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

[ArthurEld]

How about interpret the data how you want. That way everyone can believe it.

 

[DerpsyDoodler]

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.

I think the whole compression thing has been taken way out of proportion, along with other aspects of DIYing these battery packs. I will build a fixture that will hold the cells snug. When I tighten my threaded rods, I will do so with batteries anywhere from 1/2 to full charge, and I will make it “snug”. The fixture will have a bottom so compression will not “hold” them in place, a Teflon sheet will line in between the cells, and I will be using flexible 1/0 (or larger) welding cable for my bus bars. Furthermore, I will be securing my terminal studs into the terminals with jbweld on their first insert.

Taking these precautions leads me to believe I should be able to tighten nuts down on these terminals up to 35psi (even 40) with little to no concern (not that I plan on tweaking and twerking these things much really). Seriously, it’s not rocket science.

 

[Bob B]

I think the whole compression thing has been taken way out of proportion, along with other aspects of DIYing these battery packs. I will build a fixture that will hold the cells snug. When I tighten my threaded rods, I will do so with batteries anywhere from 1/2 to full charge, and I will make it “snug”. The fixture will have a bottom so compression will not “hold” them in place, a Teflon sheet will line in between the cells, and I will be using flexible 1/0 (or larger) welding cable for my bus bars. Furthermore, I will be securing my terminal studs into the terminals with jbweld on their first insert.

Taking these precautions leads me to believe I should be able to tighten nuts down on these terminals up to 35psi (even 40) with little to no concern (not that I plan on tweaking and twerking these things much really). Seriously, it’s not rocket science.

I don't see how you think things are taken out of proportion when those of us who think controlling the compression is a good idea is based on the latest data from the manufacturer. It is the MANUFACTURER who is recommending compression at specific levels to increase cycle life.

Everyone has to decide for themselves how good is good enough. If you are happy with the cycle life without the controlled compression ..... that's fine with me.
I just don't understand the need for some to deny that data.

 

[JRG]

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.

I hope I don't come off as beating up on you, I'm just questioning the overall thoughts of the group (it's healthy to have some skepticism, right?)

There's no sufficient test data that determined 35 in/lbs is "ideal". This was just an "accepted" value by the forum members based on bolt torque charts. I'm not saying it's wrong, but I would say it's wrong to use the word "ideal" without proper testing using these cells. I'm actually using this value myself as I feel it is reasonable.

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.

I haven't yet built my packs. I haven't even charged/discharged mine. I have nothing to lose in choosing a "side". As an engineer with a love for science, I just seek out what's true.

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

From the graph found in the manufacture's spec sheet. Same one you mentioned.

 

[JRG]

I just don't understand the need for some to deny that data.

Because it was grossly extrapolated!

 

[DerpsyDoodler]

I don't see how you think things are taken out of proportion when those of us who think controlling the compression is a good idea is based on the latest data from the manufacturer. It is the MANUFACTURER who is recommending compression at specific levels to increase cycle life.

Everyone has to decide for themselves how good is good enough. If you are happy with the cycle life without the controlled compression ..... that's fine with me.
I just don't understand the need for some to deny that data.

I’m not denying any data. Just saying the extent to which it is being taken is absolutely out of proportion.

 

[Bob B]

Because it was grossly extrapolated!

Well .... the new spec sheet is a year newer than the original .... so, seems like it is based on more data than the original.

 

[Bob B]

I’m not denying any data. Just saying the extent to which it is being taken is absolutely out of proportion.

The manufacturer is showing that they expect a 60% increase in cycle life by using the optimum compression ..... That's pretty distinct to me .... so, what is out of proportion?

 

[DerpsyDoodler]

Well .... the new spec sheet is a year newer than the original .... so, seems like it is based on more data than the original.

I would hope they have a long term open-ended experiment going on with this.

 

[Bob B]

I would hope they have a long term open-ended experiment going on with this.

Guess we will see if they upgrade their specs in another year.

 

[DerpsyDoodler]

The manufacturer is showing that they expect a 60% increase in cycle life by using the optimum compression ..... That's pretty distinct to me .... so, what is out of proportion?

Compression washers, springs. I bet my fixture will apply 10-12psi just by doing what I say i’m going to do.

 

[Bob B]

Compression washers, springs. I bet my fixture will apply 10-12psi just by doing what I say i’m going to do.

I'll bet the compression on your cells will vary pretty radically based on SOC and maybe even temperature .... and you will have no idea what level of pressure you have.

 

[DerpsyDoodler]

climate controlled room temp or below. And I don’t expect they will be as touchy as you imagine. Perhaps if you’re charging and discharging at high rates, but it seems excessive for solar.

 

[Bob B]

climate controlled room temp or below. And I don’t expect they will be as touchy as you imagine. Perhaps if you’re charging and discharging at high rates, but it seems excessive for solar.

Those who have actually done testing have found that each cell will expand and contract about .5mm from full to empty. So .... if you have 4 in series that will be about 2 mm .... 8 cells, 4 mm total. If the cells are just in a "tight" fixture, it probably needs to be tightened at full SOC to prevent a possible radical increase in pressure. And, that expansion is not the same from the top to the bottom of the cell.

If you don't really know what the pressure is even when you first tighten it up .... how are you going to know how the pressure will vary without some way to control it.

 

[Metalophile]

Is there a consensus that the swelling is caused by a breakdown of electrolyte and that breakdown of electrolyte is correlated with increased voltage? In that case it might be better to set the BMS cutoff voltage to a lower value - 3.4 or 3.45 volts so that the cells never see a higher voltage. I know that reducing cycle extremes is supposed to result in a longer cycle lifetime, but could this also be used to prevent swelling? That said, I plan to check on my pack next time it is near the top, disconnect everything and loosen and tighten the bus bars at the top of the cycle. Oh yeah, I just ordered a smoke alarm for my garage where my battery is.

 

[Bob B]

Is there a consensus that the swelling is caused by a breakdown of electrolyte and that breakdown of electrolyte is correlated with increased voltage? In that case it might be better to set the BMS cutoff voltage to a lower value - 3.4 or 3.45 volts so that the cells never see a higher voltage. I know that reducing cycle extremes is supposed to result in a longer cycle lifetime, but could this also be used to prevent swelling? That said, I plan to check on my pack next time it is near the top, disconnect everything and loosen and tighten the bus bars at the top of the cycle. Oh yeah, I just ordered a smoke alarm for my garage where my battery is.

You'd probably get a double benefit from reducing the range .... but I don't know if the expansion is linear across the SOC range .... @cinergi did some testing and may have more detailed information.

 

[DerpsyDoodler]

Those who have actually done testing have found that each cell will expand and contract about .5mm from full to empty. So .... if you have 4 in series that will be about 2 mm .... 8 cells, 4 mm total. If the cells are just in a "tight" fixture, it probably needs to be tightened at full SOC to prevent a possible radical increase in pressure. And, that expansion is not the same from the top to the bottom of the cell.

If you don't really know what the pressure is even when you first tighten it up .... how are you going to know how the pressure will vary without some way to control it.

At what C rate?

 

[Bob B]

At what C rate?

I don't recall .... but that may be available on another thread. I don't even know if it is the amount of expansion varies with C rate. I also don't know what happens if the cell is in a fixture that won't accommodate expansion ... it just seems like the pressure could get way too high if the cells want to expand and can't.
It is shown in a different EVE chart and from direct information from an EVE engineer that any pressure over 17 PSI is worse than no pressure in terms of cycle life .... so, if nothing else is done, it seems like it would be prudent to make absolutely sure your cells never see too much pressure.

 

[HRTKD]

I have two 4s batteries in my RV, which I've had down some washboard dirt roads. I rechecked all my screws (OEM, from supplier), bus bars (OEM from supplier) and cable lugs at the end of the camping season and everything was still nice and tight. My compression frame was installed in the garage with the batteries at a resting voltage. I then carried the batteries to the RV, using the threaded rods as a handle. The compression frame is on the ends of the batteries, with nothing under the cells. I've seen no movement of the cells. No hotspots on the bus bars.

The only thing I would change on my configuration is to use thin, double sided tape to provide additional insurance against cell shifting.

 

[Gazoo]

At what C rate?

In my case, .05C. And cell expansion is very noticeable with a 50% SOC. As has been reported by others my cells also expand mostly at the bottom. I know this is causing stress on the terminals but how much stress I don't know. I will take care of it.

I have gained much respect for those choosing to get the most life from their cells by using springs. In the beginning I thought it was silly. When the EVE engineer answered questions regarding cell compression the reasoning behind it became clear to me. Bob B came up with the idea even before EVE answered questions.

I have chosen not to go this route. My cells are being used for a UPS and they will age before I can run 2000 cycles from them.