Hi Justin, Thanks for chiming in – very useful info. Another issue and I suspect you have noticed this also: smoke and smell.
Somewhere around 1995 I had a colleague who was involved in cost reducing a product design and he too had the same idea of testing PCB traces as fuses to see if he could characterize them enough to become useful fuses. So, he grabbed an existing high density board that he had the layout files of, so he could verify the trace width of a certain trace on the board, then solder 2 wires onto the mounting pads of the components on the ends of the trace section to run current through it and experiment what trace width would blow at what current. He was amazed at the current that was needed to cause the traces to open. He only used low voltage as this was for a handheld terminal that worked from 6V battery, so the highest voltage in the system was the battery charger which supplied 9V. The other thing (as his lab was across from the break room) was the amount of smoke generated by heating up traces. Even (or especially) traces that warmed up without blowing, caused a lot of nasty smells of evaporating PCB chemicals. Traces that did (eventually) open caused smoke to pour out of the PCB, to the point that he stopped testing and evacuated his lab for a while, because he thought it unsafe to filter all those chemicals through his lungs. Many colleagues were not so happy that they were now “second hand smokers” with the results of his experiments slowly dissipating through the building halls. Several hardware colleagues came running when the smell reached their office, as this was recognized as the typical “the magical smoke was released” signal, so they wanted to know what device had blown up so spectacularly to cause this cloud. This all to say that if you design PCB “fuses” that are run so that they have a chance to break the circuit, that means that they will cause a lot of heat at rated load, which will not only constitute a loss but also likely release nasty smells in operation. If this is for a battery, you can likely contain the smoke and smell in an enclosure, just like the Leaf battery pack has a very characteristic Lithium smell when first opened, due to the deteriorating cells emitting vapors. Occasionally I smell this when walking through the parking lot with vehicles charging, apparently a battery occasionally “burps” and emits this same smell while charging. But I worry about deterioration of the PCB when regularly exposed to elevated temperatures, I have worked a lot with TVs in my past of repairing TVs as hobby and I have seen the results of not properly rated components that were running hot for years and resulting PCB deterioration: charring and deformation and the failures associated with hot solder joints cracking due to mechanical stress, corroding and pushing / pulling leads free from solder pads. That is why so many old TVs need regular “percussive maintenance” to work again for some time. Deliberately accelerating this process by heating up a PCB with traces designed for fuses does not sound like a good idea to me, even though you could carefully design a small section of the trace to be thinner and concentrate all the heat in an area that is not so affected by this heat – the smell, smoke and charring problems still remain. There is a reason that all commercial products use discrete fuses or safety resistors, designed for the job. Going back to the original question of wire to act as cell fuse in paralleled cell packs like Tesla – yes, apparently they are able to characterize it well enough. NOTE that you rely on the BMS to detect when one or more cells have “disappeared” so that a pack can be repaired before too many fuses blow and you start blowing the remaining ones, not because there is a cell failure but because you are overloading them. That is one of the reasons why EVs limit drive power when detecting a battery balancing problem. Regards, Cor. Sent from Mail for Windows 10 From: Justin Kenny via EV Sent: Monday, January 21, 2019 3:26 PM To: [email protected] Cc: Justin Kenny Subject: Re: [EVDL] 18650 cell level fuse wire I'm the owner of the Leaf Rex channel Cor mentioned earlier. I highly caution against using PCB fuses in general, I've done testing on those also: https://youtu.be/CMlpCX0bug8 If you have a relatively large open circuit voltage in the system you are trying to protect, it will create a conductive plasma and char the PCB, leaving behind a "resistor" which can continue to burn, though hopefully self-extinguish in keeping with the "flame retardant" part of FR-4. - Justin On Mon, Jan 21, 2019 at 1:09 PM <[email protected]> wrote: > Message: 13 > Date: Mon, 21 Jan 2019 15:25:38 -0500 > From: Martin Klingensmith <[email protected]> > To: Lee Hart via EV <[email protected]> > Subject: Re: [EVDL] 18650 cell level fuse wire > Message-ID: <[email protected]> > Content-Type: text/plain; charset=utf-8; format=flowed > > > IIRC I've seen "fusible links" on PCBs. They're a short section of > copper trace that is much narrower than the rest of the trace. My > understanding is that they're used for an extra level of protection or > where the designer thinks they're really clever. For all modern PCB > materials it's not *too* unsafe because they aren't (supposed to be) > flammable. > > - > > Martin K _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20190122/4f5ea5ca/attachment.html> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
