Trusting CID and picking a Bus Bar

Hello, First post here and first project(of many). I built a spot welder and ordered my chargers/testers.

I usually do IT work but I am have done light residential electrician work.

I am planning a 7s20p pack for back up power during outages(fridge etc) for my Mom and later they may buy an RV and this would be super useful with a simple addition of a charge controller and a few solar panels

I am using new batteries(although they are from aliexpress), they came in, got charged and are now sitting put away waiting for the passive discharge test, then I plan to capacity test them with the Lii-500S's and planning out balanced series.

The battery would powera 110vac inverter(1500/3000) and somebucks to 5v(3x2a) and 12v(10a).

Since my cells have CID, why would I try to fuse each individual cell? Is CID not trustworthy? I am leaning to forgo the individual fusing, alternetively I am considering spotwelding some thickness(width, and length) nickel strips to the bus bar.

Also what would be a reasonable bus bar design to handle ~60A for peak. I plan to have a breaker for 60A.

I was planning on using a doublewide nickel strip and then putting a bus bar on there of unknown size and material(2xAWG8 for easy into TX90? and plenty wiggleroom), the bus bars would go to a XT90 on both leads, so each Series would havea male and a female connector.

Please let me know if im missing some glaring issue.

batteries-ali

BMS-ali

math- google sheets

CIDs are NOT fuses. They are in case the cell overheats to the point of failure. A fuse is a safety feature against over-current in/out the cell. A CID is kind of like an Air Bag in a vehicle. Having an air bag doesn't negate the need to wear a seat belt, does it? Of course not.
One device tries to protect in the extreme whereas the other tries to protect under standard and less severe issues.

I really like the air bag / seatbelt analogy when comparing CID vs fuses.

That said, many commercial packs are not built with fuses, and depending on the application and how the pack was built, I would not fuse individual cells. For high drain applications, e-bikes, power tools, where packs are well constructed and protected, I would opt for traditional construction with spot welded nickel strips and fuse the battery pack as a whole.

Of course, you can do it how you like. You have the creative freedom as this is YOUR design.

The most useful function is saving user error. One user accidentally shorted a cell with solder, this destroyed the cell. But the fuses did save a catastrophe. This is definitely a good reason for me. If a metallic object shorts the positive top to the negative case, even a small number of parallel cells could heat that object up enough to cause that cell to catch fire. This is also why I would fuse both sides of a cell.

Given many are using second hand cells, it is well more than possible for a piece of stray debris to find its way where it shouldn't.

That alone is my only argument for fusing cells.

May I ask what cells you bought? If they where genuine new cells i would personally not fuse them in such a "small" pack. With that said Aliexpress is known to have 95% fakes

/Daniel

Also the fuses really don't add much cost or time at all on completing the packs, but the level of protection they give is well worth it. With a spot welder you may or may not be able to just weld the fuse legs directly onto the battery anode and then solder the other end to your busbar. If you can't (I understand you made your own spot welder and depending on what you used it may just be too difficult to dial in) then you could just cut a little bit of nickel strip, solder one fuse leg to the very end of the cut strip and then weld the other ends onto the battery. The latter instance does create an extra step in putting packs together but it's a very short step and, again, of time spent is well out weighed by the safety obtained.

I used the glass axial fuses but you could also just get fuse wire. Keep in mind the glass axial fuses tend to actually pop at about double their rating (2A fuses will go around 3.5 or 4A in reality). They're dirt cheap too when purchased from aliexpress. On eBay they are slightly more expensive but you also get them five times quicker.

OhmGrown said:

Also the fuses really don't add much cost or time at all on completing the packs, but the level of protection they give is well worth it. With a spot welder you may or may not be able to just weld the fuse legs directly onto the battery anode and then solder the other end to your busbar. If you can't (I understand you made your own spot welder and depending on what you used it may just be too difficult to dial in) then you could just cut a little bit of nickel strip, solder one fuse leg to the very end of the cut strip and then weld the other ends onto the battery. The latter instance does create an extra step in putting packs together but it's a very short step and, again, of time spent is well out weighed by the safety obtained.

I used the glass axial fuses but you could also just get fuse wire. Keep in mind the glass axial fuses tend to actually pop at about double their rating (2A fuses will go around 3.5 or 4A in reality). They're dirt cheap too when purchased from aliexpress. On eBay they are slightly more expensive but you also get them five times quicker.

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I was thinking of using 5a, if i do,as my surge current should be about 2.5a per cell to lower the resistance a bit. Knowing that they would not blow until 10a is concerning, im just gonna order a couple hundred of a few different ones as you said they are 1c a piece on alieexpress and try to burn some up.

EDIT:
The spotwelder i made is based on 330CCA 12v battery, starter solenoid(closingthe -), a relay to fire the solenoid and an arduino. The welds i liked were about 15ms(Lowest I can go is 12ms for the solenoid to close)in the code, but i dont know think thats the real timing, but it is consistent.

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daromer said:

May I ask what cells you bought? If they where genuine new cells i would personally not fuse them in such a "small" pack. With that said Aliexpress is known to have 95% fakes

/Daniel

Click to expand...

I grabbed theLiitokala original 18650 3400mAh NCR18650B 3.7V battery fromLiitoKala Official Flagship Store, they dont look very real (missing the authenticity sticker) and i dont think the store is official at all.

I did capacity test a few and they came back as 3450+.

out of the 134 that arrived(I got shorted 10, dispute in progress) I recall only 3-4 being lower voltage then the rest, I marked them and they are still all resting before the self discharge test. I am actually ok with that result, im just going to have to order a few more cell, as I dont think changing manufacturer is a good idea.

Next week, im gonna cap test them all and have more results.

Sounds fine then. Once again new cell i wouldnt fuse them in such small packs.

Fusing individuall cells are for when 1 cell short out due to for instance age or abuse since before. New cells dont have that and you will be able to controll it.

daromer said:

Sounds fine then. Once again new cell i wouldnt fuse them in such small packs.

Fusing individuall cells are for when 1 cell short out due to for instance age or abuse since before. New cells dont have that and you will be able to controll it.

Click to expand...

what do you think the safest way to test the CID/its presence would be?

diib said:

daromer said:

Sounds fine then. Once again new cell i wouldnt fuse them in such small packs.

Fusing individuall cells are for when 1 cell short out due to for instance age or abuse since before. New cells dont have that and you will be able to controll it.

Click to expand...

what do you think the safest way to test the CID/its presence would be?

Click to expand...

In order to test it, you pretty much destroy the cell. Similar to an air bag, you have to trust it to do its job, but you can test it by running into something with your car until it deploys.

First off, give this a quick read:https://www.nasa.gov/sites/default/files/files/345992main_NESCTB09-02_LiIonBatteryLimitations.pdf

It explains that PTC is current limiting and CID is for overvoltage/overheating. And it also explains why we fuse packs instead of just relying on PTC and CID.

There is no safe way to test for this, you have to expect thermal runaway and prepare for it. Even if the cell survives, discharge it completely and recycle it as it may have become compromised during testing.

To test for PTC, short the cell. I used a typical 4 cell holder and soldered a wire from positive to negative and left room for a clamp meter to clamp on. I used an infrared thermometer to watch the cell temperature. I did this for 2 generic cells and will do it for a lot more soon. If the cell has PTC, once you insert the cell,you will see the Amps on the clamp meter go from something like 30+amps down to whatever the PTC protection was designed to limit to. In my case, a fully charged 4.2V cell dropped from 35Amps to 1.8Amps and stayed there for 10 minutes before I ended the test. The temperature of the cell rose to about 155*F/70*C and also stayed around that temperature for the duration of the test. If the cell did not have PTC, it would have entered thermal runaway.

To test for CID, overcharge the cell. I haven't done this yet, but I plan on hooking up a 12V 1Amp plug directly to an 18650 (probably through the 4 cell holder for convenience and monitoring). I believe 12V is the industry standard as I've seen it referenced in a few data sheets.For the few CIDs I reset (dumb idea, but I was a noob at the time) all were around 4.20V upon reset, so likely the CID tripped due to overcharge. I do believe they can trip due to heat as well, so if you have a scrap toaster oven, try heating up a cell to 100*C while charging itor something like that?