HV Battery Emergency Cooling

With my car plugged in, inside my garage, without the HPWC activated, I have heard occasional clicking of what sounds like coolant pumps. I've never heard fans and/or compressors operating, but I have heard something that I believe to be pumps. Not definitive, but definitely activity outside the car.

apacheguy said:

However, while the car is sleeping and the contactors are open, I've never seen the car activate it's cooling system, even in hot weather (~100 F). For those of you in Texas or Arizona, have you ever witnessed this behavior? If so, what temperature triggered the wake up and cooling cycle?

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The rate of LiIon self-discharge is very low, so the amount of heat generated by a disconnected battery is pretty much negligible. Of course, if you heat the whole car to 70-80C then you might get thermal runaway even with disconnected batteries. But at this point you're going to have other problems...

apacheguy said:

I suppose you're right that it's just the coolant pumps passively cooling the cells. Although even for the coolant pumps to circulate I'm pretty sure the car has to be doing 1-3 above. I don't believe the coolant pumps operate with the contactors open while the car is asleep.

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In my case, I think the car was asleep. I didn't check, but there would be no reason for it not to be at that particular time.

I remember reading that Tesla has a patent on emergency battery cooldown in the event of thermal runaway which allows the BMS and HVAC to completely drain the pack (well past zero SOC) in the case of a thermal runaway event. If this is implemented this means it will do whatever it can do prevent a runaway if it believes one is occurring, even if it will completely destroy the pack (presumably "fire and destroyed pack" is better than just "destroyed pack".)

I woudn't be surprised to hear that the prior incidents of battery fires resulting from road debris were much limited by such a system, but ultimately futile as the coolant loop can only remove so much heat. Probably explains why the second vehicle managed to drive for 3+ minutes before finally giving in.

I recall reading in the manual (years ago) that the active cooling limit for the battery was around 120-130 degF. It didn't say it in so many words, but something that indicated normal operating temperature peaked around there, and the battery would actively cool itself beyond this limit. With the battery essentially in the shade (under the car), I'm not sure the pack would reach this limit temperature just sitting out on a 100 degF day--especially if there's a breeze.

As others have mentioned, I've heard the soft whir of a pump or fan, but what I can say is that I've never once noted shore power being drawn by the car unless it was actively charging or I had deliberately activated the HVAC (I have a dedicated sub-meter on the car's charging circuit). My car has sat in ambient temperatures from about 0 F to 110 F and no HVAC activity for battery management so far.

apacheguy said:

Todd - Think that 120-130 F target that you recall might be coming from leaked diag screens:

http://www.teslamotorsclub.com/attachment.php?attachmentid=20088&d=1365905905

Active cooling target is 55 C, passive 40 C. It makes sense that the battery pack has a high heat capacity and doesn't get that hot even in 100 F weather. Even so, 40 C = 104 F which seems way too lenient for the operating constraints of a LiIon pack that has the advantage of liquid cooling.

Also, I question whether even passive cooling occurs while the car is asleep. From my observations, the vampire drain is not excessively worse on really hot days compared to really cold nights (where I do indeed see greater vampire loss).

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I remember that screen, but I was blending that with what I think were operating limits for the pack in the manual.

My guess is that "Passive Cooling Target" means at 40C, circulate some coolant with the pumps, but don't actually turn on the heat pump itself (the chiller). At 55degC is when HVAC would kick on to actively cool the pack. So I anticipate the pack has to be about 131 degF before the HVAC kicks in. If your pack gets that hot sitting in a parking lot, you probably ought to drive out of Hell ASAP .

Remember that Tesla's using a special blend of additives in the electrolyte. There was a college lecture from a LiIon expert (the video is on here somewhere--you've probably seen it) that said the cells that Tesla's using are much better than other similar chemistries--so perhaps 104F is not a huge deal for the cells. Having said that though, I personally get a little uncomfortable about my pack on hot days. But having said that, I haven't actually seen any evidence of it degrading the pack, either. In the summer, my car is in the garage from 5:30pm on, and every day in the summer my garage is easily 100+ degF. Granted, my SOC is usually around 60% by that time, but still...

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apacheguy said:

Also, I question whether even passive cooling occurs while the car is asleep. From my observations, the vampire drain is not excessively worse on really hot days compared to really cold nights (where I do indeed see greater vampire loss).

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I meant to address this statement. I think even on really hot days, it's probably rare for the pack to actually reach 104F, as you mention because of its high heat capacity, being in the shade, and winds helping to cool it. I think the car probably wakes while passive cooling is on. I doubt the pumps would run purely off of the 12V battery. So I imagine that in such conditions, you'd hear the contactors engage, the pump run for perhaps 30 seconds, then the car go back to sleep. I think the draw from this event is probably small enough overall that you wouldn't notice it, and it probably doesn't happen often even on hot days.

Todd Burch said:

I meant to address this statement. I think even on really hot days, it's probably rare for the pack to actually reach 104F, as you mention because of its high heat capacity, being in the shade, and winds helping to cool it. I think the car probably wakes while passive cooling is on. I doubt the pumps would run purely off of the 12V battery. So I imagine that in such conditions, you'd hear the contactors engage, the pump run for perhaps 30 seconds, then the car go back to sleep. I think the draw from this event is probably small enough overall that you wouldn't notice it, and it probably doesn't happen often even on hot days.

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In my case, contactors were not engaging/disengaging. You could hear a pulsing, but that's it.

apacheguy said:

So just to clarify - was this at the beginning, or during a 12 V maintenance cycle? I've heard these sounds before while my car is parked on the driveway:

1. Click-clack (contactors engage).
2. Humming (presumably coolant circulating).
3. Soft grinding of a small rotor (cycling of the front right and left louvers).

Then it normally goes silent and 12 V maintenance resumes.

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I was outside in the garage when the car would have been well asleep for hours. No click-clack, just a few pulsing noises (presumably coolant circulating). Nothing else.

How about while driving with ac on,
I think I remember the energy app
showing a high spike while sitting idle for a
while ,I think it uses some of the chiller
coolant to cool the battery while ac is on.