Confusing fusewire


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Kafunkel

New member
Joined
May 3, 2020
Messages
2
Hello,

i am Constantin from Germany. Since i started to build a diy powerwall i have read everything and learned the most of you guys here.. so thank you for this.
I harvested 1400 cells that are OK and now i am trying to find the right fusewire. Most people say awg30-awg35 work fine.

I tried a thinner wire because i want a fuse that melts at about 2-2,5A.... so i openeda awg26 networkcable and soldered only 1 of the wires to 1 batterie. The wire is about awg39 or d=0.085mm. I found a list on the internet, itsays awg39 will melt at about 2.12A. I thought yes that could be correct because most of you say that their awg 35 wire melt at about 5A and thats what the list on the internet also tells me.

So i tried to discharge the batterie first with 0.5A then measure the the temperature with a flir camera... everything is fine. I repeated this with 1A then 2A, i expected to heat up the wire with 2A... but nothing... then i repeated this with 3A...4A...5A... With 5A the cell gets quite hot on the Positive +....but the wire didnt melt... i was really surprised and a littlebit confused. Of course when i shortened the cell the wire immediatly burned.

How can this be and whats your thoughts about this?

Some facts:

Cell: Samsung 18650 2600mAH
Wire: AWG 39 (0,085mm) from copper
soldered to +
AWG 26 soldered to -
charger SKYRC Ultimate Duo 400Watt
Discharge Programm with 5A max.
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,073
Kafunkel said:
Hello,

i am Constantin from Germany. Since i started to build a diy powerwall i have read everything and learned the most of you guys here.. so thank you for this.
I harvested 1400 cells that are OK and now i am trying to find the right fusewire. Most people say awg30-awg35 work fine.

I tried a thinner wire because i want a fuse that melts at about 2-2,5A.... so i openeda awg26 networkcable and soldered only 1 of the wires to 1 batterie. The wire is about awg39 or d=0.085mm. I found a list on the internet, itsays awg39 will melt at about 2.12A. I thought yes that could be correct because most of you say that their awg 35 wire melt at about 5A and thats what the list on the internet also tells me.

So i tried to discharge the batterie first with 0.5A then measure the the temperature with a flir camera... everything is fine. I repeated this with 1A then 2A, i expected to heat up the wire with 2A... but nothing... then i repeated this with 3A...4A...5A... With 5A the cell gets quite hot on the Positive +....but the wire didnt melt... i was really surprised and a littlebit confused. Of course when i shortened the cell the wire immediatly burned.

How can this be and whats your thoughts about this?

Some facts:

Cell: Samsung 18650 2600mAH
Wire: AWG 39 (0,085mm) from copper
soldered to +
AWG 26 soldered to -
charger SKYRC Ultimate Duo 400Watt
Discharge Programm with 5A max.
Fuse wire is not exact - but that's OK as the purpose is not to protect a single cell but rather protect the 'rest of the parallel cells' if a single cell shorts. So if you have 50p and 1p goes bad/shorts - then a 49p short will flow thru the 1p (bad cell) and the fuse wire will burn/stop it. Its likely a 49p will produce 100(s) of amps and you're fuse wire will melt. A youtuber named @AveRage Joe has a few videos on 'burn tests' of fuse wire such as this: "Diy Tesla Powerwall ep24 Finally Got The 3 Amp Fuse Wire BUT???" -
Search "Fuse Wire" and you'll find more. This might give a better idea of how fuse wires burn thru.

If you're trying to have an 'exact fuse' on each cell - then perhaps axial glass fuses such as this:
https://www.ebay.com/i/132542407521...MI_NyytveX6QIVFCCtBh1YLAmlEAQYASABEgKpTPD_BwE
would be a better approach as they are better designed to burn thru at a 'more exact' amperage. They come in a variety of amps.

However, as mentioned above - the general DIY pack design (based on Tesla's design) is to protect the pack from individual cells going bad rather than protect each cell.

I use AWG 30 as my fuse wire. It burns thru in the 15-25a range (from anecdotal experience) which is perfectly adequate for my 100p packs.
 

daromer

Moderator
Joined
Oct 8, 2016
Messages
5,662
Thtas not fuse Wire first of all.... Use proper fuse Wire or glass fuses. Note that the fuse on cell level should be atleast 4* your max current.

So that you dont pop that before your Main fuse
 

Kafunkel

New member
Joined
May 3, 2020
Messages
2
Thanks for the replies and the advices... i will order some thousands of the glass fuses.... seems to be the most professional solution and is not that expansive.
 

Chris L

New member
Joined
Feb 23, 2022
Messages
1
A 5 amp fuse will not pop at 5 amps it's for the cable to run a constant current a fuse will blow at aconsiderable higher amp
 

Crimp Daddy

Member
Joined
Feb 21, 2018
Messages
973
+1 for those mini glass fuses... First off its actually designed to be a fuse, and it keeps the fuse element that blows contained which is also much safer than bare wire which isnt really a fuse at all.

I did test some out... pretty decent results.

 

Crimp Daddy

Member
Joined
Feb 21, 2018
Messages
973
Before you order, remember that cell fuses are for faults, not limiting current. 0.5A, 1A, 2A, etc are all way to small and will only increase resistance - which you don't want. I think 4A or 5A fuses would be optimal, even if your plan is a max current of 500mA per cell. I wouldn't go less than 4A...

Yeah, I fought with that myself a lot. Lowering the resistance while leaving enough headroom for safe operation.

I settled on 3A for my build... the 3A fuses generally blow at about 5A, which already exceeded the max of many of the cells recommended RMS discharge rating for used laptop cells. That said, I still think your recommendation is good.

Really just depends on how large the pack is going to be and your loads as well.
 

Dr. Dickie

Member
Joined
Sep 23, 2020
Messages
341
I used 32 gauge on the positive and 28 on the negative. Welded with a maletrics welder. The only trouble I had is, my cells came from e-bike packs that were covered in rubber. IF I did not properly clean off the rubber I could hit a high resistance spot (BIG pop, scared the crap out of me). Once I figured out how to properly clear the surface, that was only very rare. The other problem is, you have to be very careful when lifting the packs, since the cells can roll around in the holders, you can easily spin a cell and break the connection. Had to go back and redo a dozen or so wires, but got everything working well now.
 
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