# cell-level fuse bond wire



## kennybobby (Aug 10, 2012)

A question about some puzzling data for Tesla cells.

It was reported by a pack dismantler on the tmc site that the bond wires were made of #29 AWG aluminum (0.28mm, 0.011" diameter). 

Some high current testing was done by another fellow who dismantled a pack to determine that these bond wires could carry 25 amps, but would fuse at 26 amps.

Numerous wire tables list the fusing current of 29 gage aluminum between 7 and 10 amps. [Similar sized Copper is between 10 and 14 amps].

25 amps makes sense from having margin since 3 to 5 C for these cells would be 9 to 15 amps or so. So the small wire size is what is puzzling me, in that the aluminum should have fused sooner.

Is there any other wire materials out there with such high fusing current at this small diameter that could explain this test data?


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## dcb (Dec 5, 2009)

are they submersed?


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## kennybobby (Aug 10, 2012)

No they aren't, although there is a cooling loop that runs throughout the pack and touches the side of every cell (7104, 96s74p).

Here's some pictures of pack, module and close up of a cell fuse.


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## Hollie Maea (Dec 9, 2009)

kennybobby said:


> Is there any other wire materials out there with such high fusing current at this small diameter that could explain this test data?


No. Even Copper 29AWG fuses at about 12 Amps. Only Silver is less resistant.

I would question if it really is 29AWG.


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## dcb (Dec 5, 2009)

well, even at 300kw, a 400v pack is putting out 750 amps, which is about 10 amps per cell @ 74P. Losing a cell bumps the rest up by 1.35% initially.

I can't imagine they are going to keep making such a complicated pack for long though. 15000 fuses and gasket seals?!?


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## kennybobby (Aug 10, 2012)

Hollie Maea said:


> No. Even Copper 29AWG fuses at about 12 Amps. Only Silver is less resistant.
> 
> I would question if it really is 29AWG.


Two different guys measured them at 0.27-0.29 mm


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## piotrsko (Dec 9, 2007)

If they are aluminum how do they get bonded to the tabs? IMHO all you can do is mechanical / crimp then there is this huge corrosion issue.


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## kennybobby (Aug 10, 2012)

electrically spot welded


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## eldis (Sep 3, 2013)

How is that fusing current defined? On a "very long" wire it can be quite different than on a ~1cm length with good thermal contact on both sides. Maybe the wire cools down quite nicely, therefore reaches the critical temperature at higher current?


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## Duncan (Dec 8, 2008)

Hi
What you have is a fuse that will resist (not fail) at 12 amps continuous - which is ok for the power required 

you also have a report that it took 26 amps to fail one
I suspect that the guy doing the testing was a bit impatient - fuses don't fail instantly when you go over their rating there is a time/current relationship

I have a 400amp fuse - and I pull 700amps for a second or two on acceleration


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## wb9k (Apr 9, 2015)

Most fuses will take up to three times their rated current for several seconds before failing. Double the rated current may take a minute or two to actually fail. You would need a detailed data sheet on this fuse arrangement to know under what conditions it will actually "blow". It's pretty much never at rated current or even very close to it unless that level of current is sustained for a long time.


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## psron (Jun 19, 2012)

eldis said:


> How is that fusing current defined? On a "very long" wire it can be quite different than on a ~1cm length with good thermal contact on both sides. Maybe the wire cools down quite nicely, therefore reaches the critical temperature at higher current?


This (above quoted comment) is extremely important... 
"Fusing" of a conductor is 100% related to its heating/melting temperature. There is a ratio of conductor cross-sectionl area to length, the heat-transfer capability of the conductor, and what each end it attached to that determines what the temperature rise will be at a given current.

If there is an effectively infinite heatsink at each end (for the given conductor material/size/length), it could carry many, many times the current of a "free-hanging" wire of longer length... simply because it is able to dissipate the heat to the attached metal at each end.

In a situation like the Tesla packs... those "fuses" are not individual pieces that are welded on... they use a wire-bonding machine with a large spool of wire, and it welds/cuts them to the cells at an incredibly rapid rate... it's very cheap.


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