Compressing cells in your battery builds is questioned in this video.

[Deleted member 23531]

That said, I would not be surprised there will be a shortage of raw materials due to increasing demand and the price might even go up.

Can you think of any situation in history where that has happened for a technology product over 10 years? (this is not a rhetorical question, I actually am asking). I think if it is not cheap it will prolong the switch from natural gas, or whatever the status quo is, to battery + renewables. But I think market forces would keep prices down and just demand would be tamped down if it is not significantly cheaper.

As far as I have followed raw material markets, which is to say casually in the news and not seriously like an investor would, shortages are really transitory - a couple years to open up a new mine or whatever if it's expected to be profitable. I think the year over year capacity improvements per dollar would offset any increase in demand or shortage in supply (meaning the cells in your garage aren't appreciating in value!) but that is obviously just a hunch. What do you guys think?

 

[ArthurEld]

And another thing.

The whole world is concentrating on making cheaper more efficient batteries and solar panels. I think it is going to happen soon.

 

[DerpsyDoodler]

Can you think of any situation in history where that has happened for a technology product over 10 years? (this is not a rhetorical question, I actually am asking). I think if it is not cheap it will prolong the switch from natural gas, or whatever the status quo is, to battery + renewables. But I think market forces would keep prices down and just demand would be tamped down if it is not significantly cheaper.

As far as I have followed raw material markets, which is to say casually in the news and not seriously like an investor would, shortages are really transitory - a couple years to open up a new mine or whatever if it's expected to be profitable. I think the year over year capacity improvements per dollar would offset any increase in demand or shortage in supply (meaning the cells in your garage aren't appreciating in value!) but that is obviously just a hunch. What do you guys think?

Whether market value goes up or not, my cells will (presumably) continue to provide their rated power for their shelf-life. By the time they need to be replaced, I am hoping new or better technology will be available at better prices. That said, it’s pretty reasonable to assume I will continue to grow my energy storage capacity over time (at least that’s the plan). I guess what I am saying is market value only matters when you’re looking to purchase or sell.

 

[ArthurEld]

Back to the fixture

One dangerous issue I see with these batteries is terminal threads failing.
Solid busbars often cause the threads to fail because of side pressure.

The side pressure can come from cells expanding or from adjusting the fixture.

Flexible busbars eliminate both issues.

If you use a fixture with solid busbars you need to understand how much damage side pressure can do.

If you don't use a fixture the side pressure isn't usually an issue. The cells are already spaced loosely when the busbars are put on. And they can rotate a little if needed to reduce the side pressure on the stud.

 

[Bob B]

I like the fixture with threaded rod. But there are so many problems caused by using the fixture improperly that I have my doubts about whether it is worth learning.
People need to concentrate on making a safe battery and the fixture seems like a distraction.

Curious what kind of problems you have heard people are having with a compression fixture?

 

[Bob B]

Back to the fixture

One dangerous issue I see with these batteries is terminal threads failing.
Solid busbars often cause the threads to fail because of side pressure.

The side pressure can come from cells expanding or from adjusting the fixture.

Flexible busbars eliminate both issues.

If you use a fixture with solid busbars you need to understand how much damage side pressure can do.

If you don't use a fixture the side pressure isn't usually an issue. The cells are already spaced loosely when the busbars are put on. And they can rotate a little if needed to reduce the side pressure on the stud.

All of the thread failures I have heard of are during the initial construction of the battery .... I haven't heard any reports of thread damage from side pressure from a fixture. The pressure is actually going to reduce the expansion and contraction which puts pressure on the terminals.

That being said .... I am going to be re-configuring my pack soon and will be installing braided bus bars.

 

[ArthurEld]

Curious what kind of problems you have heard people are having with a compression fixture?

I explained it in my last post.
If the fixture is adjusted when the busbars are tight there is side pressure on the stud.
That makes the terminal threads loose and unsafe

 

[ArthurEld]

All of the thread failures I have heard of are during the initial construction of the battery .... I haven't heard any reports of thread damage from side pressure from a fixture. The pressure is actually going to reduce the expansion and contraction which puts pressure on the terminals.

You're hearing it now because I was a dumb ass and adjusted my fixture with solid busbars. Then I started stripping threads.
And I realized my battery was no longer safe.

 

[Bob B]

You're hearing it now because I was a dumb ass and adjusted my fixture with solid busbars. Then I started stripping threads.
And I realized my battery was no longer safe.

If you think that damaged threads are from the fixture pressure ... then your case is the only one I have heard of.

 

[ArthurEld]

If you think that damaged threads are from the fixture pressure ... then your case is the only one I have heard of.

That's the problem. I assume that I'm not the only fool. So other peoples batteries have loose connections from bad threads or will have.

 

[Terrapin]

You're hearing it now because I was a dumb ass and adjusted my fixture with solid busbars. Then I started stripping threads.
And I realized my battery was no longer safe.

This I can't afford to gamble on.

 

[Q-Dog]

Any time you move the cells for any reason remove all of the bus bars first. That seems like common sense to me, but maybe not. And don't tighten down the bus bars until the cells are firmly secured in their final location. This stuff isn't that hard.

 

[JRG]

If you think that damaged threads are from the fixture pressure ... then your case is the only one I have heard of.

It could be that long-term fatigue failures haven't had time to surface yet from battery enclosures that allow movement (spring design) with solid bus bars.

 

[ArthurEld]

Any time you move the cells for any reason remove all of the bus bars first. That seems like common sense to me, but maybe not. And don't tighten down the bus bars until the cells are firmly secured in their final location. This stuff isn't that hard.

Some things aren't so obvious when you have a lot of things on your mind.
And hind sight is always 20/20
But maybe I am the only fool. I'd still rather be more fool proof in case another fool comes along some day.

 

[ArthurEld]

After realizing how much the cells move I started only adjusting my fixture when the cells are about 3.4V and with busbars loose.
That is what I came up with. I'd like to hear what others are doing about the side pressure from solid busbars on the terminal threads.

 

[JRG]

If you only consider the compression variable, the 80% capacity of the battery is increased from 2500 with no compression to 4000 with compression.

From what we know, however, this was based on extrapolated test data, where the manufacturer tested the cells to ~750-800 cycles and essentially "guessed" what 2500-4000 cycles would look like. They might have guessed correctly or, worst case, they guess poorly and compressing the cells actually diminishes cell life. We just don't know until someone does a proper test all the way to cell "end-of life".

 

[Bob B]

It could be that long-term fatigue failures haven't had time to surface yet from battery enclosures that allow movement (spring design) with solid bus bars.

That is possible .... but if the cells are compressed, there will be less movement than if they aren't.

Everyone I have seen with stripped threads is during the initial tightening of the bus bar connection. Many are reconfiguring the battery multiple times in order to parallel top balance .... and then again to put them in series. The threads don't handle repeated installation and removal very well.
If the threads strip while tightening things down .... that has nothing to do with whether or not the cells are compressed.

It has pretty much been determined that 35 in/lbs or 4 nm is the ideal torque and even at that torque some have stripped the threads .... probably a lot of inaccurate torque wrenches ... and the majority probably aren't using a torque wrench.

 

[Bob B]

From what we know, however, this was based on extrapolated test data, where the manufacturer tested the cells to ~750-800 cycles and essentially "guessed" what 2500-4000 cycles would look like. They might have guessed correctly or, worst case, they guess poorly and compressing the cells actually diminishes cell life. We just don't know until someone does a proper test all the way to cell "end-of life".

Well .... these charts are in the mfg spec sheet .... so choose to believe what their information or not ... it seems to be the data we need to use.

Where did you get the info that the data is extrapolated from 750-800 cycles?

 

[Q-Dog]

I figure all of this is mostly guesswork from the manufacturers at this point. How can anyone say how long these cells will last when they are still so new in their present form?

I looked at as many photos as I could find of the insides of factory built LiFePo4 batteries and can't recall any with a bunch of threaded rods and springs and other contraptions inside the cases. What I did see is tight-fitting metal or plastic boxes with hard foam packing or glue and an occasional metal or plastic bar screwed down to keep the cells from moving inside the case. So, I made a wood hold-down frame that constrained my cells, but did not compress them. I took the dimensions of my frame, and did the installation, when the cells were at low state of charge.

I haven't stripped any threads. I haven't used any lock tight or JB weld. I noticed during the first charge after putting my battery together, the cells were swelling up and the bottom of the cells spreading apart because the tops were held together by the bus bars, so I stopped charging and made my frame tighter. I have noticed no movement or swelling since I screwed down the tighter holder. If I get longer cell life, great. If I am replacing cells after only 7 or 8 years I hope whatever replaces them has a lot more capacity and is even cheaper.

 

[Bob B]

Sorry, but it seems that most of the comments against using compression are from those who already completed their build and don't want to accept the new EVE spec sheet data.

They updated the spec sheet after they had more time to accumulate test data .... so .... believe the data or not.