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.
 
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.
 
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
 
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.
 
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
 
+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.

 
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.
 
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.
 
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.
I have the same issue with the rubber on some of the cells from ebike packs. How did you clean the surface? I was thinking about using a Dremel to lightly buff it off.
Other thing is I ordered 30 gauge TCW for positive and 24 gauge TCW on the negative, going with spot welding too. Would this be OK on packs that are around 500Ah (14S)? I haven't settled on the final design on the packs but so far I'm leaning towards the bigger size. The weight is what other people mentioned as a hang up on larger packs, but I calculated the pack would only weigh about 20lbs with 200 cells. Doesn't seem like much.
 
I was thinking about using a Dremel to lightly buff it off.
This is not recommended as it can wear off the protective coating on the metal and lead to rust damage over time. Many hobbyists used to file both ends smooth but we have learned to do better now. That being said I have a bunch of inherited cells that are filed smooth and am using them but will give extra attention to spot any early warning signs of rust which can lead to serious cell failures.
 
This is not recommended as it can wear off the protective coating on the metal and lead to rust damage over time. Many hobbyists used to file both ends smooth but we have learned to do better now. That being said I have a bunch of inherited cells that are filed smooth and am using them but will give extra attention to spot any early warning signs of rust which can lead to serious cell failures.
I can get a lot of it off by scrapping with my finger nail but there is some that gets stuck in the groves and is difficult at times to get a good connection. I'll avoid polishing or grinding then. Thanks!
 
Important to note that proper fuse wire is not copper, it is an alloy designed:
a) to melt at lower temps
b) to have some resistance so it does heat & melt
"AWG xxx wire" is copper that's doing it's best not to have resistance & not melt!

Re filing corrosion - maybe think about what happens to the metal filings rubbed off? They are conductive metal particles & some will be falling into the top of the battery. Will this cause a short? Remember the Sony laptop batteries catching fire a while back? (might be slightly different but hey, metal bits.... If you were super careful & filed the batteries upside down, file/sandpaper flat on table, battery tip down + vacuumed might be OK?
 
At the risk of piling on :) Remember that the top has an expandable pressure plate that will push 'up' (toward the top) to allow gas pressure to escape / disconnect the + end electrically under certain (bad) conditions. You don't want any particles in there to get in the way of this plate pushing up since it's a safety feature.
1675351386140.png
 
Important to note that proper fuse wire is not copper, it is an alloy designed:
a) to melt at lower temps
b) to have some resistance so it does heat & melt
"AWG xxx wire" is copper that's doing it's best not to have resistance & not melt!

Re filing corrosion - maybe think about what happens to the metal filings rubbed off? They are conductive metal particles & some will be falling into the top of the battery. Will this cause a short? Remember the Sony laptop batteries catching fire a while back? (might be slightly different but hey, metal bits.... If you were super careful & filed the batteries upside down, file/sandpaper flat on table, battery tip down + vacuumed might be OK?
People on this forum have mentioned using TCW and stranded wire as fuse wire. Googling "fuse wire" doesn't result in anything useful for me. Do you have any suggestions? I was able to find this in a link from a video on secondlife http://www.thermalproducts.com.au/fuses-fuse-wire/
but that's not in the US. There is some stuff on ebay but also not a great option as it's either a ton or a small amount in a variety pack. I did find a few links to 0.031" fuse wire, which seems to thick. This is a bit of a rabbit hole for me.
 
At the risk of piling on :) Remember that the top has an expandable pressure plate that will push 'up' (toward the top) to allow gas pressure to escape / disconnect the + end electrically under certain (bad) conditions. You don't want any particles in there to get in the way of this plate pushing up since it's a safety feature.
View attachment 28986
The rubber is almost like silicone caulk and on some cells it is between the cap and bottom. It's relatively soft so should compress but I see your point about not getting something hard stuck in there.
 
If you google "fuse wire composition" you get heaps of information.
It's mostly a lead/tin alloy. ie basically solder!
Even there's even posts here (don't have time now to search) about glass axial fuses that glow red hot & pass current 3-5x their "rated" value rather than melt - likely cheapies made with copper vs above alloy.
To buy some maybe search for "buy alloy fuse wire", I got results there :)
 
If you google "fuse wire composition" you get heaps of information.
It's mostly a lead/tin alloy. ie basically solder!
Even there's even posts here (don't have time now to search) about glass axial fuses that glow red hot & pass current 3-5x their "rated" value rather than melt - likely cheapies made with copper vs above alloy.
To buy some maybe search for "buy alloy fuse wire", I got results there :)
That did help. Thank you. Maybe it's my crappy google skills- it's a bit of a slog finding something, but I did find one or two 4amp fuse wire spools. There were still a lot of hits on TCW as fuse wire and 1lb spools that cost upward of $300. For some reason it's easier to find this stuff in the UK.
 
I use 30awg Tinned Copper for my fuse wires - https://www.amazon.com/gp/product/B01M0AXONC/ref=ppx_yo_dt_b_search_asin_title
As discussed, this kind of wire can vary a lot (10-25a'ish + a second or 2) to burn thru. However, my purpose for fuse wire is to protect the pack in the event of a bad/shorting cell and 100p is enough current to burn 30awg tinned copper disconnecting the cell from the buss in a timely manner.

I've anecdotally verified protection by 1) accidentally shorting a pack once - the fuse wires burned thru all over the place and 2) when soldering the fuse wire I occasionally shorted to the negative side and it burned thru leaving burn marks on my fingers. I have confidence the 30awg Tinned Copper is adequate for my situation. :)
 
I use 30awg Tinned Copper for my fuse wires - https://www.amazon.com/gp/product/B01M0AXONC/ref=ppx_yo_dt_b_search_asin_title
As discussed, this kind of wire can vary a lot (10-25a'ish + a second or 2) to burn thru. However, my purpose for fuse wire is to protect the pack in the event of a bad/shorting cell and 100p is enough current to burn 30awg tinned copper disconnecting the cell from the buss in a timely manner.

I've anecdotally verified protection by 1) accidentally shorting a pack once - the fuse wires burned thru all over the place and 2) when soldering the fuse wire I occasionally shorted to the negative side and it burned thru leaving burn marks on my fingers. I have confidence the 30awg Tinned Copper is adequate for my situation. :)
I tested out the 30awg TCW and was getting it to burn through below 15amps most of the time until it popped the 15amp fuse in the variac transformer I was using to test it. That's my main purpose as well, just protection against a short. I'm planning on somewhat larger packs, maybe 140P or 180P, so should be good there as well. Thanks, again for the input.
 
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