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I don't get what is going on here. It seems like GM has gone through at least three iterations of this part: 24294004, 24291219 and 24281696. From what I can tell the part is just a huge 500 volt 400 amp Omron type fuse in a plastic shell with a lever. How hard is it to get this right? Beginning to worry about engineering mistakes as to location or type of fuse used. I realize that it is a very small percentage of blown fuses but come on now. Do they degrade slowly or blow all at once? The kW the Bolt draws or makes in regen is within this fuse's parameters. Catastrophic failure for something this essential that is not a moving part (like a serpentine belt) gets me worried.
 

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Discussion Starter #44
Almost forgot, I almost always drive in regen mode (“L”) and I was in the process of approaching a red light, so regen was engaged and I did see an error message that mentioned regen braking.
Mine also blew in L mode. I wasn’t doing anything special though just cruising around 70mph and maybe 15 to 20kw of power draw.
 

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Discussion Starter #45
I don't get what is going on here. It seems like GM has gone through at least three iterations of this part: 24294004, 24291219 and 24281696. From what I can tell the part is just a huge 500 volt 400 amp Omron type fuse in a plastic shell with a lever. How hard is it to get this right? Beginning to worry about about engineering mistakes as to location or type of fuse used. I realize that it is a very small percentage of blown fuses but come on now. Do they degrade slowly or blow all at once? The kW the Bolt draws or makes in regen is within this fuse's parameters. Catastrophic failure for something this essential that is not a moving part (like a serpentine belt) gets me worried.
Working extensively in electronics, typically a fuse “should” never blow unless there is a failure downstream causing an excessive current draw.
 

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Working extensively in electronics, typically a fuse “should” never blow unless there is a failure downstream causing an excessive current draw.
Exactly my point. This fuse should handle 200 kW. GM needs to test every fuse they install up to 95% current and s**t can the ones that fail. Maybe the fuse gets confused because electrons flow both ways and gives up.;)
 

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I'm not sure that type of testing would solve the problem. These fuses are probably defective and the defect causes them to increase in resistance slightly over time. When the resistance gets high enough to trip the computer, you get the failure message and can no longer drive the car.

Mike
 

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It was indeed the MSD. I am considering trying this instead:

29674
 

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Just an observation before we begin to pontificate on what GM should or should not do or why fuses never blow at less than their rated current.

Have you noticed the dates when these MSD discussions start in this forum? This is not a comprehensive list:
  • July 3, 2018
  • Aug 6, 2018
  • Aug 12, 2019
  • Sep 3, 2018
  • Aug 26, 2018
  • Jun 5, 2020
  • Aug 2, 2018
These suggest to me the common theme: Summer-Northern Hemisphere. Warm environment and increased driving.

The hypothesis that the MSD is failing due to thermal stressing and micro-cracking gets a bit more support. Why?

  • Summer driving adds to thermal stress. MSD fuses rated at 20C are suddenly facing 40C+ from hot asphalt and longer drives.

  • Cyclic stress of a mechanical fuse element does not necessarily result in the fuse 'blowing'. It just builds up enough resistance to cause a voltage drop detected by the control system. It could be a 40V drop (0.1 Ohm @ 400 Amp current). There is a manufacturer's engineering paper showing that up-down-up current cycle causes heating and cooling stresses that eventually cause a sudden increase in resistance. A second paper supports the same conclusion.
29675


Note that the curves do not rise to infinite resistance. They jump to ~1000 micro-ohm (0.001 Ohm) and then rise from there. The rise is precipitous and sudden; it's just metal crystallizing after a certain number of cycles. Add some current and the Bolt EV controller will shut down due to voltage drop.​
  • Current cycling happens very frequently in an EV. It's not just when to turn the car on. It's every acceleration, deceleration, DCFC charge, and regen. (Everything goes through the MSD, and current goes in both directions). There may be 5-10 cycles per minute of driving--the number of cycles can build up quickly and a susceptible MSD may fail.

  • It's generally hard to test for fatigue without a destructive test.

If so, this may be a weak point for the Bolt EV MSD design in general. Engineers may have designed the MSD based on GM hybrid experience with the Volt that may have fewer, lower stress cycles than the fully electric Bolt EV. If you design based on fuse manufacturer's specs, the MSD should 'blow' on a time-current graph like this:

29676

At 1x rated current (400A) the MSD should never blow! But there is no thermal cycling information from the fuse supplier.

There are ways forward however...

  • There appears to be an awareness of the problem. Eaton launched a new line of EV fuses in May. Their sales sheet includes design factors to test under driving profiles and simulate pulse current profiles. Those tests are new and show that EV fuses are subject to very different stresses than distribution fuses. Perhaps these new EV fuses are being included as new GM part numbers.

  • There is always the Tesla or Toyota approach of using a Pyroswitch to break the HV circuit. A Pyroswitch does not rely on heating to 'melt' and break the circuit. A separate monitor circuit check for current, or crashes, and triggers a pyrotechnic device that cuts the fuse link. It costs more, does more, but will not suffer thermal stress cycling. Maybe the next generation?
https://www.pyroswitch.com/fileadmin/Assets/Downloads/PSS_Function_Animation.mp4
 

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Great info Pike!

I wonder if you can answer a question; earlier in this thread someone mentioned that accessories like the infotainment system and phone chargers run off the 12V battery, but I would think that the 12V battery serves the same purpose as that of an ICE engine in that once the drive train is powered on it would then provide power to the 12V components through a step-down transformer. Or am I totally off?
 

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This video
can shine a light on how high current high voltage fuses work, with time the fuses deteriorates and it can break open circuit open of have higher resistance that in fact will trigger codes on the computers.
 

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Just an observation before we begin to pontificate on what GM should or should not do or why fuses never blow at less than their rated current.

Have you noticed the dates when these MSD discussions start in this forum? This is not a comprehensive list:
  • July 3, 2018
  • Aug 6, 2018
  • Aug 12, 2019
  • Sep 3, 2018
  • Aug 26, 2018
  • Jun 5, 2020
  • Aug 2, 2018
These suggest to me the common theme: Summer-Northern Hemisphere. Warm environment and increased driving.

The hypothesis that the MSD is failing due to thermal stressing and micro-cracking gets a bit more support. Why?

  • Summer driving adds to thermal stress. MSD fuses rated at 20C are suddenly facing 40C+ from hot asphalt and longer drives.

  • Cyclic stress of a mechanical fuse element does not necessarily result in the fuse 'blowing'. It just builds up enough resistance to cause a voltage drop detected by the control system. It could be a 40V drop (0.1 Ohm @ 400 Amp current). There is a manufacturer's engineering paper showing that up-down-up current cycle causes heating and cooling stresses that eventually cause a sudden increase in resistance. A second paper supports the same conclusion.
View attachment 29675

Note that the curves do not rise to infinite resistance. They jump to ~1000 micro-ohm (0.001 Ohm) and then rise from there. The rise is precipitous and sudden; it's just metal crystallizing after a certain number of cycles. Add some current and the Bolt EV controller will shut down due to voltage drop.​
  • Current cycling happens very frequently in an EV. It's not just when to turn the car on. It's every acceleration, deceleration, DCFC charge, and regen. (Everything goes through the MSD, and current goes in both directions). There may be 5-10 cycles per minute of driving--the number of cycles can build up quickly and a susceptible MSD may fail.

  • It's generally hard to test for fatigue without a destructive test.

If so, this may be a weak point for the Bolt EV MSD design in general. Engineers may have designed the MSD based on GM hybrid experience with the Volt that may have fewer, lower stress cycles than the fully electric Bolt EV. If you design based on fuse manufacturer's specs, the MSD should 'blow' on a time-current graph like this:

View attachment 29676
At 1x rated current (400A) the MSD should never blow! But there is no thermal cycling information from the fuse supplier.

There are ways forward however...

  • There appears to be an awareness of the problem. Eaton launched a new line of EV fuses in May. Their sales sheet includes design factors to test under driving profiles and simulate pulse current profiles. Those tests are new and show that EV fuses are subject to very different stresses than distribution fuses. Perhaps these new EV fuses are being included as new GM part numbers.

  • There is always the Tesla or Toyota approach of using a Pyroswitch to break the HV circuit. A Pyroswitch does not rely on heating to 'melt' and break the circuit. A separate monitor circuit check for current, or crashes, and triggers a pyrotechnic device that cuts the fuse link. It costs more, does more, but will not suffer thermal stress cycling. Maybe the next generation?
https://www.pyroswitch.com/fileadmin/Assets/Downloads/PSS_Function_Animation.mp4
Great info and research, but once again exactly what I said in my first post.. "Begining to worry about engineering mistakes regarding the location or type of fuse used." We don't know how hot it gets under the rear seat and there is no obvious heat sink. The fuse appears to be rigidly mounted in its case with no way to gently expand. Do we know for a fact that the control system can even detect a small voltage drop due to micro cracking? We do not.
And I'm pretty sure a normal battery cycle can be 40 volts from 100% SOC to 20%. Maybe the control system can nuance this out, we just don't know.
As for the new part number being an advanced fuse design, well the OP as far as I can tell had the newest part number. These comments not at all to dis your post which is very informative and most likely what is going on.
So what do we do? Replace the fuse every few years preventatively? Trust in probability and statistics or "Are you feeling lucky today" GM will probably not issue a recall until someone dies. To me, this is a really big deal. Part is $111.00 and if you disconnect the 12 volt battery there should be no codes to clear. Oh yeah, electricians gloves too and the key fob needs to go far away.
 

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Well I placed another order from another GM parts warehouse hoping they might have it in stock and I'd get a different answer. It's been a day and no update on status so I suspect I'm going to get the same story: "Your order has been canceled. This part is on national back order and we have no estimate for when they will be available."

Pretty bad when you are willing to pay the price for the part to correct a known weakness and give you some peace of mind driving the car... and GM says "Sorry. You can't have that part." :( Guess I'm forced to do like the others if this happens to me: wait until I break down on the road.

Mike
 

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There is an DC-DC converter that reduces the power from 380v+ to 12v
That’s what I thought, but given all the info above, I doubt my keys falling into the wireless charger opening caused this. But now I wonder if on a hot day it is better to NOT use regen, avoiding additional cycling.

Keep us posted. I really want to know if the MSD you ordered has the fuse inside it or not.
I was told by my dealer two to three days for the part to arrive.
 

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It seems I can't make the quote function work right now so my comment is in the quote box.....
If the fuses are not totally blowing and getting micro-cracks which cause resistance but still flow current then GM/LG are negligent for not writing software that show a service needed light and pops codes but allows the car a limp home (or a least to the side of the road) mode at reduced speed. I once had my cruise control shut off at 70 on a busy highway in L and it was no laughing matter where seconds can make a difference. I can't imagine what a joy it must be to go dead stick in a similar or worse situation.
 

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We don't know how hot it gets under the rear seat and there is no obvious heat sink.
We should have a pretty good idea. The fuse is located on top of, and actually connects/straddles, modules 5 and 6. It is directly under the rear seat cushion.

AC 72F- amb. 95F.jpg Leave Fincastle.jpg

These are both shots showing battery module and air temperature...one leaving an EA 150 kW charger on a 90 F+ day. You will notice that the modules never get as hot as 90 F, and that module 6 is slightly warmer than module 5, because module 6 has the heat generating BMS sitting on top of it. I have no shots like this for a Bolt sitting in an asphalt parking lot for ten hours...something we have never done, but the car does run the AC to cool the battery on some schedule. Can anybody tell us how hot the battery gets in that situation? I can imagine the battery being in the 80 F range, and the cabin being over 100 F.
 
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