LiFePO4 Compression

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LiFePO4 Compression

LiFePO4 Compression Jan. 04, 2024

LiFePO4 Compression

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LiFePO4 Cell Compression

What is Cell Compression?

Cell compression is quite literally squeezing/fixturing the outer aluminum case of LFP (LiFePO4) cells to prevent delamination of the internal components.  Delamination leads to a reduced lifespan of the cells, as well as additional stress on electrical terminals. Internal forces are constantly acting on a thin aluminum case that is not strong enough to resist these forces.  Lack of compression leads to damage of the cells, as indicated by swelling and premature battery failure.


Why is Cell Compression Needed?

With the latest technology of LFP cells, many advances have been made to increase how much energy can be stored in a given volume.  Engineers in general regularly face the challenge of fitting “more” into “less”.  Sometimes to accomplish such a challenging task,  compromises must be made.  In this case, to create more space, a thinner, less structurally integral material has been chosen.

The thin aluminum case cannot withstand the internal forces exerted when the cells are charged, but the new design never expected the cells to be used on their own.  The greater majority of these LiFePO4 cells on the market today are originally intended for use in Electric Vehicles (EV’s).  Automakers and pack manufacturers are already designing the packs with the compression in mind, because the demands of a solidly held pack in an electric vehicle where vibration is guranteed are much more intense than a stationary solar power system.  By using a single compression fixture on an entire pack as compared to many thick plastic cases, significant space and weight is saved, leading to higher capacities in the same space.

When is Cell Compression Needed?

More times than not, cells will require compression. 95% of all aluminum cased LFP cells on the market require compression.  While the exact wording varies, most high-quality cell manufactures will make note in their data sheets.

Aluminum case batteries that do not require compression do in-fact exist, however they are rare.  Asking the vendor you are purchasing the cells from is usually a good idea, however not all vendors are fully educated with regard to the engineering considerations of each and every battery and manufacturer requirements.

How to Build a Cell Compression Fixture

Cell compression fixtures will vary extensively.  Every application is different, for example a large array of cells in an electric vehicle will have many more cells to fixture, than a simple 4 cell pack used in an RV or camper.  Advanced builds like this will have far more details than we can cover in this article.  Some fixtures take advantage of plastic or metal banding, while others utilize threaded rods and end plates.  Care must be taken in selecting materials that are able to resist the forces exerted by the cells.


A simple cell compression fixture that is sufficient for most stationary packs used in solar and off-grid applications can be built using some simple materials sourced at a local hardware store.  In the video shown here, you can learn our recommended procedure for building a compression fixture that will provide enough compression to get you started, and prevent premature degradation.  This design is not perfect for every application, but can be used as a starting point.  Members of the DIY community like this design due to its simplicity, however, other members have added coil springs, additional reinforcement, and many other methods to create a smoother and more even compression fixture..

Important Considerations

Cells will swell if they are charged over 3.4V regardless of the rate of charge.

Cell-swelling is based on state of charge, not rate of charge.  Once a battery swells, it’s permanent damage that will make your pack a headache for the rest of its life.  When you are preparing your battery pack for use, you may choose to top-balance your cells.  When preforming this process, you may think that you can opt out of compression since it’s a slow rate of charge.  This thought process is a MISTAKE and will cause major difficulty for your cells.  Swollen cells that are re-compressed apply immense pressure to the outer layers of plates, where keeping an already-flat cell still flat applies even pressure to all plates throughout the battery.


 

LiFePo4 battery pack questions



The plan after top balancing them is to build a 2p16s pack to feed my inverters with.  some of the questions I have are as follows:

Compression:  after top balancing the cells I need to rearrange them into the 2p16s configuration is releasing the cells form compression after the top balance charge is complete advisable?  will the cells swell as I release the compression?  is this safe for the batteries?

if so then I need to stop the top balance and finish my battery box first so that I can put them in in under compression and then top balance.

BMS:  the wires on the BMS appear to be too short for the battery packs shape, can I lengthen them (solder/heatshrink) with the same or thicker gauge wire?  will this cause the BMS to not read correctly?

With the info that Mcivor gave me he suggested a 54.4 charge voltage which puts the individual cell at 3.4vdc and then use the LVD function of my inverters to set a shutdown thresh hold high enough to prevent the BMS from activating for instance in the 44 volt range which effectively leaves the individual cell at 2.75.  the end result seems that it would comfortably put me in the 20-85% range for the batteries ensuring a longer life.

So the last question I guess is the various settings with in my solar controller  Morningstar  TSMPPT-60.  I have downloaded MSView and played with the available settings just to help familiarize myself with the system.  

anybody familiar with the setting could help walk me through it? so the bulk or absorb setting would be the above mentioned 54.4, but what about float?  set it the same?  

One thought I had was that as there is no need to equalize why not use the equalize function (in manual mode only) to occasionally let the cells be top balanced say 3.65 volts per cell vice 3.4 (58.4 vs 54.4)  this would still be just below the BMS cutoff but might it help to top off or balance occasionally without the need to disassemble the pack.  Thoughts?

with my current system the heat from the batteries, inverter, and solar controllers was enough to keep the battery shed at 5° or 6° Celsius on average with a couple of dips down to 1 degree during the coldest hours.  I do not think I will get the same amount of heat with the lithium's so I have decided to make a battery warmer to keep the batteries warm enough 4 x 12 volt 25 watt heating pads wired in series will be Kapton taped to an aluminum plate along with 4 temp sensors taped to the lower portions of the cells insulated with foam and a temp controller set to turn on at 6° and turn off at 8° or 9° C this might be also on a timer to keep the heaters turned off from1800 to 0400 to save power.  I will have to play with that portion to figure out what's best and too be honest as I have no desire to pull the pack apart after the build, I will probably tape several other size heating pads under there in case these are not enough 








Thanks to forum member Mcgivor I have managed to select and purchase some Calb 200 a/h lithium cells. importing them into Japan was easier than expected the Seller was very helpful with some of the additional paperwork that was required here.The plan after top balancing them is to build a 2p16s pack to feed my inverters with. some of the questions I have are as follows:Compression: after top balancing the cells I need to rearrange them into the 2p16s configuration is releasing the cells form compression after the top balance charge is complete advisable? will the cells swell as I release the compression? is this safe for the batteries?if so then I need to stop the top balance and finish my battery box first so that I can put them in in under compression and then top balance.BMS: the wires on the BMS appear to be too short for the battery packs shape, can I lengthen them (solder/heatshrink) with the same or thicker gauge wire? will this cause the BMS to not read correctly?With the info that Mcivor gave me he suggested a 54.4 charge voltage which puts the individual cell at 3.4vdc and then use the LVD function of my inverters to set a shutdown thresh hold high enough to prevent the BMS from activating for instance in the 44 volt range which effectively leaves the individual cell at 2.75. the end result seems that it would comfortably put me in the 20-85% range for the batteries ensuring a longer life.So the last question I guess is the various settings with in my solar controller Morningstar TSMPPT-60. I have downloaded MSView and played with the available settings just to help familiarize myself with the system.anybody familiar with the setting could help walk me through it? so the bulk or absorb setting would be the above mentioned 54.4, but what about float? set it the same?One thought I had was that as there is no need to equalize why not use the equalize function (in manual mode only) to occasionally let the cells be top balanced say 3.65 volts per cell vice 3.4 (58.4 vs 54.4) this would still be just below the BMS cutoff but might it help to top off or balance occasionally without the need to disassemble the pack. Thoughts?with my current system the heat from the batteries, inverter, and solar controllers was enough to keep the battery shed at 5° or 6° Celsius on average with a couple of dips down to 1 degree during the coldest hours. I do not think I will get the same amount of heat with the lithium's so I have decided to make a battery warmer to keep the batteries warm enough 4 x 12 volt 25 watt heating pads wired in series will be Kapton taped to an aluminum plate along with 4 temp sensors taped to the lower portions of the cells insulated with foam and a temp controller set to turn on at 6° and turn off at 8° or 9° C this might be also on a timer to keep the heaters turned off from1800 to 0400 to save power. I will have to play with that portion to figure out what's best and too be honest as I have no desire to pull the pack apart after the build, I will probably tape several other size heating pads under there in case these are not enough

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