Fuses and busbars

Hi everyone ....New member here .... I've only recently become aware of the 18650 scene ....I'm sure it holds great promise , I've spent most of my life tinkering with electronics , playing with leds , making solar panels , so I hope I can offer some new insights , and also learn a lot from this forum ...

One thing that got my attention from watching a few videos on youtube is the question of fuses and busbars ...

Fuses ..... requirements for a fuse are ....

1. Low resistance under normal conditions ... and a 'runaway effect ' when abovenormal current , that's to say the increase in temp will cause a big rise in resistance , causing it to blow quickly ... all metals will have increased resistance as temp rises , but some alloys have a bigger one .

2 . A low blowing temp(to prevent fire of damage)... information is that some fuses are made from an alloy of lead and tin , this will melt around 300C ... copper wire (tinned or pure) will melt at 1000C ...I'm not sure this is suitable for fuses , although conflicting information from a search . Galvanised steel wire would certainly not be suitable havingan even higher melting point , and a high room temp resistance , wasting energy under normal conditions ...

3 . Sable composition ... Many wires , like copper ,oxidize under the elevated temps a fuse operates at , under normal current ... over time, more of the copperbecomes non conductive oxide , increasing the resistance and making the fuse blow at a lower current.

Fuses is big subject .. I'm still searching and learning .... are fuses really necessary for 18650's , I would guess not really ... And how much power is wasted under normal conditions heating up theses fuses ? an easy calculation , but will vary for each wire type.

Also in some videos constructors solder fuses onto busbars , keeping them as short as a few mm... these will not blow at the expected current , before the heat can build up in the wire to blow , it's conducted away to the busbar and cell ... I estimate about 10mm of fuse length is required ...

Which leads us to Busbars...

I've seen some very serious looking busbars in youtube videos , Are they really needed ??? I suspect not , and they're a pain to solder onto ...let's go through a calculation to see how much energy is saved in using these ....

Let's consider copper wire 2.5 square mm cross section ,this is the wire in UK ring main cable , about 10AWG , this has a resistance of 6.5 milliohms per meter

Imagine we have a block of cells 20 cm long , that's 10 cells long .. so a wire/busbar runs between the cells , one wire/busbar is fed by 20 cells ... the max current in this wire at C discharge (2amp for a 2AHr cell ) is 40 A ...but the average current in the wire , because cells feed in at different points is 20A ... the wire length is 0.2meter , resistance is 1.3mOhms ...

Power loss is I squared xR = 400 x 1.3/1000 = 0.52W ...double this sinceeach side needs a busbar = 1.04W

So if 10AWG wire is used 1W is lost , when the 20 cells are delivering 20x3.7x2 = 148 Watts .... About 0.67% power is wasted when using 10AWG as bus wire ... not really significant ... a copper strip might reduce wastage to 0.17 % .. ...

Can 10AWG handle 40A? ... easily @ 40A each meter has to dissipate ... 40xx40x 6.5/1000 = 10.4W , bare wiremight just get a little warm.

Regarding fuses: Are they needed? Its up to everyone to decide. I dont see the reason not to use them if one cells goes bananas totally. And your points on the fuses are important.
I did a calculation in another thread and came up to that if i use my system fully i would waste 8-10w in total on the fuses. And thats for the 10kw.... Is it worth that? I think so. If 1 cell goes dead short (Yes i have seen it happen) and you dont have the fuse it will drag that whole pack down. But yes it cost to fuse and it takes time...

The length of them are crucial as you states!! And this is very important.

Regarding busbars:
Dont forget that you can have 14 blocks in series. That's 14watt Nevertheless i will not say you are wrong in any of the points but i would say that if you can reduce the wasted power and add some safety i think that is good. Though fuses is wasted power... :-/

A 1.5mm2 will handle 40A but you also get some voltage drop among the wire. Ie in the end you have a higher voltage drop that also causes the first cell to have to deliver alot more current than the last one. This can also affect the performance of the overall pack. If its significant Im not sure? Perhaps you know? (I have not calculated eventual resistance in beginning vs end and the voltage diff and if this could cause when charging/discharging (Though when no load is applied they will balance them self agian)

Im designing my bus bars based on 1.5mm2 = 10A and 2.5mm2 = 16A. This is very conservative and you get a very low drop. Yes you need thicker bus bars but not that much. My packs are made to stand 80A (1A per cell) max. Then i also use 6*2.5mm2. In my case its very simple to make those busbars so for me its not a big deal to go a bit bigger to save me some watts.

So my busbars by my principle would do 96A... But on the other hand i have a short-current i have in mind to. In my case i do design that if there is major issue i should be able to run up to 4x the current shorter times. Thats 320A.. Thats when we get close to what the busbar can handle. And frankly if i pull 320A through mine i think the contact between packs is the biggest issue....

Either way i think you have a good thought in what you write! Its all about you want to spend in money and time compared to what you get back.

This is an interesting table ....http://www.powerstream.com/wire-fusing-currents.htm ...it gives the fusing current for copper, iron , aluminium and Tin wire ...

It shows for 5amp blow we need 0.16 Cu ... 0.192 Al ... 0.35 Fe .... or 0.54 Sn (tin) those numbers are diameter mm for the wire..

It shows pure tin wire is the best 1/2 mm diameter means it has the lowest resistance in normal operation and a very low blowing temp (231C) ...but not easy to find this wire

(galvanised) Iron wire is the worst , high resistance and red hot blowing temp....

But there's not a great deal of difference between them all , and tinned copper wire is fine ...

Lets see what the power wastage is in using one 10mm long fuse on each cell

Resistance 34AWG (0.16mm dia) 5.1A blow is 8.75mOhms at 25C .....16mOhms at 250C .....24.7mOhms at 500C that's for 1cm wire .

What is the temperature of this wire under 2 Amp load? It blows at 1000C under 5 A ...but there is a runaway effect , I would estimate it could be around 150C ...perhaps 13m Ohms..@ 2A.

I squared R = 4 x 13/1000 or ....52mW this is the power lost in the fuse , when power from cell is 3.7 x 2 =7.4 W that's 0.7%

So fuses waste about the same power as all the other connecting busbars and cable put together total overall power lost in all wires is about 1.4%

The reason for the fusing the is use of large numbers of cells in parallel. Tesla came up with the idea for their battery modules, for instance the Model S has 16 modules of 6s74p. HBPowerwall uses 80p in his packs. The odds of any cell failing, when thousands of cells are used, is high enough to be an issue.

Should a single cell fail short circuit, the fault current is Max current x 79, so if a cell can do 10A then you get 790A into the failed cell.
The failed cell can only convert that to heat.
Power = Voltage x Current = 3.7V x 790A = 2,923 W

Aluminium will melt at 660C, lower for some alloys. Tesla use a 0.3mm diameter aluminium wire in their battery modules.

This is threads I like. I will respond later tonight or tomorrow.

Station240: Your point there is something many missed including myself. Its not about the current from that cell but from all the other cells... And as you said thats HUGE...

station240 said:

The reason for the fusing the is use of large numbers of cells in parallel. Tesla came up with the idea for their battery modules, for instance the Model S has 16 modules of 6s74p. HBPowerwall uses 80p in his packs. The odds of any cell failing, when thousands of cells are used, is high enough to be an issue.

Should a single cell fail short circuit, the fault current is Max current x 79, so if a cell can do 10A then you get 790A into the failed cell.
The failed cell can only convert that to heat.
Power = Voltage x Current = 3.7V x 790A = 2,923 W

Aluminium will melt at 660C, lower for some alloys. Tesla use a 0.3mm diameter aluminium wire in their battery modules.

Click to expand...

Ahhh.... i've been trying to find out what tesla use for a fuse !!! 0.3mm Al (from the table in link) blows at 10 Amps ...do you have a link for that tesla information?
I wonder why they chose Al over tin? ... trouble with aluminium is it doesn't solder with normal solder !! I think there is a special expensive solder that can bond with aluminium.

They dont "solder" it.

I love the fact that we have several really good mathematical folks in here. The fact that you guys are working this out and sharing it with the rest of us is invaluable! Great discussion!

If you havent seen any of my 18650 fuse style videos yet. You also need to consider that most fuses are for a higher voltage and we need them for 3-4v.

ozz93666 said:

Ahhh.... i've been trying to find out what tesla use for a fuse !!! 0.3mm Al (from the table in link) blows at 10 Amps ...do you have a link for that tesla information?

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https://endless-sphere.com/forums/viewtopic.php?f=9&t=67661&start=313

Since you are regularly ripping apart Tesla batteries I wonder if you can tell us the diameter of the fuse wires Tesla is using on their positive terminal on each 18650 battery. I would be interested in trying a design that uses fuse wires and I wonder what size wire Tesla chose.

They are aluminum with 0.3mm diameter

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https://teslamotorsclub.com/tmc/threads/tesla-cell-level-fuse-bondwire-material.45892/
This thread say it's 0.29 to 0.27mm

station240 said:

ozz93666 said:

Ahhh.... i've been trying to find out what tesla use for a fuse !!! 0.3mm Al (from the table in link) blows at 10 Amps ...do you have a link for that tesla information?

Click to expand...

https://endless-sphere.com/forums/viewtopic.php?f=9&t=67661&start=313

Since you are regularly ripping apart Tesla batteries I wonder if you can tell us the diameter of the fuse wires Tesla is using on their positive terminal on each 18650 battery. I would be interested in trying a design that uses fuse wires and I wonder what size wire Tesla chose.

They are aluminum with 0.3mm diameter

Click to expand...

https://teslamotorsclub.com/tmc/threads/tesla-cell-level-fuse-bondwire-material.45892/
This thread say it's 0.29 to 0.27mm

Click to expand...

Thanks ....'oshiera' sounds confident , but I don't know how reliable that information is .....

"wirebonded " What's that ? like spot welding ? from daromer's picture it seems so ...can you spot weld aluminium onto the (stainless?) cell +ve terminal ? [spot welders , the type some use to spot weld nickel strip onto cells are very easy to make , I've done it with one supercapacitor (2.5V)... but if you have a big pack of cells in parallel, I would imagine that would do it ... just a pulse of a few volts @ 20A or so]
Looks like tesla use an aluminium bus bar , and from the picture the cells seem to be spaced further apart than we're used to, I don't see why that's necessary whatever cooling fluid you use .Is this picture of a car battery ?? did someone say these are liquid cooled?

Nickel strip for spot welding onto cells .... beware most on ebay is nickel plated steel (very high resistance)....

Pure nickel strip is available 0.2 or 0.1 mm thick , 8mm wide ... but pure nickel has 4 times the resistance of copper .

So these strips are the equivalent of 0.2 square mm and 0.4 square mm cross section copper wire , high resistance and not suitable for making large 3.7V packs ... it probably couldn't handle the current , but for a 3.7V pack size of 80 cells could waste perhaps 20% (thin strip) or 10% (thick strip) of the power.. ridiculous would cause battery to heat up

Might just be usable where the 3.7 packs have no more than 4 cells , and these are wired in series , in this configuration resistive losses are much less.

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AveRageJoe said:

If you havent seen any of my 18650 fuse style videos yet. You also need to consider that most fuses are for a higher voltage and we need them for 3-4v.

Click to expand...

Great videos Joe ... i'm working my way through them .... good hands on practical experiments !!

In the first video you measure the 5A at 0.19mm and the 15 A at 0.49 (This figure seems high are you sure 0.49wasn't 30A?)

I've a feeling this wire could be tinned copper wire , can you scrape the surface and see if a copper colour is underneath ?

Voltage shouldn't effect the blowing of fuses ...the wire only knows about the current going through it until the wire melts , then if high voltage , arcing occurs making a spectacular flash.( I think that's correct)

With the length of fuse you are using I don't think the thickness of the busbar will affect things at all , it might be worthwhile seeing how short a fuse we can use , the shorter the less power loss , but not really much difference , my guess is 10 mm is good.

It seems first we have to decide what current we want the fuse to blow at , the higher current fuse , the less power wastage in normal running ... I guess tesla knows best , but some confusion about what they use , it seems at least 10A ....

If we can first decide on the required blow current for the fuse , this will reducethe amount of experiments we need to do .

ozz93666 said:

Nickel strip for spot welding onto cells .... beware most on ebay is nickel plated steel (very high resistance)....

Pure nickel strip is available 0.2 or 0.1 mm thick , 8mm wide ... but pure nickel has 4 times the resistance of copper .

So these strips are the equivalent of 0.2 square mm and 0.4 square mm cross section copper wire , high resistance and not suitable for making large 3.7V packs ... it probably couldn't handle the current , but for a 3.7V pack size of 80 cells could waste perhaps 20% (thin strip) or 10% (thick strip) of the power.. ridiculous would cause battery to heat up

Might just be usable where the 3.7 packs have no more than 4 cells , and these are wired in series , in this configuration resistive losses are much less.

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AveRageJoe said:

If you havent seen any of my 18650 fuse style videos yet. You also need to consider that most fuses are for a higher voltage and we need them for 3-4v.

Click to expand...

Great videos Joe ... i'm working my way through them .... good hands on practical experiments !!

In the first video you measure the 5A at 0.19mm and the 15 A at 0.49 (This figure seems high are you sure 0.49wasn't 30A?)

I've a feeling this wire could be tinned copper wire , can you scrape the surface and see if a copper colour is underneath ?

Voltage shouldn't effect the blowing of fuses ...the wire only knows about the current going through it until the wire melts , then if high voltage , arcing occurs making a spectacular flash.( I think that's correct)

With the length of fuse you are using I don't think the thickness of the busbar will affect things at all , it might be worthwhile seeing how short a fuse we can use , the shorter the less power loss , but not really much difference , my guess is 10 mm is good.

It seems first we have to decide what current we want the fuse to blow at , the higher current fuse , the less power wastage in normal running ... I guess tesla knows best , but some confusion about what they use , it seems at least 10A ....

If we can first decide on the required blow current for the fuse , this will reducethe amount of experiments we need to do .

Click to expand...

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I didnt think voltage would play a part in the testing but it does. The first 2 vids i did testing, the voltage drop was around 2 to 2.5v and it req more amps to pop them. Those 2 vids are in the forums here and on my youtube. Ill scrap that wire when i get home

It occurred to me we still haven't exhaustively covered the busbar issue ... let's see if I can nail it in this thread so we can see what the optimum cross section is. Daromer's twisted wire method is getting very popular .... and I have to agree it is the best I've seen , easily obtainable 2.5mm2 wire ...can be twisted into place ...so what are the pro's and cons of 2 or 3 or 4 wires ...80 p seems standard in a 4x20 layout , so I'll use that for the figures ....

The length of busbar on each side of the pack is exactly 1 meter ... there's one on each side ... but the average current carried by the whole wire is half the output (wire at the back end will carry no current ) so to calculate loses we can assume 1 meter is carrying the full output current...
The table is for wire at 31C the resistance is 7 mOhms/meter ..... it doesn't rise much with temp at 67C it is 8 mOhms/meter ...

...........................................Powerloss at 20A .............. Powerloss at 40A ..........powerloss at 80A .......... powerloss at 160A

One strand 2.5 ......................2.8W .... 3.9% ...................11.2W.....7.8%..............44.8W ....15.5% .............179W .....31%

Two strand 2.5 ......................1.4W.... 1.94% ....................5.6W ..... 3.9% .............22.4W ....7.8% ..............89.6W.....12.5%

Four strand 2.5 .....................0.7W .....0.97% ....................2.8W.....1.94% .............11.2W.....3.9% ...............44.8 W ....6.25%


The % number is the percentageof power lost in the bus at that particular current so with four strands in the buss (cross section 10mm square), when the pack is delivering 40A , 2.8W is lost , which is 1.94% of the power output.

The loss is not insignificant ... the shape of thepack doesn't help ...long and thin(4x20)means more buss wire per cell ...

This lost power has a monetary value!!!... athigh currents the % loss is higher so if high discharge rates are expected it may be worth going thicker than 4 strand!

Ideally the bus would get thicker near the output terminals , could be one strand thick at the other end of the pack where only a few amps is being carried ... and putting cells with a higher AHr capacity at the terminal end of the pack will also help.

EDIT .... I've just realised if output terminals were at BOTH ends of the pack .... wired to the next pack at top and bottom , this would HALVE the above losses !!!!..... very worth considering.

Nice! Yes i went for 3*2.5mm. (2*3*2.5 in total). I based it on 16A per wire is something we generally go by in households. And that would end up to 90A without big issues. If i wouldnt have had enough cells i would most likely have gone up with more wires. It all depends on how much loss you can take and heat. The heat atleast in my area is not an issue or a problem. Good ventilation and rarely above 20c

Right now i have 40kWh so my max load wont even come close to above.

Also note that some cells sit significantly closer to the connector than the rest. Even if you have contacts in both end so your number is a tad higher than in real life.

Ur suggestion to wire in another wire closer to the connectors was something i had in mind but due to the extra work doing that i instead made sure that the whole bus bar was thick enough for my purpose. Only reason i would have done it is if didnt have enough wires or

ozz93666 said:

EDIT .... I've just realised if output terminals were at BOTH ends of the pack .... wired to the next pack at top and bottom , this would HALVE the above losses !!!!..... very worth considering.

Click to expand...

Combine that with a tapered bus towards both ends, thinnest in the middle and thick at the ends. Our closest equivalent to 2.5mm^2 is 14ga and it's not terribly heavy and as you point out very common.

I'm envisioning parallel 14ga strands making up a double tapered bus, wrap the bus with smaller wire and solder at every short end of your strands.

Maximum amps for power transmission for 14ga is 5.9A according to this useful chart.. http://www.powerstream.com/Wire_Size.htm

So 40/6 = 7 strands each end (rounding up) if you consider that either end of a double ended bus will see half the total amps. One full length strand and six successively shorter ones on each side.


http://www.powerstream.com/Wire_Size.htm

Has anyone used the little pico fuses.
Sorry I dont have a link at the moment but if you type 5 amp pico fuse in ebay you will find them.

I just found a 1a one in a HP laptop battery and have heard them mentioned before

Microfuses become expensive fast if you need 3000 of em

daromer said:

Microfuses become expensive fast if you need 3000 of em

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Yeah thats fair enough

But say for a smaller pack, has/ doesanyone use them?

daromer said:

Nice! Yes i went for 3*2.5mm. (2*2.5 in total). I based it on 16A per wire is something we generally go by in households. And that would end up to 90A without big issues. If i wouldnt have had enough cells i would most likely have gone up with more wires. It all depends on how much loss you can take and heat. The heat atleast in my area is not an issue or a problem. Good ventilation and rarely above 20c

Right now i have 40kWh so my max load wont even come close to above.

Also note that some cells sit significantly closer to the connector than the rest. Even if you have contacts in both end so your number is a tad higher than in real life.

Ur suggestion to wire in another wire closer to the connectors was something i had in mind but due to the extra work doing that i instead made sure that the whole bus bar was thick enough for my purpose. Only reason i would have done it is if didnt have enough wires or

Click to expand...

Please clarify something for me. Are you using 3 2.5 mm wires twisted together or 2 2.5 mm wires twisted into a single wire?