18650 fusible link benefits

[11spokes]

Hi, I'm looking into building my own 18650 battery pack. Can anyone tell is it important to have fusible links to every cell. I have noticed most of the people on here don't bother with it. So what are the possible risks in not using fusible links? I have noticed em3ev now use fusible links and have bought a pack from them before, there batteries seem to be of a very high standard. Are there any other e bike battery manufacturers building packs in the same way? I am also wondering where the best place to buy 18650s would be I would like to get Samsung 30qs. I have noticed a few people buying from Nkon are they any good? What would be a good spot welder to buy and where? Also nickel strip and cell holders. If anyone could give me some advice on a firs timer building a pack it would be greatly appreciated!
Thanks guys!

 

[amberwolf]

So what are the possible risks in not using fusible links?

One cell in group fails shorted, unfused parallel cells feed all their power thru that cell and heat it, potentially to fire point, then it's burning sets off the other cells, etc.

If you don't get any specific responses, some pointers:

There are a number of threads and posts around the forum discussing fused cells, methods of doing it (wire, PTC, cuts in the interconnect strips, etc), but there's no single search that finds them--takes a lot of poking around.

There's also dozens of good threads about building packs, including one or two about how *not* to build them.

A search on Nkon will find various discussion about them, good and bad. Simiilarly, about other places to buy (and not to buy) cells.

 

[jonescg]

I've only ever seen individually fused cells in 18650 packs (mainly Tesla). I've never seen a pouch cell or prismatic cell be individually fused (like say, Leaf packs, Volt packs, Zero MC packs, Mitsubishi iMiEV packs...).

Perhaps its because the risk of a short is higher in an 18650 pack? Dunno...

 

[jonyjoe303]

I have built 3 different 18650 battery packs a 94ah no-fuses (168 cells), a 65ah fuses (90 cells) and a 32 ah fuses (60 cells), I notice that the 94ah battery pack where I use cells from 1100 mah all the way to 2600 mah stays in perfect balance, while the other 2 battery packs where I used cells very close in mah are never in balance when charging.

Also 2 lifepo4 110 ah battery packs (80 cells) each I also use fuses on both packs and they also don't stay in balance. The lifepo4 cells where brand new cells never used.
I been using 5 amp ampere pin fuses, I don't know if that is the reason the packs don't stay in balance. The pack with no fuses never gave me any problems.

Doing some research I did notice that the high amp discharge 18650 (power tool batteries) are better able to tolerate being shorted out without catching on fire. I read the data sheets on some of them, they are tested brutally.

Below is a section of a test they did on one high discharge 18650 cell:
-Overcharge test
Test method: To charge with 20A-20V at 25℃ for 2hr.
Criteria: No fire, and no explosion.
-External short-circuit test
Test method: To short-circuit the standard charged cell (or 50% discharged cell)
by connecting positive and negative terminal by 50mΩ wire for 10min.
Criteria: No fire, and no explosion.
-Reverse charge test
Test method: To charge the standard charged cell with charge current 10A
By 0V for 2.5 hours.
Criteria: No fire, and no explosion.
-Heating test
Test method: To heat up the standard charged cell at heating rate 5℃ per minute up to
150℃ and keep the cell in oven for 10 minutes.
Criteria: No fire, and no explosion.
Many of the packs here at the forum use those high discharge type of 18650 cells and rarely see fuses on any of those packs.
Laptop 18650 batteries don't handle abuse well. I would definitely use fuses on those.

When I build another pack I think I will use fuse wire instead of actual fuses, maybe the fuses I'm using have different tolerances that are unbalancing the pack.

 

[Hillhater]

None of those manufactures tests cover the situation that the individual fuse links are designed for.
The fuse link is intended to protect Parallel groups of cells from a thermal runaway , should a INTERNALshort occur in any one cell. If that happened, the shorted cell would be exposed to the full reverse current load from all the other cells in the parallel group.
In the case of large parallel groups like Tesla ..74-86p.. That would be over 1500amps applied in reverse polarity to one single cell ! Without the individual fuse links to isolate that shorted cell, that would be pretty much a guaranteed plasma situation.
Even in small scale 6-10P paralell groups, the same internal short situation could mean 100s of amps across the bad cell...still pretty much a guaranteed thermal event !
Fortunately , the risk of internal shorts is very low with modern quality branded cells, and of course the risk is reduced again with fewer cells in the pack (Tesla has over 8000, for Ebikes we rarely have more than 1-200 cells.)
EVs like the Prius have no parallel cells, and the Leaf has only a 2P cell configuration so i guess they have tested and shown there is neglible risk of either internal shorts, thermal runaway, or their BMS can somehow detect such issues.

 

[jonyjoe303]

One thing I wonder if there is a collision and the battery pack gets damaged which results in shorted out cell, maybe the fuse in that situation will come into play?
Maybe thats the reason tesla uses the fuses.

But one thing I have notice from opening up ryobi 18 volt li-ions powertool packs to recycle , is alot of overheating damage, many cells were vented, but I never saw any fire damage, those cells had no fuses and took the full brunt of whatever catastrophe happened to them. Laptop batteries on the other hand never once saw a damaged cell but found many at 0 voltage. Also they had no fuses. But both type of packs had at most 12 cells.

Myself I would recommend the fuses just for worst case scenario expecially if your building your own.

As far as spot welders , I bought the arduino spot welder, theres a thread on it here in the forum. I got the version 2, he has come out with version 3 which is a major improvement, cost is 76 euros (about 95 dollars) without the probes, it was around 110 with shipping. I think thats the cheapest spotwelder that runs off a 12 volt car battery/lipo pack. Don't get the ones that need house current I hear they pop fuses and might not last you long. If you build your own probes use 6 gauge pure copper welding cables, they pass all the current to the probe tips letting you use a lower setting which keeps the cables from getting hot.
They are available at the Malectrics website , it ships from germany and I got mine to california a week after I ordered it. I build 2 packs with it so far on the large 32650 lifepo4 which don't solder at all, but tab welding worked great on them. Made quick work of putting the pack together.

 

[11spokes]

Thanks for the replies guys!!

So fusible links is more for the parallel packs this makes sense especially in the case of tesla packs being 86p!

So I have found some battery packs built with fusible links by soldering fuse wire to the cells. This would be an easy way to do it, but what if you want to avoid any heat being transferred to the cells.
It looks like em3ev use some sort of laser cut nickel with fusible links so it can still be spot welded. Does anybody have any pics or ideas on how to do this with a home made packs while still spot welding the cells? I would love to see some alternative ideas to soldering fuse wire for the average diy battery builder? Thanks jonyjoe I will look into the arduino spot welder too!
If I can't find a good idea for fusible links, I won't worry and just spot weld nickel strip.

Anyone have any good ideas for this?

 

[jonescg]

I see that fusible links on prismatic cells are covered in this patent:
https://www.google.com/patents/US20130101892

But I haven't seen many packs made of prismatic cells in parallel - just make a bigger cell.

 

[jonyjoe303]

this is how I did it on my lifepo4 battery pack, I tab welded a small piece of nickle strip to the positive side of battery then solder the fuse to the nickle strip. solder sticks extremely well to the nickle strip and you won't spend too much time soldering it.

 

[11spokes]

Nice! Has anyone else used fusible links or have any pics? I’m still interested in the best way to do this.