telefunken
New member
- Joined
- Jun 21, 2017
- Messages
- 5
So I am in the planning stages of my own build and of course safety is my foremost concern. I've been looking at several options for temperature monitoring and cooling, but that's beside the point.
Anyway, onto the Nth topic about the controversial fuse wire. The whole fuse wire ordeal seems to have started with the Tesla packs, Tesla has fuse wires and thus it must be a good idea right? Let me first make it clear that I am not an opponent of the extra added layer of security, I say extra layer because the fuse wire goes on top what is (should be) already present there.
So let's first reflect on what security measures are already there built into the cell. A properly manufactured 18650 has a fake top, in that fake top you will find a PTC (Positive Temperature Coefficient) and a CID (Current Interrupt Device).
1. The resistance of a PTC will rise according to the temperature of the cell, this means the amount of current which can flow through the cell will become a function of the temperature effectively limiting and eventually cutting the cell off when it starts to overheat.
2. The CID is a mechanical device, effectively it's just a pressure release valve. When pressure builds up inside the battery the CID will pop out and thus mechanically cut the battery off.
Note that it doesn't matter if your cells are protected or unprotected, protected just means there's an extra chip at the bottom of the cell which adds another layer of protection, but using protected cells in a powerwall setup has other disadvantages.
So at some point along the line Tesla decided to add external fuse wires to cells in their packs. I am struggling to understand why they did this. The whitepaper of the Tesla Roadster BMS mentions that they are using cells with a PTC and CID, it doesn't mention anything about fuse wires besides saying they have some active and passive measures (can't post link because rules do not allow me to-do that just yet).
Keep in mind that the Tesla Roadster was their first prototype platform, when they built that they didn't have their own batteries. When the Model S came around they were producing their own batteries. It could be they decided to ditch the PTC, I could think of several reasons for doing this. The battery pack is actively cooled, maybe they found the PTC doesn't do much because of the cooling. Maybe they found the PTC affects the battery performance too much in extreme weather conditions.
The reason for this is unknown to me and I would appreciate the comments from somebody who has more insight on the matter.
So onto the fuse wires used by some people in their own builds. Is this extra added security measure necesary? The best guess until now is that we do not know. So the Tesla engineers must have thought about this right? They did the math, designed a robot which uses a properly defined wire and welds that wire onto the cells according to a properly defined procedure. The Tesla engineers can say with some amount of certainty that their fuses conform to narrowly and specifically defined properties.
So this has been said a few times before and I will repeat it again, the wires used by most people here are just rule of thumb wires with no thought put into them. When push comes to shove nobody can exactly say when and if the wire is going to fail. When I look at some of the pictures on this site, I see people have gone to great lengths to put the wires in but then made it all obsolete with a botched soldering job. The wires are covered in tin which runs up the wire. So the wire might have had some property but that property is now gone because it's covered in tin. On top of that when I look at some of the builds I see cells which have a rated continues output current of 20A. That cell gets soldered onto a teeny tiny little wire as a measure of protection, the only thing you're doing there is building an expensive light bulb.
So let us come together and figure this one out once and for all. Let's start collecting information how these DIY systems behave so we can start defining best practices.
Any comment from people who have more insight in the matter (other then the standard, I did it this way and it works or the I did it this way because my gut tells me to) would be greaty appreciated.
Link to Tesla Roadster BMS whitepaper:
http://sa.indiaenvironmentportal.org.in/files/TeslaRoadsterBatterySystem.pdf
Anyway, onto the Nth topic about the controversial fuse wire. The whole fuse wire ordeal seems to have started with the Tesla packs, Tesla has fuse wires and thus it must be a good idea right? Let me first make it clear that I am not an opponent of the extra added layer of security, I say extra layer because the fuse wire goes on top what is (should be) already present there.
So let's first reflect on what security measures are already there built into the cell. A properly manufactured 18650 has a fake top, in that fake top you will find a PTC (Positive Temperature Coefficient) and a CID (Current Interrupt Device).
1. The resistance of a PTC will rise according to the temperature of the cell, this means the amount of current which can flow through the cell will become a function of the temperature effectively limiting and eventually cutting the cell off when it starts to overheat.
2. The CID is a mechanical device, effectively it's just a pressure release valve. When pressure builds up inside the battery the CID will pop out and thus mechanically cut the battery off.
Note that it doesn't matter if your cells are protected or unprotected, protected just means there's an extra chip at the bottom of the cell which adds another layer of protection, but using protected cells in a powerwall setup has other disadvantages.
So at some point along the line Tesla decided to add external fuse wires to cells in their packs. I am struggling to understand why they did this. The whitepaper of the Tesla Roadster BMS mentions that they are using cells with a PTC and CID, it doesn't mention anything about fuse wires besides saying they have some active and passive measures (can't post link because rules do not allow me to-do that just yet).
Keep in mind that the Tesla Roadster was their first prototype platform, when they built that they didn't have their own batteries. When the Model S came around they were producing their own batteries. It could be they decided to ditch the PTC, I could think of several reasons for doing this. The battery pack is actively cooled, maybe they found the PTC doesn't do much because of the cooling. Maybe they found the PTC affects the battery performance too much in extreme weather conditions.
The reason for this is unknown to me and I would appreciate the comments from somebody who has more insight on the matter.
So onto the fuse wires used by some people in their own builds. Is this extra added security measure necesary? The best guess until now is that we do not know. So the Tesla engineers must have thought about this right? They did the math, designed a robot which uses a properly defined wire and welds that wire onto the cells according to a properly defined procedure. The Tesla engineers can say with some amount of certainty that their fuses conform to narrowly and specifically defined properties.
So this has been said a few times before and I will repeat it again, the wires used by most people here are just rule of thumb wires with no thought put into them. When push comes to shove nobody can exactly say when and if the wire is going to fail. When I look at some of the pictures on this site, I see people have gone to great lengths to put the wires in but then made it all obsolete with a botched soldering job. The wires are covered in tin which runs up the wire. So the wire might have had some property but that property is now gone because it's covered in tin. On top of that when I look at some of the builds I see cells which have a rated continues output current of 20A. That cell gets soldered onto a teeny tiny little wire as a measure of protection, the only thing you're doing there is building an expensive light bulb.
So let us come together and figure this one out once and for all. Let's start collecting information how these DIY systems behave so we can start defining best practices.
Any comment from people who have more insight in the matter (other then the standard, I did it this way and it works or the I did it this way because my gut tells me to) would be greaty appreciated.
Link to Tesla Roadster BMS whitepaper:
http://sa.indiaenvironmentportal.org.in/files/TeslaRoadsterBatterySystem.pdf