batterywall designs

[spacecabbie]

Hello,

Spoiler: orignal post

I was wondering if there are build guides for batterywals.

I was thinking of doing something with PCB 18650 holders i like the no spotwending approach.
But as this is quite a big project I dare not design it my self just of safety concerns.
So i am looking for build guides preferably including shopping list, why invent the wheel twice ?

If there is none available are there commercially pack designers that can provide me with a build to my specs ?

Initially was thinking way ahead and scalled it back to a simple first build:

12v 50ah battery for my trailer to power flood lights etc when not connected. So it wil need to provide 12v automotive range witch is from my experience between 11.6 and 14 (low battery and max alternator)
Since this pack wil need very good insulating/protecting not only for cold/heat but rain as wel and a possible impact of tiny rocks. I am so ademend on monitoring or beter said isolating independent cell if they fail.

 

[floydR]

Spotwelding is the way to go, PCB holders take up too much space, Something like this https://secondlifestorage.com/index.php?threads/paragon-powerwall.9156/ would work better. Much denser packing of the cells.
Later floyd

 

[Korishan]

PCBs would also be very expensive to have made for a large project. The 18650 holders will cause problems after a time as the springs used are cheap steel with nickel coating. The springs will get weak over time, and will possibly corrode. The only types of springs I would recommend would be Beryllium Copper. This is a better spring material and will not corrode like the nickle coated steel. Also, it will carry current more efficiently as it has lower resistance.

As floyd mentions, spot welding is the better option. It creates a much better connection. And lowers the resistance drastically.

There isn't really a pack designer, per se. There are suggestions all around the web on how to design them, to meet certain criteria such as voltage/amps needs, and pack layout.

There are many different designs here on the forums

 

[cak]

I agree with FloydR and Korishan, spot welding 1s_p packs wired up in series are the way to go for any larger packs as implied by you asking about battery walls vs more portable pack designs. If you are set on a PCB based design I would look at Jag35s kits https://jag35.com/collections/kits and ether buy those or use them as inspiration but they will be more expensive and have other trade offs to the types of powerwall designs most folks here usually end up doing.

I would also caution against the type of design in floydR's link from personal experience inheriting a similarily designed powerwall which has been working fine for me for months but it annoying and hard to maintain without meaningful advantages. I am building my new packs off a fairly standard 100p blocks wired in 7s for 25vdc system and monitored by the DIY BMS. Still tweaking things but it has been working well so far and I can relatively easily swap out a block for a new one if I run into issues so upkeep should work well. Pic is of 7 blocks just waiting for the 4awg copper busbars.

 

[spacecabbie]

Ok thanks all for your input some folowup questions if i may.

Lets go with a 12v design.

3s20p as far as I understand BMS it wil only monitor the 3 groups ?
Is it posible to monitor all 20 18650 per group ? or atleast more then just all 20 ?
Can you for example create 3s2p packs with a simple bms then place those in paralel under a bigger bms ?

To summarize what design configuration has the maximum safety and reliability ?

 

[Korishan]

Lithium-Ion cells are bad for 12V systems. You either go too low voltage, or overcharge.

If you want a bms monitor "every" cell, then you want 1p configuration, regardless of the series, and then have a bms for each string. This is highly overkill, to be honest. Not to mention the logistics headache and the wiring pathing. You would most likely pull out all your hair before you finish building a large enough configuration to actually be useful.

There could be a way to do a pcb for this particular application. Again as I mentioned earlier, it would be pricey. However, you could design a pcb thin strip, barely wide enough to run traces to carry the current and a few return lines. Probably would need to be a 3-layer so that you could send signals back in the middle layer. Then use an mcu with a multi-plexer and an ADC so that it can scan all cells connected.
This would be very tedious either way.

 

[spacecabbie]

Lithium-Ion cells are bad for 12V systems. You either go too low voltage, or overcharge.

Don't understand this is in a car application for a test build and practical application

If you want a bms monitor "every" cell, then you want 1p configuration, regardless of the series, and then have a bms for each string. This is highly overkill, to be honest. Not to mention the logistics headache and the wiring pathing. You would most likely pull out all your hair before you finish building a large enough configuration to actually be useful.

This makes sense but what then, if i go 3s10p and the first battery in that line fails all those behind it go as wel and might cause dangers in heat and inbalance ? How would you design it ?

There could be a way to do a pcb for this particular application. Again as I mentioned earlier, it would be pricey. However, you could design a pcb thin strip, barely wide enough to run traces to carry the current and a few return lines. Probably would need to be a 3-layer so that you could send signals back in the middle layer. Then use an mcu with a multi-plexer and an ADC so that it can scan all cells connected.
This would be very tedious either way.

you lost me here tbh but i am intrigued although i doubt its within my skillset.

 

[Korishan]

Don't understand this is in a car application for a test build and practical application

Lithium-Ion cells have a voltage range of 3.2 - 4.2V
For a 3S configuration this would be 9.6 - 12.6V
For a 4S configuration this would be 12.8 - 16.8

With a 3S configuration your alternator will overcharge the cells beyond 4.2V. Charging voltage is 14.5 - 15.5V. At best, 14.5 / 3 = 4.8V, at worst that's 5.17V. This will cause the cells to explode possibly causing severe damage.
Also with a 3S configuration you will need to pull at least 300A to start the vehicle. You would need at least high drain cells capable of 30A each, or you would need a lot of cells in parallel to be able to deliver the required amps. Do NOT attempt to do that with recycled cells from laptop or other similar devices.

With a 4S configuration if your battery is ever left without being charged because the engine isn't running (leaving the lights on, using the radio, etc) then the voltage can drop below the safe limit for these cells and they will be compromised and weaken each time this happens. Possible situations include drastic decrease in capacity to high internal resistance that can cause overheating during charging, which can lead to a fire.

The only safe Lithium based cells you should use in a vehicle that utilizes at 12V system at which the battery will also be at 12V nominal, is Lithium Iron Phosphate, or LiFePO4, cells. These types of cells operate at a lower voltage range than Lithium Ion cells.

Their operating ranges are closer to 2.8 - 3.6V, or for a 4S configuration 11.2 - 14.4, which is extremely close to Lead Acid batteries. Lead Acid batteries operate between 11 and 14.5V.

This makes sense but what then, if i go 3s10p and the first battery in that line fails all those behind it go as wel and might cause dangers in heat and inbalance ? How would you design it ?

I recommend that you read the FAQ that is located on the home page. There are several threads that cover cells, arrangements, bms configurations, etc.
In short, if a cell in a parallel group goes bad, the BMS (a smart one, not a cheap non-reporting one) will be able to report that a group a cells are operating out of spec.

you lost me here tbh but i am intrigued although i doubt its within my skillset.

This requires learning how to design and build PCBs and understanding how to program microcontrollers, even the fundamental basics. I would not recommend going down this path if you have no idea how to do these things. Learn the theory and watch lots of videos and read lots of articles on the topics before trying to do such a task.
I say this because of safety concerns. I don't want anyone's house, garage, vehicle, etc to be destroyed or anyone's life injured, or worse.
Do not just take some Joe Smo's devices, programs, pcbs, bms, etc., off the internet and slap them together with your cells/packs and not have the basic understanding of what's going on.

Please ask more questions, for sure. That is how we learn. I just don't want someone diving into the deep end of the pool and not know how to swim.

 

[Redpacket]

To clarify, are you:
- trying to build an electric car powered by lithium cells?
- trying to build a starter battery for a regular petrol, etc engine car ?
- trying to power other devices in a vehicle & recharge from the car's 12V system?

As Korishan & others have said above, lithium Li-ion (typical 18650 cells) need the right voltages & current flows or they will definitely be a serious fire/explosion hazard.
Only the last item in the above list is practical with 3s 18650's & then still need the right charging & BMS systems to be safe.

 

[spacecabbie]

Lithium-Ion cells have a voltage range of 3.2 - 4.2V
For a 3S configuration this would be 9.6 - 12.6V
For a 4S configuration this would be 12.8 - 16.8

With a 3S configuration your alternator will overcharge the cells beyond 4.2V. Charging voltage is 14.5 - 15.5V. At best, 14.5 / 3 = 4.8V, at worst that's 5.17V. This will cause the cells to explode possibly causing severe damage.
Also with a 3S configuration you will need to pull at least 300A to start the vehicle. You would need at least high drain cells capable of 30A each, or you would need a lot of cells in parallel to be able to deliver the required amps. Do NOT attempt to do that with recycled cells from laptop or other similar devices.

Ah now I understand what you are getting at.
Yes this definitely had occurred to me I had thought A BMS would protect the cells from this. Also this battery wil by no means provide starter power for the exact reason you mentioned it is to much amps I am comfortable with it providing. I am wondering though the whole explanation above, as I understand is this not (i thought) exactly what a BMS i suppose to prevent ? Drain and overcharge ?

I recommend that you read the FAQ that is located on the home page. There are several threads that cover cells, arrangements, bms configurations, etc.
In short, if a cell in a parallel group goes bad, the BMS (a smart one, not a cheap non-reporting one) will be able to report that a group a cells are operating out of spec.

I had read them all i thought, checked them again i do not seem to find one about cell arrangements ? Although the fuse part made a few things more clearly and developed the following idea:

https://imgur.com/a/24li0jD

View: https://imgur.com/a/24li0jD
(don't laugh at my design skills)
This way all cells can break without impacting the others witch is the most important for me.

Please ask more questions, for sure. That is how we learn. I just don't want someone diving into the deep end of the pool and not know how to swim.

I completely understand I do hope that via the questions i have been asking, Shows i do not take building these packs lightly. I learn 1000x fold by watching, listening and understanding the base-logic. I can read 2 textbooks and understand nothing while sitting down with an expert for a 1h chat I am able to learn more then 10 books combined.
So intrigued should not be read as cool lets build one, but more like I am already looking into PCB's. Din't think about the fact you could build your own BMS interested to see how it looks. Shit I can't even fix a very simple LCD driver pcb witch i have been using as demo practice thingy.

To clarify, are you:
- trying to build an electric car powered by lithium cells?
- trying to build a starter battery for a regular petrol, etc engine car ?
- trying to power other devices in a vehicle & recharge from the car's 12V system?

As Korishan & others have said above, lithium Li-ion (typical 18650 cells) need the right voltages & current flows or they will definitely be a serious fire/explosion hazard.
Only the last item in the above list is practical with 3s 18650's & then still need the right charging & BMS systems to be safe.

Righto I keep doing this I am a shit writer and communicator in writing... So for me its obvious I am not going to use it as a starter or power a car
So yes this is a test build and at the same time to fulfill a need i have. basically want to make a about 50ah battery for my trailer to power lights etc when not connected. So it wil need to provide 12v automotive range witch is from my experience between 11.6 and 14 (low battery and max alternator)
Since this pack wil need very good insulating/protecting not only for cold/heat but rain as wel and a possible impact of tiny rocks. I am so ademend on monitoring or beter said isolating independent cell if they fail but found a solution to this i think. see above.

 

[Korishan]

what a BMS i suppose to prevent ? Drain and overcharge ?

Yes, a good one can do this. However, they aren't always 100% failsafe. You don't build your battery to let the failsafe device work as your breaker. This will cause problems as the BMS will "disconnect" the battery from the load/charger. In all intents and purposes, this would be like you removing the battery terminal or throwing a switch. I'm sure you don't want this kind of operation.

I had read them all i thought, checked them again i do not seem to find one about cell arrangements

What does S and P stand for? What is Xs, Yp or XsYp with X and Y being numeric digits?

What does S and P stand for? What is Xs, Yp or XsYp with X and Y being numeric digits? This is the notation for the configuration of the battery. S means Series, P means Parallel. XsYp means X cells in series (to get the desired voltage) and Y cells in parallel (to get the desired capacity). If...

secondlifestorage.com

What's all this about the different chemistries?

11. What's all this about the different chemistries? This whole topic has a backpack full of naming issues, not only because people use it wrong or simplify it, but also because the manufacturers don't always state the complete chemistry and because there are "in between / hybrid chemistries"...

secondlifestorage.com

What lithium configuration do I use for 12V, 3s or 4s?

What lithium configuration do I use for 12V, 3s or 4s? In a perfect world, Neither This is the major case described under Why-are-12V-24V-and-48V-widely-used-defaults where the chemistry prohibits the effective use within the 12V system: 3s lithium batteries have a range of ~9.0V when empty to...

secondlifestorage.com

This way all cells can break without impacting the others witch is the most important for me.

Yes, fusing is recommended, especially on recycled cells that have signs obvious wear (meaning they no longer have the same capacity as when they were new, so their life cycles in unknown).
Fusing is so that if a cell goes Self-Discharge Dead Short, becoming a heater, it will pull enough current to pop the fuse. This is why the fuse needs to be rated for around 2 - 5A.

Din't think about the fact you could build your own BMS interested to see how it looks.

Yeah, this is the difficult part. But I wasn't referring to building the BMS itself, but more the power rails that would bridge the cells together. The BMS would still be a premade device that is mass produced, or one similar that has already stood the test of time and many users, like the diyBMS by Stuart Pittaway. There are few others that are discussed here on the forums as well

basically want to make a about 50ah battery for my trailer to power lights etc when not connected. So it wil need to provide 12v automotive range witch is from my experience between 11.6 and 14 (low battery and max alternator)

Ok, this makes things a LOT easier to work with.
First off, go with 24VDC, not 12VDC. So that'd be 7s.
Secondly, you'd use a 'Buck Converter' to lower the voltage down to 13-14V (this is the operating voltage while the vehicle is running).
Third, you'd use a charge controller that can take 12VDC and charge output 24VDC.
3a: Or you use a 'Boost Converter' to bump the 12V up to 30V and then use a regular 24VDC charger to charge the battery.

This seems a bit more complicated than it really is. But, it would be a lot safer in the end, more reliable, and easier to work with. Plus you get the added bonus of learning more skills as you'd need to build these things together.

need very good insulating/protecting not only for cold/heat but rain as wel and a possible impact of tiny rocks. I am so ademend on monitoring or beter said isolating independent cell if they fail but found a solution to this i think. see above.

Easy, don't put the battery pack under the vehicle

For insulation, you'd be looking more for large heatshrink wrap. This would help protect the battery pack from weather fluctuations and small debris and splashes of liquid. But the unit should still be inside of a protective box of some sort with all the other equipment for monitoring.
You might want to look into the BMS systems that have Bluetooth connectivity. That way you can monitor what's going on with the battery/bms on your phone/tablet.

 

[spacecabbie]

Yes, a good one can do this. However, they aren't always 100% failsafe. You don't build your battery to let the failsafe device work as your breaker. This will cause problems as the BMS will "disconnect" the battery from the load/charger. In all intents and purposes, this would be like you removing the battery terminal or throwing a switch. I'm sure you don't want this kind of operation.

Well. Its exactly what I want it to do just like pbc in the makita battery's Iwant it to stop charging or warn if there is an issue.
Ps. I understand I am new here but lets assume I wil not use shit BMS cause imho you might as wel not use a bms.
Any bms I want to use in a pack with more then 10Ah would need to have monitoring for:

Temperature
Voltage's Charging / Load / Both
Current Charging / Load / Both
Voltage and Current per S lines so 7 in this case.
Shortcircuit Protection
Its what I concider a must have these things scare the living shit out of me. You should see me work any spark and I am of running.

What does S and P stand for? What is Xs, Yp or XsYp with X and Y being numeric digits?

What does S and P stand for? What is Xs, Yp or XsYp with X and Y being numeric digits? This is the notation for the configuration of the battery. S means Series, P means Parallel. XsYp means X cells in series (to get the desired voltage) and Y cells in parallel (to get the desired capacity). If...

secondlifestorage.com

What's all this about the different chemistries?

11. What's all this about the different chemistries? This whole topic has a backpack full of naming issues, not only because people use it wrong or simplify it, but also because the manufacturers don't always state the complete chemistry and because there are "in between / hybrid chemistries"...

secondlifestorage.com

What lithium configuration do I use for 12V, 3s or 4s?

What lithium configuration do I use for 12V, 3s or 4s? In a perfect world, Neither This is the major case described under Why-are-12V-24V-and-48V-widely-used-defaults where the chemistry prohibits the effective use within the 12V system: 3s lithium batteries have a range of ~9.0V when empty to...

secondlifestorage.com

Ow duh Yes I know the base of this I thought you meant a specific layout like the little drawing i made how to exactly place the fuse wires and battery's
But I understand S and P and how you make different voltages or paralel

Yes, fusing is recommended, especially on recycled cells that have signs obvious wear (meaning they no longer have the same capacity as when they were new, so their life cycles in unknown).
Fusing is so that if a cell goes Self-Discharge Dead Short, becoming a heater, it will pull enough current to pop the fuse. This is why the fuse needs to be rated for around 2 - 5A.

Yes I like the fuse Idea kinda love it then with a bms I could read out capacity occasional or set a warning threshold if a certain lane has to many cell's gone.

Ok, this makes things a LOT easier to work with.

I really should have been more specific sorry about that, Its a bad habit I always assume people think what i am thinking

First off, go with 24VDC, not 12VDC. So that'd be 7s.
Secondly, you'd use a 'Buck Converter' to lower the voltage down to 13-14V (this is the operating voltage while the vehicle is running).
Third, you'd use a charge controller that can take 12VDC and charge output 24VDC.
3a: Or you use a 'Boost Converter' to bump the 12V up to 30V and then use a regular 24VDC charger to charge the battery.

Shit. I was ready and set to go Yes.... But I can't charge 24v with 12v its why I din't look into it (there I said it anyway )
Anyway thats brilliant so this would do for example ?
https://nl.aliexpress.com/item/1005003229120906.html?channel=twinner
If thats the case thats briliant II could basicaly input 5v-60v to charge it or if I vind a dual input one have 220 and 12v charge.

This seems a bit more complicated than it really is. But, it would be a lot safer in the end, more reliable, and easier to work with. Plus you get the added bonus of learning more skills as you'd need to build these things together.

Not at all apart from the charge controller the rest i know but the added soldering and building a electronic center is cool

Easy, don't put the battery pack under the vehicle

Wel duh, that's not even legal unless protected bij metal case.
But my point was more why I needed it fail safe because once wrapped and placed in case its painful to open up and don't plan to unless allot of cells died.

You might want to look into the BMS systems that have Bluetooth connectivity. That way you can monitor what's going on with the battery/bms on your phone/tablet.

I kinda think is a must with the requirements i have but why do they have to be so damn expensive.
And also why is there no BMS-Charge Controller thingy !

 

[Korishan]

If any cells are going to fail, you'd need to open up the pack anyways. You need to get the bad cell out that's causing the issue. Otherwise you're just adding fuel to the fire and wasting energy.

A BMS that cuts off power during overcharge is what you want in a failsafe. Meaning that you don't want it to keep happening every time you charge the battery. You want the "charger" to stop charging once the battery is full.
One of the reasons is that if the BMS cuts power during a full amp charge, it could damage the BMS and/or the Charger. Disconnecting under heavy amp load is not a good idea and best to stay away from it if at all possible. Doesn't matter if the switch is a relay or a bank of mosfets. High amp disconnect is hard on electronics. It'd be like throwing your vehicle into park every time you wanted to stop. Your transmission could probably do it, at least for a short period of time. But eventually, it's going to be overloaded and fail. That's why we have brakes on vehicles instead, to slowly bring the vehicle down over a period of time.
The other reason is that loads are usually after the BMS, not between the BMS and battery. So if the BMS disconnects, then any loads connected will loose power. Even if the charge controller is still powered, depending on the controller, it could disconnect because it doesn't detect any battery voltage.

Easy, don't put the battery pack under the vehicle

This was a joke, btw I knew it wasn't going under the vehicle. Just that you mentioned you wanted to protect it from rocks and water, which makes it sound like it won't be inside of a vehicle.

Anyway thats brilliant so this would do for example ?
https://nl.aliexpress.com/item/1005003229120906.html?channel=twinner

Well, kind of. This might work. I have not tested these using a 12VDC stable input voltage. I suppose theoretically it should work, though.

 

[Redpacket]

Yes any battery pack you make needs to be properly electrically insulated but you'll find over wrapping them is not really helpful.
For trailer use, you also need to consider vibration & movement.
Maybe make robust smaller packs & link with flexible short cable links vs stiff bus bars.
Re the environment, using one or more metal boxes, eg ammo cases or tool boxes, etc, etc is a good idea.
You could thermally insulate the outsides of those or the whole battery system area. You need to keep the cells in the 0 to 40 degC range.
Water & flying stones (or other physical damage) can't get anywhere near your pack. One dent = unsafe cell, remove from service asap.
Moisture, even condensation is also a problem & will destroy your packs.
Plan to use circuit breakers (separate for charge & load sides) & cell fusing in the packs.

 

[spacecabbie]

If any cells are going to fail, you'd need to open up the pack anyways. You need to get the bad cell out that's causing the issue. Otherwise you're just adding fuel to the fire and wasting energy.

Ok I have to keep remembering what I consider a cell is possibly not what you consider a cell. So let me rephrase cell to 18650 cell. (I find it hard to redefine names)
Just a quick recap for my self: cell is the ones that make up the voltage combined so in the current design all 18650's under the 7 groups. The 18650's them self how are they referred to when in a s7p10 config for example ?

Just to make clear if I have s7p10 pack and one or 2 18650 fails in any "cell" that would be ok for me the fuse wire wil cut them of. If of-course a whole cell cuts out this of course means you have to go in.(Did I apply the correct terminology this time ?) Brings out a interesting question though how many 18650 can you let fail before you would go in and replace? In theory this design could handle 60 failed 18650 as-long as they are in there separate cells. The capacity would be greatly diminished but voltage stable.

Ps. I see you can, unsure if you are allowed. But If the above use of cell is still used incorrect, I have no objection for you to edit my post to adjust it. Since I do believe you will understand what I mean.

A BMS that cuts off power during overcharge is what you want in a failsafe. Meaning that you don't want it to keep happening every time you charge the battery. You want the "charger" to stop charging once the battery is full.
One of the reasons is that if the BMS cuts power during a full amp charge, it could damage the BMS and/or the Charger. Disconnecting under heavy amp load is not a good idea and best to stay away from it if at all possible. Doesn't matter if the switch is a relay or a bank of mosfets. High amp disconnect is hard on electronics. It'd be like throwing your vehicle into park every time you wanted to stop. Your transmission could probably do it, at least for a short period of time. But eventually, it's going to be overloaded and fail. That's why we have brakes on vehicles instead, to slowly bring the vehicle down over a period of time.

This is something that I din't knew but along with the common used visualization of "electricity" as flowing water, it makes allot of sense (wide open hose suddenly closed do it often enough the hose wil break). This stuff is gold for me cause when i can follow the logic like that its the most clear to me thanks.
Still though I would expect a BMS to do both actually but from this I deduce that proper BMS do this (ie. smart etc) and cheap one's just wait till overcharge happens then cuts off?

The other reason is that loads are usually after the BMS, not between the BMS and battery. So if the BMS disconnects, then any loads connected will loose power. Even if the charge controller is still powered, depending on the controller, it could disconnect because it doesn't detect any battery voltage.

Yea got it I think BMS is more like main breaker you don't want it to trip. Still would suspect these 2 functions to be in all BMS and am surprised for example there is not even a BMS with charge controller in one. Seems that like always producers are slapping a this is a BMS on anything remotely related......

This was a joke, btw I knew it wasn't going under the vehicle. Just that you mentioned you wanted to protect it from rocks and water, which makes it sound like it won't be inside of a vehicle.

Wel it won't be But i got the intent my reply din't properly reflect that.

Well, kind of. This might work. I have not tested these using a 12VDC stable input voltage. I suppose theoretically it should work, though.

Why would it not work in practice? Logic would dictate fluctuating input is more likely to break it even though designed for it.?

Btw is there a device capable of dealing with input power/monitoring/failsafe of the whole battery? They all communicate to each other I figure why seperate them.

I got enough info to start building the base cells so will show case my design when done might be a while though.

 

[Korishan]

Just a quick recap for my self: cell is the ones that make up the voltage combined so in the current design all 18650's under the 7 groups. The 18650's them self how are they referred to when in a s7p10 config for example ?

A cell is the smallest base component that creates the voltage/current. Some times cells are not exterior. An example would be a 12V lead acid battery, that has 6 cells internally.
However, with 18650's, or similar types of batteries, are considered the cells when using them in groups. Same is said for AA, AAA, C, D cells as well. If they are used singularly, they called batteries. When they are used in groups, especially increasing voltage, they are called cells.
Groups connected to increase voltage/capacity would be 7s10p. This is also called a 'string' of cells.

Just to make clear if I have s7p10 pack and one or 2 18650 fails in any "cell" that would be ok for me the fuse wire wil cut them of. I

If a cell goes "short circuit", it may blow the fuse. It will only blow the fuse if it pulls a large amount of current to cause the fuse to pop. Not all cells will fail this way, though. Others may just loose capacity rapidly. This causes a cell to get hot during charging. Not that it pulls more current, it just burns the energy sent to it into heat. This is why thermal sensors/probes are recommended as well.

Brings out a interesting question though how many 18650 can you let fail before you would go in and replace?

You don't want to let any of them stay in the pack failed because it will cause other issues. Even if the fuse blew and is never used again. It causes capacity imbalances.
If you have 7s10p, and each cell (individual 18650) is 2000mAh, that means that each parallel group of 10 is 20Ah. If a single cell dies and is disconnected, then a single pack in the string now is 18Ah. So during charging, that 18Ah pack will reach full capacity before the other 6. Now the bms balancing has to kick in to burn off the energy from that pack to maintain the voltage until the other 6 can catch up.
Capacity, Ah and mAh, is similar to a bucket. If you have a 2L bucket vs a 1L bucket, the 1L bucket will get filled up faster than the 2L bucket given that the flow is 1L/m. To keep them balanced, you poke a hole in the top of the 1L bucket to let water out until the 2nd 2L bucket is full. The water that is let out is wasted and cannot be reclaimed (this is the simplest form, and there are ways to reclaim, but will discuss it here atm. and it's not recommended really).
So, you fix the problem before it becomes more of a problem. If those cells aren't bled off by burning the energy then the lower capacity cells could become over charged and cause a fire.

Still though I would expect a BMS to do both actually but from this I deduce that proper BMS do this (ie. smart etc) and cheap one's just wait till overcharge happens then cuts off?

There are different types of BMS's, yes. Some just disconnect. Some tell the charger to back off. Others can divert the excess energy somewhere else. Some have all these features, some have none.

Still would suspect these 2 functions to be in all BMS and am surprised for example there is not even a BMS with charge controller in one.

There are charge controllers that have BMS's, to a degree. They are a lot more expensive, though, than having the units separate. We're talking into the several of thousands of USD$$

Why would it not work in practice? Logic would dictate fluctuating input is more likely to break it even though designed for it.?

It should work in practice. Yes. But I'm not sure how well it will work. Only one way to test it out
The fluctuating voltage really doesn't make a difference on the components. If you compare again to a water hose, then you could say the output has a spray nozzle that allows 1L/h. The source provides up to 5L/h. as long as the pump doesn't drop below 1L/h, the nozzle output won't be affected. But if the pump does drop below 1L/h, then then output pressure drops at the nozzle.

Btw is there a device capable of dealing with input power/monitoring/failsafe of the whole battery? They all communicate to each other I figure why seperate them.

Yeah, the Batrium BMS is one of these types of units. It can communication with other components in the system using the CAN protocol.

 

[spacecabbie]

If a cell goes "short circuit", it may blow the fuse. It will only blow the fuse if it pulls a large amount of current to cause the fuse to pop. Not all cells will fail this way, though. Others may just loose capacity rapidly. This causes a cell to get hot during charging. Not that it pulls more current, it just burns the energy sent to it into heat. This is why thermal sensors/probes are recommended as well.

We agree already think temps are a must.

You don't want to let any of them stay in the pack failed because it will cause other issues. Even if the fuse blew and is never used again. It causes capacity imbalances.
If you have 7s10p, and each cell (individual 18650) is 2000mAh, that means that each parallel group of 10 is 20Ah. If a single cell dies and is disconnected, then a single pack in the string now is 18Ah. So during charging, that 18Ah pack will reach full capacity before the other 6. Now the bms balancing has to kick in to burn off the energy from that pack to maintain the voltage until the other 6 can catch up.
Capacity, Ah and mAh, is similar to a bucket. If you have a 2L bucket vs a 1L bucket, the 1L bucket will get filled up faster than the 2L bucket given that the flow is 1L/m. To keep them balanced, you poke a hole in the top of the 1L bucket to let water out until the 2nd 2L bucket is full. The water that is let out is wasted and cannot be reclaimed (this is the simplest form, and there are ways to reclaim, but will discuss it here atm. and it's not recommended really).
So, you fix the problem before it becomes more of a problem. If those cells aren't bled off by burning the energy then the lower capacity cells could become over charged and cause a fire.

Ok unsure to disagree or misunderstanding usage of reclaimed cells.
This would mean its best to just use new cells with reclaimed ones you always have a risk of failure. This is why you take it into account there is inbalance but a bms would compensate this indeed.

Don't get me wrong but I fear this is more professionalism vs practical vs Cost discussion. Still I wil not ignore advise I am hoping you are flexible to sidestep with me here: Is it absolutly mandatoy to take them out or is indeed de bms to make up for it a perhaps not best but ok cost solution ? Or if course am in missing something al together

There are different types of BMS's, yes. Some just disconnect. Some tell the charger to back off. Others can divert the excess energy somewhere else. Some have all these features, some have none.

Yea I looked afther door nr1 then 2 3 and all upto 1000 I gave up for now. Damn there is shitloads but seems to be eigther cheap or expensive no middle way Would you be able to recommend 3 BMS ? 1 for budget 1 for average 1 top of the line most importaintly why you would choice them so i can see what bms options to look for cause its a real maze out there.

It should work in practice. Yes. But I'm not sure how well it will work. Only one way to test it out
The fluctuating voltage really doesn't make a difference on the components. If you compare again to a water hose, then you could say the output has a spray nozzle that allows 1L/h. The source provides up to 5L/h. as long as the pump doesn't drop below 1L/h, the nozzle output won't be affected. But if the pump does drop below 1L/h, then then output pressure drops at the nozzle.

I am going to find out orderd one.

Yeah, the Batrium BMS is one of these types of units. It can communication with other components in the system using the CAN protocol.

Yea batrium looked nice till i saw the prices thats not even expensive that something new altogether.

 

[Korishan]

The BMS isn't designed, or meant to be, a device to "compensate" for damaged cells. It's there to protect the system for a faulty cell(s) until they can be removed/replaced (or in the world of commercialism, the whole pack tossed and replaced).
For a BMS that reports the data, it's there so you can detect the fault early and get it fixed early.

Of course, it's always best to purchase and use new cells compared to reclaimed ones, for cost, reliability, safety, etc. There is an inherit risk by using reclaimed cells. That is why there are tests that should be done to ensure the best ones from the get go.
What I'm stating is that each pack of parallel cells should be as close to the same capacity as the others in the string. This is critical and crucial. This means that if cells fail in a pack, especially a small pack, they need to be replaced asap to restore proper balance. Otherwise you are putting the rest of the cells at risk, not to mention wasting energy that could otherwise be put to good use. Also putting undo stress on the BMS that isn't needed.

There are some inexpensive BMS's out there. as I mentioned the one called diyBMS is a good one. It's very DIY, though. You buy the components and assemble them yourself, and then flash the code to the chip. And then you can use a web browser to look at and configure the device.
There's others as well. Batrium is more on the end of professionally done with loads of extra features most ppl wouldn't use. A bit overpriced, in my opinion.