are fuses a good idea?

[Jonisingt35r]

well i did plan a really good design where the fuses would be protected, as for the correct fuse it could be reaally difficult to fit the correct one as im sure cells characteristics would change a bit i may just fit fuses and see how it runs, i have two motors on the way, i do intend just using the same cells recycled ones only my best ones, will definatly find out what their discharge current is and use that, didnt think about that so thanks, i will use these fuses or simular if i can get a speck sheet, and lay out a small pcb it will be reayy well protected

https://www.aliexpress.com/item/33012231...b201603_52


yes i am going to use a bms, a lot of skateboards just use new cells without a bms, think it would be ok for a while....

i did concider no bms, just thought about balancing it every couple of charges, but that cheap chinese bluetooth one is pretty good

[daromer]

As Said i would not use fuses...
I would either Charge with RC Chargere
With balance function or CC cv with bms

Just make sure having Main fuse.

That way you have a solid battery that Will work

[Crimp Daddy]

For skateboard no fusing.  That pack need to be rigid and support high current.

I would never put cell level fuses on such a device.

Just spotweld the pack together and make sure it can take a beating. Charge it with RC charger or with proper CC CV with a good BMS that handle the balancing and if something critical happen.
If you use RC charger a BMS is really not needed unless you are keen to protect it from overdischarge.

And for people now guessing and such this is how you do it in the industry.

Whats important is that you have cells that can cope with the stress and load. No fuse can save you from a poor design from the start.

This, 100%

---

For skateboard no fusing.  That pack need to be rigid and support high current.

I would never put cell level fuses on such a device.

Interesting - I thought fusing was always recommended.   Can you explain a bit why fusing is not recommenced in this case?   What's the thought process or guideline to go fuse or non-fuse.

Its extremely rare to find cell level fusing on packs for applications like this "high performance batteries"... ebikes, skateboards, RC, hoverboard, powertools...  

The packs are not large enough... meaning that your current requirements are large enough that you cant reliably put a small enough fuse for it to really matter.

The fuse would be sized such that even if the cell shorted out, it might not blow with the inrush from the other cells.

All you are doing at this stage is adding a point of increases resistance, heat, voltage drop, fragility in construction.

I do however treat and maintain my packs very similar to how people would in the RC world.  Balancing charger, with no BMS, and diligent monitoring of general health.  Noting capacity, balance, and internal resistance... just like many do in RC.

[OffGridInTheCity]

Have you ever seen a commercial pack have per-cell-level fuses?. Its not answer to your question but the the cell level fuses was started by us due to coupling 100s  of cells in parallel and in an area for instance a shed or workshop. This along with what Tesla did to their packs. Their intention on other hand was most likely reason on swapping out the CID against a cell level fuse. On their packs they have a different system so cant be fully compared but in some areas they can

Doing that you wanted an extra level of security. That security extra added is of course not bad but you need to weight in pros and cons.
Those fuses are fragile and do not make up for any rigidity. They also only work for low current applications.

Ebike packs and small other packs i would never use cell level fuses due to this. I want rigid packs. In my world a fuse wire or other can create more issues than they solve on such a pack.

Cell level fuses in my world exist on packs for powerwalls where you have this criteria fulfilled
* They are not moving around
* You have huge amount off cells in parallel like 80p or more
* Low current
* Random built packs
* Tested with low cost testers and where you might not use ir as a factor
* 2nd hand cells
* mixed age and abuse
* Permanently mounted

Ebikes for me are
* High current packs
* They are moving around and need to be rigid
* should be built with considereation of ir and type of cells. May not always be the case though...
* lower number of cells
* Packs arent permanently mounted. (I dont call an bike or skateboard pemanent)

You can mix this a bit as you want but this is my stand in the point. Security and safety are good but you need to weight in how and why you do it against risks not doing it.

Same goes with BMS... You dont need a BMS on a pack you run the RC-Style = Charged with balancing charger.  In worst case you just drain it way beyound the max... Of course you need to make sure you cant run to high current but that can be done with proper fusing.

I have no BMS on any of my RC gear. I dont have it on most of my Ebike gear either that is swap-in out.

BUT
I have it on all ebikes charged with CC-CV or statically built packs.

This recap makes perfect sense - thank you!

[Earlonics]

Sorry for butting in here, but since this post is talking about fuses, I'm going to use 2amp on my 18650's 14s100p cells pack.

Can anyone offer advice on the best place to purchase such fuses or fuse wire, I see such a variety of sources, would be good to go to a reliable and guarantee source.
Thanks in advance

[OffGridInTheCity]

Sorry for butting in here, but since this post is talking about fuses, I'm going to use 2amp on my 18650's 14s100p cells pack.

Can anyone offer advice on the best place to purchase such fuses or fuse wire, I see such a variety of sources, would be good to go to a reliable and guarantee source.
Thanks in advance

Sounds like you want something like this:   https://de.aliexpress.com/item/32739650661.html?spm=a2g0o.detail.1000023.13.20a12a39HgK3Zd
(I can't endorse this specific product - just using it as an example)

FYI - Here's AveRage Joe soldering in Axial Glass Fuses - https://youtu.be/5vKJhTEqpyU

[not2bme]

Sorry for butting in here, but since this post is talking about fuses, I'm going to use 2amp on my 18650's 14s100p cells pack.

Can anyone offer advice on the best place to purchase such fuses or fuse wire, I see such a variety of sources, would be good to go to a reliable and guarantee source.
Thanks in advance

If you're looking for fuse wire, search for tinned copper wire. A 30awg wire will fuse at around 7-10A. That's safe enough yet but large enough that there won't be enough resistance to cause too much loss. I prefer fuse wire because it's easy to spot weld in a row. A glass fuse seems quite difficult to apply.

If you're in the US, you can purchase it on amazon or ebay. Here's just one link.

https://www.amazon.com/Remington-Industr...B01M0AXONC

[MattsAwesomeStuff]

I know it's a bit of an older thread... but... I don't agree with what some people are posting here. I think there's some misconceptions.

1 - The purpose of cell-level fuses is not to protect the cell if you tried to short the output, the way you typically think of a fuse. Most cells will be just fine if you try to abusively-quickly discharge them. Their internal resistance will spike and they will self-limit (somewhat) the amount of current that can flow out of them because the chemistry is limited at the rate at which it can act. They may get hot, but they'll soon be empty. They may be damaged by the heat and the higher-than-designed discharge, but that's okay.

In a many-parallel design, as soon as one cell gets hot, its resistance goes up, which means, relatively, it has lower voltage than the rest of the pack, so any current demands put on it by the load will be lightened on that cell and picked up by the rest of the pack. This also means that the entire pack is self-balancing load-wise. Cells in parallel will naturally share the load (and heat) equally.

It is electrically impossible for one cell to attempt to hog more of the load than others. That's what being in parallel means. Ditto for in series, they're forced to have the same current draw, that's Kirchoff's laws.

2 - That means that there's no point in having a cell-level fuse to protect each cell, only a single fuse for the entire pack. If they all share the load equally, then the only load you have to worry about being excessive is the total load on the output terminals of the battery. So yes, use one big fuse there.

3 - The purpose of cell-level fuses is to protect the entire pack from burning down if one cell fails. Most of the time when a cell fails (which happens somewhat randomly) it will fail "open", meaning the circuit through it is dead, like if you cut the ropes to a bridge, electricity will not get to the other side. Occasionally, rarely, when a cell fails it will fail "closed", meaning the circuit is shorted. If that happens, it means the entire energy of the entire parallel bundle is going to instantly discharge through that cell (which is basically equivalent to a wire across the + and - terminals). Instantly, that cell is going to reach ignition temperatures and liquify/blow up. The heat from which may set off its neighbors too in a chain reaction. So you have individual cells fuses to protect this. If a massive load is suddenly trying to discharge through that cell, something is wrong with that cell, and the fuse blows so that it fails "open". This removes one cell from the circuit, but otherwise has no impact.

...

So I don't get any of the logic which says that E-bikes and power tools should not have cell-level fuses.

Teslas have cell-level fuses, in high-current applications, that aren't rigid, with permanently mounted packs, that are massive numbers of cells. Pretty much shoots that argument in the foot.

However, the fuse only has to be thin enough to blow when the whole pack discharges through it, before that cell would overheat. Just about any connection, including nickle strips, will act as a fuse when an ungodly amount of current ramrods their way through it.

I wouldn't worry with actual fuses or delicate fuses. I don't know that I'd bother with anything smaller than 24g copper wire. Might as well just rely on the connections to the cells in most cases.

[daromer]

Tesla dont have CID that protects against a short and will therefore not ahve any over pressure protection
A shorted cell will get DEAD hot untill the CID actually disconnects the positive end. On tesla they added a cell level fuse to protect against such element to happen.

Cells will not have equal current unless they have 100% matched resistance. Ohms law. This is perfectly true on powerwalls built from 100s of different cells. Also notes as that dendrites built up and the unknown factor the cell level fuse add an extra level of protection "if". It doesnt really cost any extra not adding it. There is also a thread here when many tests have been done showing how unequall cells distribute current if not built properly

The cell level fusing is only to protect towards 1 cells being shorted or had issues where the other cells will push current in or out of it. It can of course protect against full discharge but that should be protected by the main fuse. ANd as you said in #1 short on the total battery pack you have main fuse to protect towards. Cell level fuses have very little interaction in that moment and should never popp.

Ebikes dont have that many cells and is generally built from new high quality cells. Powertools as well. A powerwall is built from 2nd hand cells and some times only 50 and some have 500 in parallel.

You can always argue if to have or not to have

[MattsAwesomeStuff]

Cells will not have equal current unless they have 100% matched resistance. Ohms law.

Yes, but you missed the part about them being self-balancing.

The thermal coefficient of resistance is positive. When a cell gets warmer, its resistance changes. So a warmer cell will have higher resistance, which means less current flows through it, which makes it cooler and the rest of the pack slightly hotter by carrying the load.

The miniscule amount that cells will have their resistances mismatched is completely swamped by the responsiveness of a conductor's resistance with respect to temperature.

The greater the imbalance, the stronger the change in resistance, and the stronger the balancing effect.