussed about it because I can just adjust the bottom cut off up a bit and sacrifice a few ah because I’ll never use 100% of the ah anyway.

I wouldn't worry about it either. I have 8 cells coming and will never get to use the full capacity unless we have a major power outage.

I have my BMS set to cut off at 2.875V and 3.5V per cell.

Those are good settings and I am going to do the same.

 

[Sgt Raven]

Those are good settings and I am going to do the same.

It's not like they are written in stone. If I felt I needed to change them because of a situation I was in, then it's only a few clicks away.. That is the beauty of building my own battery and the smart Bluetooth BMS, I have complete control over it.

 

[Sharky722]

What was different? The voltage, internal resistance, or both?



I would exercise it in the pack like it is. No need to separate it.



I wouldn't worry about it either. I have 8 cells coming and will never get to use the full capacity unless we have a major power outage.



Those are good settings and I am going to do the same.

The resistance on 7 cells was .11-.12 then 1 cell was .15