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On today's TMC Podcast, we ask the question "What is the Tesla Cybercab?". Join us on YouTube live at 1PM and participate in the chat.
glhs272
Unnamed plug faced villian
My guess would be resistance develops at the contact point inside one of the contactors. That higher than normal resistance becomes heat. Therefore when at full amps it sees full heat. That heating may lead to more resistance. Suddenly poof... contacts melt and contactor opens up. Considering how many amps the pack can put out, it doesn't take much resistance across the contactor to make it fail.
I would ratify what others have said. Have it in the back of your head that if the car goes clunk you will need to find a safe place to glide it in to to park, and place the car into "tow mode"/Neutral right away and then chock the wheels. Mine took a few tries till the rear brakes released and I couldn't get it to raise the car up every. If you really want to be proactive kick the car in neutral once or twice to feel just how far it will go with no power (It holds it's speed damn well when coasting, I managed to complete a passing maneuver even though it failed half way through), and I would think twice about 80% or more acceleration when away from cell communications, we had a couple of mile hike to get back to cell service when ours went.
Peter
Peter
The 'clunk' might actually be an 'unclunk' since likely the relay (contactor) spring is returning it to the OFF position due to operation outside of normal parameters. But that begs the question: where is the clunk when the car is turned ON or shifted out of P or N? Do we hear this?
Redundancy could be built in by use of duplicate control circuitry, twin relays, etc to keep the car going: "Car is now being controlled by Circuit B - please visit Service asap." Finding space for this inside the existing batt packs will be a challenge however. But any future redesign should allow it.
Redundancy of Power Units has been answered with the AWD "D" which can limp home using either front or rear unit if one should fail to produce power. Looking for a good excuse to go AWD??
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Redundancy could be built in by use of duplicate control circuitry, twin relays, etc to keep the car going: "Car is now being controlled by Circuit B - please visit Service asap." Finding space for this inside the existing batt packs will be a challenge however. But any future redesign should allow it.
Redundancy of Power Units has been answered with the AWD "D" which can limp home using either front or rear unit if one should fail to produce power. Looking for a good excuse to go AWD??
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That's my guess. I've also wondered if marginal voltage or DC-DC converter power output in the 12V side is a contributing factor - the "squealing balloon" noise that some report on hard acceleration is said to come from the contactors, which I'm assuming must be moving slightly from the fully open position to induce the noise.
Walter
I think the 'clunk' might be a little bit of gear lash after acceleration has dropped off suddenly?
The 'clunk' might actually be an 'unclunk' since likely the relay (contactor) spring is returning it to the OFF position due to operation outside of normal parameters. But that begs the question: where is the clunk when the car is turned ON or shifted out of P or N? Do we hear this?
Redundancy could be built in by use of duplicate control circuitry, twin relays, etc to keep the car going: "Car is now being controlled by Circuit B - please visit Service asap." Finding space for this inside the existing batt packs will be a challenge however. But any future redesign should allow it.
Redundancy of Power Units has been answered with the AWD "D" which can limp home using either front or rear unit if one should fail to produce power. Looking for a good excuse to go AWD??
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You know, the contactors are one of the parts that probably has to be changed to unleash the full 700 horsepower of the P85D. If they chose to change it by putting a pair of the legacy contactors in parallel, it'd achieve most of your purpose, get the output they needed, and let them keep using the existing parts from the same suppliers. If one contactor failed, you'd be able to drive on the other one (but with only ~400 HP available. The horror!) until you got it fixed, and you'd know there was a problem.
The unique parts in the battery and charging system are about the only single points of failure left in the drivetrain of the D cars (of course, there's lots of single point failures in suspension, steering, etc - we don't think about them because they haven't been failing.)
Walter
SteveS0353
Member
The 'clunk' might actually be an 'unclunk' since likely the relay (contactor) spring is returning it to the OFF position due to operation outside of normal parameters. But that begs the question: where is the clunk when the car is turned ON or shifted out of P or N? Do we hear this?
Redundancy could be built in by use of duplicate control circuitry, twin relays, etc to keep the car going: "Car is now being controlled by Circuit B - please visit Service asap." Finding space for this inside the existing batt packs will be a challenge however. But any future redesign should allow it.
Redundancy of Power Units has been answered with the AWD "D" which can limp home using either front or rear unit if one should fail to produce power. Looking for a good excuse to go AWD??
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I believe the clunk is indeed an "unclench". The contractor has to carry very high current at full "throttle", and the noise is undoubtedly the contractor opening while hundreds of amps are flowing through the contact points. A household thermal magnetic circuit breaker makes a heck of noise when it opens under load -- I suspect it's the same thing here.
I suspect that the contacts stay closed all the time in normal operation -- there seems no need for them to open unless there is a fault condition detected. However, if indeed the contractors close when you touch the break pedal to "start" the car, there is no noticeable "clunk" because there is very little current flowing at that time.
Most modern EVs I know anything about have the contactors open when shut off, though they may close it under remote control or computer guidance for HVAC and TMS operation. However, the vampire losses on the Tesla cars (something else most other EVs don't do) cause me to suspect that you're correct that Tesla has chosen to keep the HV battery live most of the time.
Walter
Cottonwood
Roadster#433, Model S#S37
Thanks for the advice.
I will carry my satellite text message box with me, just in case. In addition, I will keep the passing punches a little lighter when I am outside of cell coverage.
Early adopters...on the bleeding edge...
You can hear contactors closing/opening when locking/unlocking the car with sleep mode enabled. It's obviously not as loud as the clunk, because there is no load.
So why would that be dependent on vehicle age? A number of us used to have the squeal under load, but have since lost all sign of it. And I'm not quite sure I understand the 12 V argument either. The DC-DC converter doesn't draw much power at all compared to power required to accelerate the car - it's more like a drop in the bucket.
stevezzzz probably got my original A pack with only 4500 miles on it.
My refurb "A" has never had the range of my original and is degrading rapidly. 4500 miles later, I only have 240 rated miles at range down from 258 in 1/2014 with my original pack. Tesla is still thinking about what to do about this 9 months later . . . This is despite a warranty that states you will receive a battery with the same or more miles on it.
Congrats stevezzz on the great range.
It's just an idea, with little proof so far - it could be quite incorrect.
Keep in mind, the high voltage contactors are fairly similar in design to a relay - 12V power is going to an electromagnet that holds the contactor closed while a spring tries to push it open.
Thus, what I was suggesting is that if the car's 12V voltage becomes low or has a lot of ripple in the voltage from an overloaded DC-DC converter, it could cause the contactor to not stay fully closed, possibly leading to bits of sparking and then the overheating and failure suggested above.
I'm pretty sure what I just described can happen with the types of pieces the Model S has in its system. I have no idea if that is what is actually happening to the cars experiencing the failures.
Walter
Correct me if I'm wrong, but isn't a solid state relay just a glorified transistor? Do you know of any of those that can reliably hold 400V back against all failure modes?
Walter
glhs272
Unnamed plug faced villian
The pack has good 'ol heavy duty contactors (name given to Big Relays). See the pictures of the pack taken apart: http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-pack
spaceballs
Member
Reason is SSR, SCR, IPM, IGBT, MOSFET, transistors etc.. gate/device failures do not guarantee the device will short open, i.e. they can short closed.
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