Wooden Storage Case for 24V DIY

[OffGridForGood]

My Overkill BMS has two temp probes. They are pretty small in diameter. The official Overkill recommendation is to tape them to the side of a cell with Kapton tape. Does the Kapton tape exclude ambient temperature somehow? Does it work to place them between cells (which would give a more accurate reading of the actual temp inside of the battery)? I'm thinking with compression this could be something that would damage the protection around the cells as they expand, no? If so, could they be integrated with some sort of separation material to prevent that?

I tape one sensor on top of a cell in mid pack, and one on an opposite side of a cell, and call it good. All my cells are in double rows in complete boxes - enclosed to keep out dust, insects, rodents, fingers...
You need to add separation between every cell. The forum has a number of suggested materials for this.
The shrink wrap the cells come with is very thin, and the Aluminum case beneath is electrically continuous with one of the battery terminals, POS IIRC.
Generally you will want to top balance to get the cells close enough to the same potential near the fully charged state, that the BMS can just maintain this balance. If you're in a hurry, charge them up and get the pack working - in your set up you have access to all the cell terminals so you can 'bleed' a cell to lower voltage with an automotive bulb easily enough at any time. If you have a bench source you can charge a low cell (you don't need to disconnect the cells from each other to do this) to bring it's voltage up if needed on a cell or a few. If you have the app showing you all the cells's individual voltages, it is an easy matter to pull some low ones up, and pull some high ones down while monitoring all cells on the BMS app. Takes some time.

 

[740GLE]

My Overkill BMS has two temp probes. They are pretty small in diameter. The official Overkill recommendation is to tape them to the side of a cell with Kapton tape. Does the Kapton tape exclude ambient temperature somehow? Does it work to place them between cells (which would give a more accurate reading of the actual temp inside of the battery)? I'm thinking with compression this could be something that would damage the protection around the cells as they expand, no? If so, could they be integrated with some sort of separation material to prevent that?

My 8s is in a block shape, I was able to sneak the JK sensors down in between 4 cell corners, didn’t take any force at all.

I figured if ambient temps rise, the cell internal temp would be cold soaked and it’d take some time for them to rise up.

But then again I’m only worried about October before the battery is brought back south to live the winter in the basement.

 

[curiouscarbon]

My Overkill BMS has two temp probes. They are pretty small in diameter. The official Overkill recommendation is to tape them to the side of a cell with Kapton tape. Does the Kapton tape exclude ambient temperature somehow? Does it work to place them between cells (which would give a more accurate reading of the actual temp inside of the battery)? I'm thinking with compression this could be something that would damage the protection around the cells as they expand, no? If so, could they be integrated with some sort of separation material to prevent that?

the temp probes with overkill/JBD BMS are relatively hard, and i advise against any compression that would push them into the cell casing. that would damage the cells and increase likelihood of failure.

kapton or polyimide tape is good at electrical and thermal insulation, so it will provide some degree of "rejecting ambient temperature influence" so that the cell material is what will be mainly measured. try to ensure the wire is positioned so that the tape is not being pulled away over time. just gentle contact with the outside of the cell should suffice for most situations.

with two sensors, one near the center and one near a "far side" seems common and fine.

the cell terminals themselves are often the most leading indicator of overall cell temperature, but the busbar and interconnect also make that difficult both electrically and physically.

when in doubt, post a photo or drawing of a proposed thermometer placement and surely a few folks will share feedback.

 

[Bananaman321]

Cut the pieces.
View attachment 222258
My first problem was I put the pocket holes on the inside of the end pieces, thinking it would come out nicer with the holes hidden by the batteries. Then I realized I wanted to secure the batteries between the ends before screwing them to the bottom, otherwise the compression would not equalize properly, so drilled another set of holes on the other side. I designated an inside to the ends based on how the grain would cup, so that any warping would add to the compressive forces.

Fed the all thread into one screw post, through one end board, covered it with latex tubing, and threaded it into another post in the other end.
View attachment 222259
My current problem is that I cut the bottom to match the length of 8 2 3/4" cells, plus 1 1/2" for the ends. It appears the cells are bulging enough to add just under an inch. I tried to compress them with the all thread but the post shoulders are starting to dig into the pine.

I tracked down the specs and these cells are supposed to be 71mm thick, meaning my 2 3/4" estimate brought me about a half inch short calculating the total assembly length. So the cumulative bulge is about 1/4". Is this to be expected? I bought these just over a year ago and have had them sitting around with whatever charge 18650.com sent them with.

I'm thinking I will cut a new bottom board at the extra 3/4" length, insert washers under the screw posts to prevent them digging into the wood, reassemble with the present level of 'snugness', and screw the ends down there.

My next step will be to figure out if I can/should do top balancing. I had read many posts for other builders saying the BMS will eventually work out the balance with enough cycles itself. But I also read that the Overkill BMS is not a strong charger in that regard. the 6.5k Ahs is probably way more than what I will need until next winter, so should have more than 100 cycles for the BMS to work on it before I might be looking for some extra Ah. Does the BMS eventually arrange for max capacity itself without top balancing first?

I agree with what @OffGridForGood said about cell separators but also include eva foam between each cell and at each end these cells expand and contract when charging/discharging also be careful not to over compress just give a nice hug leaving some breathing room.

another thing if you ever plan on lifting that pack by those handles I recommend adding a 4in or taller board down the length of of each side of your pack and fastening the sides and bottom to it. This will prevent deflection when being lifted wich would cause unwanted crushing force at top of cells deflection will also put damaging forces on terminals if you are using solid busbars.

One more thing I would predrill and run a 4in screw through the solid material on each side of the handle cutouts. It would be a shame if that wood cracked along the grain and gave way wile being lifted or carried.

Anyway looking good hope this helps I'd like to see how it turns out.

 

[Coydon]

... also be careful not to over compress just give a nice hug leaving some breathing room.

another thing if you ever plan on lifting that pack by those handles I recommend adding a 4in or taller board down the length of of each side of your pack and fastening the sides and bottom to it. This will prevent deflection when being lifted wich would cause unwanted crushing force at top of cells deflection will also put damaging forces on terminals if you are using solid busbars.

After I did the tweaks I mentioned above, the wood around the handles felt a bit flimsy. I also didn't care for the board across the top that Offgrid Power Solutions use in the video above so I went back to my original design intention for a 5 sided box and added a full 11.25" board to the near side and will mount the BMS there. Not sure if I will do the same on the other, or use some sort of 'cleat' on the wall where I plan to mount this to do the same thing.

I grew weary of internet forum advice on torques learning to wrench on my F650GS. It seemed some people considered a bolt should be "nice and snug" whether that meant 5Nm or 100. Anyone care to throw out a ballpark number on how tight I should make the threaded rod before fastening this whole thing up and charging the cells? I tried both 10 and 20 in-lb, seemed like 10 (a little over 1N.m) was enough to keep things in place when I picked it up, think it'll be enough?

Now I just need to verify that all of my wiring can handle the loads. I am switching from a 12V 225Ah FLA battery to these LFPs in a 24V config and using the same 30A charge controller. My understanding is the controller limits the load on the wires going to the battery and upping the voltage won't require any thicker wires, or do I need to recalculate this? I'm also swapping in a 24V 2000W inverter for the 12V 600W one, but the new inverter docs call for 6AWG for a run less than a meter and I have that covered. I think my only problem is the 40A breaker I had on the old system. [Edit: going back over the old system, i think i had a 90A ANL coming off the battery, then the 40A braker going to the charge controller.] The new inverter will need to draw more current and the voltage increase won't cover that, right? Could I put a 70A ANL fuse in there? [Guess the breaker to the CC is still good, what fuse should i have on the negative terminal?] Do I need something a little bigger or am I calculating this all wrong?

 

[Coydon]

Here's what I've been working with for the last four months:

Seems to have just worked, no top balancing. I've yet to set up the Overkill app and check any status, but as is it has carried me through multiple cloudy days without having to unload the fridge into the smaller cooler.

I haven't moved the box yet either. We are about to have our days drop below freezing in the next week or two, so need to move it into my living space. Unfortunately the present 10' run of 10awg that came with the charger is 4' short of reaching from the panels, so have to pick up some more wire tomorrow.

Not wanting to leave this in the living space, I'll have to come up with something for an insulated box and heating pads. Of the heating pads I purchased a year ago, I could only find one when assembling the box, and stuck it under with the wires running up the side. Now I can't even find any records from purchasing the pad, no idea on specs, so will have to start from scratch on that...

 

[cs1234]

I like your battery, but I'm not so sure about that type of breaker in general. What brand/model is it and what are you depending on it to do?

 

[Coydon]

I like your battery, but I'm not so sure about that type of breaker in general. What brand/model is it and what are you depending on it to do?

https://www.amazon.com/dp/B07D7VHJYK?ref=ppx_yo2ov_dt_b_fed_asin_title

I was following Will's diagram for his 40A system (The Classic 400W), which has a 50A fuse between the Controller and the 100A fuse on the battery's positive terminal. Since I have a 30A CC I used 40A, and made it a breaker in case I wanted to isolate the battery from the CC. Not sure what made me think I might want to do that, I made that initial design over 2 years ago...

 

[cs1234]

https://www.amazon.com/dp/B07D7VHJYK?ref=ppx_yo2ov_dt_b_fed_asin_title

I was following Will's diagram for his 40A system (The Classic 400W), which has a 50A fuse between the Controller and the 100A fuse on the battery's positive terminal. Since I have a 30A CC I used 40A, and made it a breaker in case I wanted to isolate the battery from the CC. Not sure what made me think I might want to do that, I made that initial design over 2 years ago...

The general consensus here on the forum is that those style breakers don't have a high enough AIC to work properly in the event of a Lifepo4 battery short circuit, likely not even for a 100ah battery.. let alone a nearly 300ah battery. It will most likely function properly for a 24v battery under light load for manual disconnects though, but under short circuit your battery could possibly be pushing 10,000 amps or more and that breaker just won't cut it.

Consider looking at the MidNite Solar DC breakers available on Amazon. They have them in both polarized and non polarized and the price is roughly the same as what you paid for that breaker. They have much higher AIC ratings (especially the bigger ones).

Search here on the forum for discussions about t-class fuses and high AIC ratings and you'll see why a different style breaker is recommended, not just for a connection between your battery and charge controller.. but for all breaker/fuse positions.

 

[740GLE]

I’d keep an eye on the temp of that pop style fuse, if there’s any voltage drop on it it’ll also mess up your SCC charging and your battery will never get to 100%.

I do second the recommendation of midnights DC breakers for SCC, a little babybox and one breaker is a little expensive but can’t be beat.

 

[OffGridForGood]

but I'm not so sure about that type of breaker in general.

I have one of those breaker-switches on a small 12v system - I find they can get hot, and I don't leave it unattended for long.
With only one battery pack (not two, three or four in parallel) I don't see a problem in your set up since you also seem to have the mega-fuse after the breaker.
If you ever parallel up multiple battery packs, then get better fuses (higher AIC rating) to protect from one pack shorting, and three other packs dumping a zillion amps trying to feed into that short.

 

[robbob2112]

The real bussman brand of that style breaker has an AIC of 2ka. Not high enough. They have a high contact resistance and the contacts aren't hardened or coated to be used while current is flowing. So they run hot, are easily damaged which makes them run hotter, and the copies and fakes have a habit of melting or bursting into flames. The real ones are plastic that won't sustain a fire so if they melt you only need to worry about live wires being loose.

 

[OffGridForGood]

The real ones are plastic that won't sustain a fire so if they melt you only need to worry about live wires being loose.

Only...be right back, gotta do something...

 

[Coydon]

The general consensus here on the forum is that those style breakers don't have a high enough AIC to work properly in the event of a Lifepo4 battery short circuit, likely not even for a 100ah battery.. let alone a nearly 300ah battery. It will most likely function properly for a 24v battery under light load for manual disconnects though, but under short circuit your battery could possibly be pushing 10,000 amps or more and that breaker just won't cut it.

Consider looking at the MidNite Solar DC breakers available on Amazon. They have them in both polarized and non polarized and the price is roughly the same as what you paid for that breaker. They have much higher AIC ratings (especially the bigger ones).

Search here on the forum for discussions about t-class fuses and high AIC ratings and you'll see why a different style breaker is recommended, not just for a connection between your battery and charge controller.. but for all breaker/fuse positions.

I have a 40A ANL fuse and holder I can put there. Given the rate at which I've needed to isolate that area (0 times in 3 years), I think I can live with having to manually disconnect.

What is the general consensus of ANL vs ANM fuses?

 

[cs1234]

I have a 40A ANL fuse and holder I can put there. Given the rate at which I've needed to isolate that area (0 times in 3 years), I think I can live with having to manually disconnect.

What is the general consensus of ANL vs ANM fuses?

It's all going to come down to the AIC of the fuse and what you have connected to it. If you can't find the AIC rating of the fuse anywhere online, that's not a good sign. You just want a fuse/breaker that is capable of stopping a dead short in your system causing the battery to dump it's full amperage out. I'm not sure of the exact calculations on a 280ah 24v lifepo4 battery. Somebody here can probably give a good indication of the potential amperages involved.