compression plan for a small (120ah) CATL 4s battery.

rickst29

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My first DIY battery will be a small one, 12.8v (4S) of CATL prismatic 120Ah cells. I bought the cells from Basen. On the forum, I read that I should probably compress these cells to about 6 PSI. Can some of you please review this scheme, and advise if I'll be making a 'misteek' by proceeding with this plan?

In the listing, Basen gives cell dimensions as 48mm x 174mm x 168mm. The width x height area is 45.3 square inches, calling for compression of about 270 lbs. I bought a couple of cheap 8x8 inch baseplates, in thickness 3/16". They're larger than I need, but I don't have good cutting equipment for heavy steel. Some of the excess area between the plates will be taken up by a thermostat-controlled heater pad (which is enabled by both the thermostat and a separate 'enable' switch). The rest will be taken up by foam insulation, protecting the battery cells from RV Trailer vibration during travel. (The inner floor and sides of the outer battery box will also have anti-vibration foam, except for an area where the BMS can release heat.) BMS will be mounted on one of the end plates. With another el-cheapo thermostat unit, I could cover that BMS heatsink with a small computer fan to assist in pulling heat out when the battery gets warm, but I'm not sure that I want to bother with that. Each added thermostat creates a phantom load of about 0.3 Watts.

The 3/16 steel baseplates should be quite inflexible. If ordered pre-drilled, they would have been pre-drilled with 9/16 holes supporting 1/2" threaded rod. But I don't want to use rods that large, unless the inner diameter of suitable compression springs require that I go there. Being soft steel, I can drill my own holes (for either 1/4" or 3/16" rod). In the case of the small ones, each 1/4" rod can support about 200 lbs of tension before stripping its nut or stretching too far. I will require only 270/4 lbs of tension on each one (just under 70 lbs), so even 1/4" looks OK to me -- if that's not too tight radius for the controlling springs.

I have no idea what springs to buy, and will be reading whatever messages Threads I can find on that topic shortly. Thank you all for reading!
 

Bob B

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There's a thread over here ..... if you end up using springs please add a link to your thread in that thread.
 

rickst29

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There's a thread over here ..... if you end up using springs please add a link to your thread in that thread.
Thanks. I've just read it, although I'm having a very hard time finding suitable springs without paying a $40 minimum order charge. Maybe I should institute a group buy, in which I buy the springs in bulk and then snail-mail out to other people?
 

Bob B

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I'm not sure if your 120 ah cells have the same requirement as the 280 EVE cells.

It would just be common sense, however, that compression to stabilize the pack will be a good thing ... and the springs will provide a somewhat consistent compression.
Don't know if it will have any effect on the cycle life like it does for the EVE cells.
 

rickst29

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No 'messages' in reply to my post on the main Thread, concerning a group buy. So I spent the $40 minimum a bit inefficiently, buying extra-fancy $10 each springs instead of the $4 alternatives which could have been made to work just as well (The $4 ones would have required larger diameter rods, and a just bit more length.) I'm now waiting - hopefully not more 3 months - for delivery on the cells.

Slightly OT: All parts for my cold weather 'auto-heating before charging' system have arrived, although I will install those heater pads (and thermostat and relays) into my battery storage area only after I have built the new battery, there's no need to add them now. (They would only be supporting 'legacy' pre-built LFP batteries during the summer, when I won't be needing the heater functionality.) The thermostat and relays will be external to the batteries. For the new battery, I could put just the heating pad inside the battery case or box - which need not be waterproof or airtight..
- - -
But I have a general question about assembling this battery. I see a lot of "inside-the-RV, water-resistance case NOT required" battery packs being built with plywood, and with some open exposure to "within-the-compartment" dry air. Does 1" plywood likely have enough bending resistance to serve as compression plates, rendering my 3/16" steel plates as Meaningless Overkill"? But doesn't plywood also introduce some level of fire hazard?

Plywood certainly looks a lot quicker, for attaching the compression "side plates" to a bottom plate, and perhaps even constituting an entire vented case. I see some experts (from here) building batteries that way on videos
 

rickst29

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Pictures would probably help with visualization of your project.

What's a picture worth ?
OTA, thank you for asking!

My cells are scheduled to arrive in the port of LA/Long Beach this very day. I'm expecting to receive them in 4-5 days (although customs clearance and UPS shipping delays could stretch to an entire week). I've decided to go with the steel plates. Between cells, I have some very thin but solid plastic sheeting material which I will cut to keep the aluminum surfaces from abrading each other during compression and expansion.

By using only 4 springs on the outside of one plate (rather than 8 screws on both plates), there will be more sideways movement of that adjacent "first cell" when the spring is expanded and compressed by increased/decreased cell swelling. The "last" cell will hardly move at all. Underneath, rather than a solid base assembly, I intend to loosely fit a plywood panel into grooved "end brackets", so that the plywood does not fall out of place (while space in the end grooves expands and contracts with the battery assembly above). The plywood panel should prevent middle cells from sliding downwards, due to road vibration (this is going into a Travel Trailer).

The weight of the battery is taken entirely by the end plates and the two "end brackets" attached at the bottom. Below the plywood panel, but above the lower portion of the end brackets, there is empty space - into which I plan to place a motorcycle seat heater pad, run by a thermostat. If my design works without further mods, I should have pictures done within 2 weeks.
 

Gazoo

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Curious. What is your reasoning for compressing the cells using springs? I admire your ambition but there is no data I am aware of cycle life will be improved using compression with 120ah cells. I don't know how much they expand if at all. Is there anything in the CATL spec sheet that mentions compression and the recommended compression force? Too much compression is worse than no compression.

What do you plan to use for busbars? I have braided busbars I am going to use when I rebuild my 8S EVE pack. The EVE's do expand and contract but I just don't know why anyone would be using springs with 120ah cells?

Generally speaking a cells cycle life is 2000 cycles to 80% capacity. And that's charging and discharging full cycles at 1C rates according to the spec sheets. If one does that every day the cells will last apx. 5.5 years. Since most of us go much easier on the cells than the manufacturers tests, the cells will last much longer and might age before before degrading due to capacity cycle loss.

I am trying to visualize your fixture and if my brain is working your fixture is sound :) No doubt a fixture should be used no matter the size of the cells. Looking forward to photos when you do your build.
 

rickst29

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Battery is done. This photo is from before adding a 12v "motorcycle seat heating pad", underneath the bottom and lower sides of the cells. And also from before encasing 4 "working sides" (top, bottom, and compression spring ends) in 1" * 8" pine board. Image is with a small plug-in-the wall charger attached for cell high-balance testing, the actual lugs will be 8mm and the wire size is 1/0.

After encasing in boards in pine boards (not under compression), the pine "side" with springs the long springs has a bit more than 1/8" of "free space" before the adjacent steel plate. This allows the length of the compressed rods between the plates to vary a bit (with the steel plate on that end moving slightly). The bottom is not attached to the cells, except by gravity. HomeBuilt-120Ah-number-one.jpg
 
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rickst29

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The BMS is now anchored to the "fish tank separator" with a loop of electrical tape, and it's against the right side (away from the "hot" terminal lugs and main fuse). That's the fuse on top, and I haven't tightened the screews of the case cover all the way in yet.
HomeBuilt-120Ah-number-one-with-heating-pad-and-partial-wood-case.jpg
 

rickst29

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I like the grid you used to protect the terminal and bus bars from stray tools.
Thanks for noting that! I wanted a combination of non-conducting "space" while still allowing for some ventilation if/when they get too warm. These are very cheap "fish tank dividers", and they can be cut with a normal scissors to fit a bit better. The "width" is the original size (and I would have liked a bit more), but I did cut down the length to fit.

In comparison to widely used wooden panel, they are also likely to melt at a point of spark or burn, rather than catch fire.
 
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rickst29

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Curious. What is your reasoning for compressing the cells using springs? I admire your ambition but there is no data I am aware of cycle life will be improved using compression with 120ah cells. I don't know how much they expand if at all. Is there anything in the CATL spec sheet that mentions compression and the recommended compression force? Too much compression is worse than no compression.

What do you plan to use for busbars? I have braided busbars I am going to use when I rebuild my 8S EVE pack. The EVE's do expand and contract but I just don't know why anyone would be using springs with 120ah cells?
Gazoo, I have new and probably surprising information for you: These two pictures are actually of a rebuild, in which I took cells from a distorted 110Ah battery pack which I have been using since only late 2019.

My new set of '120Ah' battery cells is finally unloading at the Port of Los Angeles TODAY :). That entirely "new" battery pack was the original subject of this Thread. But my second battery pack (with the new cells), using the new cells, will be 100% identical in construction and appearance to this 'first' one, with this same compression scheme. So I have posted the "rebuild" pictures, and I plan to skip from adding any photos of the second pack, because they'll be exactly the same.

The late 2019-vintage, pre-built battery pack had only closed cell foam, providing minimal compression in a relatively weak plastic case. I noticed that case was swelling out of shape, and decided to rebuilding it (also using this new BMS to allow for higher current levels).

If you look very carefully, you will see (in spite of phone-camera distortion) that the width of cells in the bottom half is already expanded to more than the width of cells at the top. Even after adding some extra compression on the lower springs, the steel plates ARE NOT vertical. In just two years of moderate use, these cells have already become distorted - and that's the problem which I hope to first reduce, and also prevent, by using active compression.
- - -
My Busbars: The pre-built pack was constructed with 2x2 cells lengthwise, so the old busbars couldn't fit the new layout. They were also grossly undersized. I went to a big-box hardware store and bought a 3-foot length of aluminum (not copper), 3/4" wide by 1/8" thick, along with new and longer M6 screws. I cut and drilled the aluminum bar to 6 segments of the proper length, and then cleaned with rubbing alcohol, before using a PAIR of bars for each of my three bus connections. Although aluminum is less conductive than copper, the massive cross-section of my oversize bars can handle a ton of current.

My new cells will arrive with screws and bus bars (plated copper) included in the package, and I'll probably just use them - even though they will be much thinner, and less capable.
 
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rickst29

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Now for the new 120Ah battery build, already under compression. It's ready for taping (a safety issue on the bus bars), the big fuse holder, and another wooden box. The Shenzen-Basen provided busbars were too wimpy, so I again did my own. I will be adding another heating pad to this one, sharing the same low-temp heater temp controller unit as I use with the previous battery.
120ah-battery-ready-for-taping-and-case.jpg
 
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