The Question; Compression or not

[JanVJ]

As a DIY´er with too much spare time on my hand;
I am bulding a 12 VDC 320 Ah LiFePo4 based on 4 pcs 3,2 volt cells.

And the reason why would be to save some money.
My wild guess is that the cost will be aprox 1/3 of the price for a 300Ah ready made battery.

How ever; I would like a battery that looks "clean".
I dont want a battery with a look like a piece of something you find at the dumpster.

I am just out from this morning management project meeting about compression or not.
(Management consist of 1 person; ME...)
At this moment the vote went for NO compression.

- I believe compression would be kind of "finetuning". I dont have the tools to do that kind of accurate adjustments.
- Dont think my ready made of the shelf 12V 200 AH battery (bought for big bucks from a local store) got any compression (have not looked inside)
- Add lifetime? The seller claim 6000 cycle. That would be 16 years. In 16 years (2038) I probably got new hobbies.
- A set up with compression would add a lot of extra bits and pieces and most DIY solutions appear like a sputnik satellite.

Until I do get the "aureka" moment I will continue without compression.

What would you do?
And why?

The sun is back for the season.

 

[Bubba1]

What is going to hold the cells together? The bus bars and tape? With smaller cells compression is not as critical. If you ever need to move the battery you will need some sort of compression. The strain on the terminals if or when the cells swell could be enough to cause problems.

 

[John Frum]

What would you do?
And why?

I would not worry about adding life cycles with elaborate springs or foams.
I think there are more important considerations when assembling a DIY battery.

I would make a box or fixture frame to stop my cells from ripping their terminals out if they were to swell.
I would also put a very thin plastic insulator between the cells because the blue shrink wrap is thin and the aluminum underneath is connected to one of the terminals.
I forget which one.

I would do the all the terminal joinery meticulously.
That means cleaning all mating surfaces.
Carefully crimping and testing connectors.
Torquing fasteners.

I would wear eye protection and use insulated tools when working on the battery.
I would try to make a safe battery that is fit for its intended use.

 

[HRTKD]

I think you should replace the project management team. If these cells will go in a mobile they should be compressed, if only to eliminate the stress on the cell terminals. You don't need to high end tools to make a compression fixture. Put something together that is snug enough to hold the cells together when you lift the fixture, but not so tight that the cells are crushed. The cells are pretty strong so crushing them would require you to be really stupid when applying torque.

 

[JanVJ]

Thank you guys for your comments....

This is my first DIY LiFePo4 build ever.
Earlier experience is ready made batteries only.
I have ordered my 320 Ah cells, but not yet received any.

I guess the draw of A´s will be contributing to the swelling of my cells.
During a abnormal day there will be a charging at maybe 30A. (Max)
A typical continuous draw will be at 5A...
Every 3. or 5. day I will switch on my inverter.
The inverter I will limit to 1500 watts... for 5-10 minutes.. let say 15 minute
The inverter will be protected with a 150A fuse.
I would guess that them 150 watts would be the main contributor for swelling.
Your experience?

I am building my battery in tight fit a enclosure made from wooden sheets.
At this moment I got a 12 mm sheet at home.. so 12 mm it will be.
I will not use no treaded rods, springs or any other adjustment devices for compression.
The enclosure will be a tight fit, tight enough that the cells wont move.
Not compressed, but tight enough that any expansion will think twice.

From a number of DIY`ers we have seen that the cells might swell and retract again
For that reason I was hoping to get hold of the busbars that has a "flex zone"
This to protect the internal mechanics.
The cells will then be able to move slightly without causing any damage.
Sounds good?

Some suppliers provide data sheet with number of cycles.. or life time....
If we do some research; maybe we will find numbers for uncompressed... and compressed set ups.
Extreme case would be 1 cycle every day... then we are talking about years.
And if we are in the range of 10-15 years of a uncompressed set up
Would I need to extend the batterylife with 2 years?
For the time being I am thinking I will not bother.
Why? In 2037 I got new hobbies, there are new technologies available...

The single cell casing.... extra isolation or not
If the exterior of the cell is same potential as one of the connection points.. the poles..
(probably the negative...but then again... chineese design)
of course it would be smart to isolate them cells from each other.
Placed next to each other them cells will touch... if the factory exterior plastic film open..
The two layer of blue plastic film is not good enough?
Would not be fun to release all them fury A´s. - yeah, them safety goggles could come handy
Probably I will add extra insulation..... its easy

My DIY battery build will be installed in a trailor.
The trailor will be towed behind my motorhome.
Most of the time it will be towed in a developed part of the world.
In Europe we got good asphalt... (most of the time)
But of course... it will be vibrated.
But so would my of the shelf 12 V LiFePo4 battery bank in my motorhome.
Should I really worry?

Replacing management? You sounds like my wife


Another sunny day is emerging in northern Europe.

 

[Frick]

I think you should replace the project management team. If these cells will go in a mobile they should be compressed, if only to eliminate the stress on the cell terminals. You don't need to high end tools to make a compression fixture. Put something together that is snug enough to hold the cells together when you lift the fixture, but not so tight that the cells are crushed. The cells are pretty strong so crushing them would require you to be really stupid when applying torque.

Oh noes, did you just assume everyone here isn’t stupid? the “oh crap my battery caught fire“ threads are entertaining.

in particular, I would be careful about using threaded rods and nuts for compression and “gently snugging” them down. The force multiplier of nuts on threads can put a tremendous compression force without using a wrench. Now factor in the cells may crown like a slightly overfilled waterbed and the compression box is flat and you have another force multiplier of only contacting on a tiny patch in the center of the battery. Could be a 1000’s of lbs right there, or more, or less. Hard to say.

a solid plywood box, some wood screws holding it together, some plastic between the cells. Prob good. I went with neoprene between the cells as they are not a machined flat surface , but a landscape of “looks flat from 10’ away“. The wood protects from damage, the screws apply gentle compression, something between the cells for insulation and allow the uneven surfaces to mate. Do a good job on contacts and bussbars so you don’t have to be in there again and Don’t forget a nice cover to keep accidental discharges at bay.

 

[Zil]

It is awkward to call this compression. It should be considered restraint. If the cells swell much, they can not be compressed back to good.
With most LiFePo cells such as CALB with plastic cases. New unswelled cells must be restrained, prevented from swelling while being charged. I would not use any foam plastic between the cells as there is a question of movement as well as low ignition temperatures of the foam..
There is no fancy torque setting. There is a plate on the side of the battery, a plate on the other side, and 3 or more threaded rods on each side of the plates. I use 1/4 aluminum or SS plate. Bring the cells into firm contact between the plates, holding them together with the rods with nuts. Using Ny-Lock nuts everything will be fine, as far as restraint from swelling. It would not hurt to torque each nut to some shared value, as that is always "Good Practices". That can be done with the same inch/pound tool as you must use on the terminals.
Protection from short circuits is a different subject and must be prefect. Keep the metal below the top of the cell cases and covered with electrical insulation along the top if there is any possibility of negative and positive coming into contact.
That might be fun on the Fourth of July, Bastille Day, Or other High Holidays.

 

[HRTKD]

It is awkward to call this compression. It should be considered restraint.

I think the term "restraint" is reasonable. But that brings back memories of the special jacket the guys in the white coats may or may not have used on me.

 

[Diesel Pro]

If I were doing that project i would temporarily compress and then fiber tape them. I would use flexible busbars and build into a premade case that resembles a battery. If my project was not 8s that's what I'd have done. I'd probably consider 280ah cells as better cost per ah especially when 320ah is an overrating of a 304?ah cell. Docan should have US stock of 280's and a case that should fit 4 cells properly.

 

[robby]

Cells should not be compressed. Just make sure they are discharged down to a low state like 10% SOC and then restrain them in goups of four with wraps of fiber based tape or put them in a box that they fit in tightly. The idea is that when they charge up they cannot expand.

 

[JanVJ]

"Restriction" sounds reasonable,
and I believe I will achieve this by making a tight fit in a rigid 12 mm enclosure.
and from your comments I do this when the batteries are in a low charge.

Compression or applying any force does not sound like a good idea for me.

As mention earlier I already got two "off the shelf" 12 V 100 Ah LiFePo4 prismatic batteries with Bt and Heating elements.
We got a saying "you get what you pay for", and I payed top ?? for them batteries....
The word on the street between the one salesman is that the brand is best there is on the marked in our part of the world.
The exterior of the LiFePo4 batteries enclosure is the same as my AGM leisure battery.
Them LiFePo4´s are made to replace the GEL or AGM in a typical European motorhome or van.
I did fit both in parallel in my van.

I would be tempted to sacrifice one of them expensive batteries and open to look inside.
What do the "professionals" do inside their up scale batteries?
I would be very supriced if I found any treaded rods or similar.

Anyone opened a good quality battery.. just driven by curiosity?
What do the professionals do in their batteries?

 

[luckielab]

What do the professionals do in their batteries?

Have you seen @Will Prowse's YouTube channel where he opens them up and shows what's inside?

 

[JanVJ]

Have you seen @Will Prowse's YouTube channel where he opens them up and shows what's inside?

I will do ?

 

[kshopper2084]

If I were doing that project i would temporarily compress and then fiber tape them. I would use flexible busbars and build into a premade case that resembles a battery. If my project was not 8s that's what I'd have done. I'd probably consider 280ah cells as better cost per ah especially when 320ah is an overrating of a 304?ah cell. Docan should have US stock of 280's and a case that should fit 4 cells properly.

This is exactly what I've decided to for my 280Ah 12v mobile battery. Planning on using thin neoprene between the cells, temporarily compress with clamps and fiber tape together to secure them before ever charging them. Connections with flexible bus bars. Then it all goes into a regular plastic 12v battery box, or possibly an ammo case.

 

[Diesel Pro]

This is exactly what I've decided to for my 280Ah 12v mobile battery. Planning on using thin neoprene between the cells, temporarily compress with clamps and fiber tape together to secure them before ever charging them. Connections with flexible bus bars. Then it all goes into a regular plastic 12v battery box, or possibly an ammo case.

Use a board or something to spread the load. i put some small dents in the cases of mine where the rubber pads of my clamps made contact. I used shelf liner in double layers.

 

[kshopper2084]

Use a board or something to spread the load. i put some small dents in the cases of mine where the rubber pads of my clamps made contact. I used shelf liner in double layers.

That's good advice. Will do.

 

[OffGridInTheCity]

Here's a pretty detailed youtube discussing "compression". @LithiumSolar has made a serious effort to investigate various options/approaches and presents several things to think about and his conclusions so far. He's a detailed, trustworthy source of info in my experience / has my highest recommendation. Here's a key youtube - but he has others if you find this interesting.

 

[VideoPipeline]

When in doubt: Read the cell manufacturer's specs/guidelines & comply with them to the maximum extent practical.

Pages 2, 3 & 4 of the 3/23/2021 LF280K specs state "under 300±20kgf, 30%～40%SOC" & "Under the 300kgf clamp" but there are no similar entries without that force applied. It seems clear to me that the manufacturer intends for these cells to be constrained by something stiffer than their very thin aluminum casings.

Though not mentioned in this spec., previous specs indicated that these cells gain about 0.5mm thickness between 30%-40% SOC & 100% SOC. That's typical, and easy to accommodate, especially if the operational temperature is kept reasonably close to the specified 25°C.

It is simple to build a container of the appropriate size to incorporate foam or very soft rubber isolators of the appropriate thickness & stiffness* to provide '300kg clamp force' (which works out to only about 12 psi) fairly evenly over the large & not-perfectly-flat sides of the cells, as they expand & contract slightly, while leaving the terminal center-to-center spacing unchanged. These also provide some vibration isolation & shock absorption, which is important in vehicular applications.

Given that the manufacturer's specs indicate that at 25°C cycle life is ≥6000 but at 45°C cycle life is reduced to less than half that, temperature control may be a bigger priority if the the pack is going to be used in high temperature environment. If that requires active cooling between the cells, it might be necessary to use something besides foam or rubber to provide the 300kg clamp force.


* i.e.: so that the intended pressure of around 12 psi is maintained over the very small change in thickness caused by cell expansion.

 

[John Frum]

practical.

^

 

[noenegdod]

I find it really interesting how disconnected we can become from what practical is. What is practical for me is not necessarily practical for some one else. One persons skill set/intellect, finances, facilities, access to information and equipment may make one approach completely practical for them but absolutely unreasonable for another.
What is practical varies from one person to another. Do what you can.

 

[Frick]

The first time I built an engine as an apprentice, I wanted to Do all the hot mods to it. Bigger cylinders, cams, high volume oil pump, compression bump…

my mentor wouldn’t let me, even though I was more than able to execute it correctly. He said
”frost you learn to play by the rules, then you learn to break the rules”.
followed by a discussion of the manufacturer’s decisions to make it That way which is a series of compromises made by talented engineers. You can make different compromises, once you understand Why they did it the way they did.

compression is a factory recommendation, these are not Diy batteries, these are made for companies to build batteries from, including proper fixturing, bussing, safety measures and such. unless we begin to approach the knowledge of the engineers who are the intended clientele, we should simply follow the set forth protocols of best practices.

yes they work without compression, or wrapped in tape, but that is a compromise for simplicity and economy with different results than a properly engineered and built enclosure.

this is a roll your own kind of place, we share knowledge and do what we feel is the best compromise for our own use, at our own expense, at our own risk. My answer is different from your answer, because I chose different compromises. As noenegdod said above, do what you can. I Would add, do so carefully, with much thought and research.

 

[S Davis]

The first time I built an engine as an apprentice, I wanted to Do all the hot mods to it. Bigger cylinders, cams, high volume oil pump, compression bump…

my mentor wouldn’t let me, even though I was more than able to execute it correctly. He said
”frost you learn to play by the rules, then you learn to break the rules”.
followed by a discussion of the manufacturer’s decisions to make it That way which is a series of compromises made by talented engineers. You can make different compromises, once you understand Why they did it the way they did.

compression is a factory recommendation, these are not Diy batteries, these are made for companies to build batteries from, including proper fixturing, bussing, safety measures and such. unless we begin to approach the knowledge of the engineers who are the intended clientele, we should simply follow the set forth protocols of best practices.

yes they work without compression, or wrapped in tape, but that is a compromise for simplicity and economy with different results than a properly engineered and built enclosure.

this is a roll your own kind of place, we share knowledge and do what we feel is the best compromise for our own use, at our own expense, at our own risk. My answer is different from your answer, because I chose different compromises. As noenegdod said above, do what you can. I Would add, do so carefully, with much thought and research.

I don’t think enough people building these batteries have a good appreciation for how much energy these larger cells hold, and how bad things can get in a hurry.

Now put them in a mobile application and your chances of issues goes way up, these cells have a lot of mass and will move around in a mobile application. Add expansion on top of that and the possibility of putting too much pressure on the cells when they expand in a fixture that is too ridged.

I decided for me using die springs I could get the most clamping force I could to restrain the cells without too much pressure when fully charged. I am using four cells so 160lb die springs are perfect to get me just below 12psi and not go over the 17psi max when fully charged.

My system is a little different I will have twelve 12 volt 280ah four cell batteries, this is a combination stationary/mobile application. All the batteries mount the same on vibration dampers. I have two on my work truck with 100amp charging circuit and will have a cradle for 8 stationary in my garage. They will all be built the same and be easy to move between the truck and house if needed.

The truck will have one Outback VFXR 2800 watt inverter and the house will get two of them, the system should be pretty resilient and can absorb some failures and still be functional.

 

[JanVJ]

Since I was the one that raised the question in this tread;
Compression or not?

I have been watching some disassembly videos;
Videos showing new batteries...taken apart just to look inside;
Have yet to see treaded rods or other components for compression purpose.
Most probably I have not seen all them videos, but I have seen a few.
Seen no rods.. no springs.. no nuts.. no nada

Yes I do know that some manufacturers got some lines about compression in their datasheets.
But then again; others dont. Some of them dont now the meaning of the word compression.

Me on mounting batteries in a moving vehicle;
I already got two factory batteries moving around in my van.
Van moving... and batteries moving and shaking with it.
Them batteries are mounted under one of the seats.
If any of them expensive batteries do funny shit; JUMP!
Good insurance is a key word.
But my experience so far is that I can trust them batteries.

My DIY home brew will probably be in mounted on a trailor.
(I say probably because every now and then new ideas emerg)
More specific it will be mounted in a protective aluminum enclosure (truck bed tool box)
Inside there will be a tight fit box made from 12 mm wood. (1/2")
The final box will be in the front of the trailor, near the trailor hitch.
If any issues lets hope its possible to unhitch the trailor from the van... in a hurry.

Have faith in the many brothers and sisters out there.
Might be many of them got a lot of skills related to wires and amps.
People who buy a set of LiFePo4 battery cells most probably got some basic skills.
I do believe they got enough skills to know that them blue boxes could be som fury bastards if provoked.
I do believe they know the importance of thickness of wires and suitable fuses.. and were to place them fuses.
I do have faith in most of them DIY´ers playing with LiFePo4 cells.


Another beautiful sunny moring in northern Europe.

 

[mdatot]

@RCinFLA made a very informative post in one of the big threads about compression that's worth highlighting, I think: https://diysolarforum.com/threads/e...spaced-for-expansion.7892/page-37#post-413461

Essentially (and assuming I'm understanding him correctly), the reason you'd want to compress large prismatic cells (like the ubiquitous EVE 280Ah ones) is that when you run high currents through them - from about 0.5C and up, say - they get hot, and when they get hot you start risking delamination. Compression is intended to prevent that. However, the cells also "breathe", they get a bit thicker when they are at a high state of charge and thinner when they are discharged, but this doesn't happen evenly over the entire side of the cell. So, compression also risks creating very high pressure spots which can damage the cells. So, if you're not running high currents (and you probably shouldn't be, with these cells) then you really don't need compression.

 

[Rednecktek]

Mind if I jump in with some questions?

I'm doing the research right now to build my first LiFe pack, I have visions of a 8s 24v 280Ah setup. My goal is to replace the 6 FLA's my cabin currently uses due to weeks on end of below freezing weather. Here's my "plan" in my noodle:

"Coleman" Cooler for the housing
8x 280Ah EVE cells
Daly 24v 200a or JBD 7s-20s 200a BMS, not sure which yet

4x 7w 12v heating pads (Left side, mid pack, right side, bottom. I'll probably throw an Aluminum plate in the bottom just to help the heat spread evenly
12v thermostat, on at 3c, off at 6c?
120Ah FLA battery to power warming system (because I have a few already) on its own 400w panel/MPPT setup independent of the LiFe array
Fuses, wires, crimps

2x 80mm 12v fans, intake and exhaust on another 12v thermostat for when it gets hot. On at 30c, off at 25c?

So, my question comes back to compression/retention/holding it all together. As this will be at my cabin there's no vibration to worry about (unless a volcano erupts, but then I got bigger issues ) and I'm trying to find the simplest way of sticking this all in the box. I'm not sure if I can just tape the cells together for installation ease and leave it, if I need to put some mouse pads between the cells, or if there need to be steel plates and threaded rods installed? All of Will's videos he just uses the yellow tape to strap everything together, and then we've got threads like this that talk about Inch/Pounds and KiloNewtons (which I understand aren't a delicious treat) and such.

Yeah, A bit lost here guys...

Since this battery is going to be going through pretty significant temperature swings between summer and winter, and it'll be unattended 50 weeks a year, what do I really need VS what would be scientifically perfect in an optimum world?