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I have a two-month old 2019 Premier. Everything has been working great. Last Thursday night, I was getting ready to leave for a 4-day weekend trip. I was taking another (older) car to leave at the airport and so put the car cover on the new Volt and locked it. Battery was at about 40% charge. The car cover is a top quality custom cover from California Car Cover company and has a mesh vent where the grill is on the lower front of the car.

I got back yesterday evening and noticed a small puddle of liquid under the front of the car on the driver's side. On closer examination, I opened the hood and determined that it had come from the battery reservoir, which was totally full to the cap. I decided to pull the cap off which caused coolant to surge out of the tank. I immediately screwed the cap back on and the fluid stopped spilling out. The overflow tank was pressurized even though the car had not been run for 4 days and was at ambient outdoor temperature, which was about 85-90 degrees F.

I rinsed the coolant off the driveway and rinsed off the engine compartment and went in for the night. This morning, there was no more coolant under the front of the car and the car ran fine back and forth to work today. When I got to the car to go home tonight, I popped the hood and checked the battery coolant tank and it was at a normal level; filled up to the seam like the other two coolant tanks. Coolant was no longer up to the cap.

So the question is: What happened??? It is hot here this time of year (high temps in the 90's) and the car was under a car cover for that 4 days. Is that a factor in what happened here?

Any suggestions to ensure this doesn't happen again? I really would like to use the cover when the car is going to sit out for days in the hot sun.

Thanks for any suggestions.
 

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Was the charge cord plugged in.
It shouldn't matter. The car is set up to run the battery chiller if it gets too hot with a full charge, whether plugged in or not.
 

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It shouldn't matter. The car is set up to run the battery chiller if it gets too hot with a full charge, whether plugged in or not.
Point being, it could under the right circumstances be running the battery chiller when sitting. I am sure it can do this when plugged in. And this would pressurize that coolant circuit. And if you would remove the cap with it pressurized, juice might squirt out. Sounds like this reservoir might have been over filled. In any case, my reservoir levels are always at the or close to the recommended line, not full to the cap.
 

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Maybe it was some combination of being over-filled, having an air bubble in the circuit, and running the battery cooling system. I don't know if it has an overflow or over-pressure valve (possibly part of the cap), but if so, it may have vented as designed and possibly corrected whatever condition caused it. The fact that it is now showing the proper fill level is a good sign. I would keep an eye on the level and take it in if it goes up or down significantly or produces any more puddles.

I don't see any reason to avoid using the car cover. I don't think that caused any problems. If anything it helped keep the car cooler than it would have been in full sun.
 

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Was the charge cord plugged in?
It was not plugged in. Battery was at about 40% as I had driven the car 20 miles with an elevation gain of 1300' before it was parked for the 4 days. I did not put the cover on until the car was at ambient temperature.
 

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Maybe it was some combination of being over-filled, having an air bubble in the circuit, and running the battery cooling system. I don't know if it has an overflow or over-pressure valve (possibly part of the cap), but if so, it may have vented as designed and possibly corrected whatever condition caused it. The fact that it is now showing the proper fill level is a good sign. I would keep an eye on the level and take it in if it goes up or down significantly or produces any more puddles.

I don't see any reason to avoid using the car cover. I don't think that caused any problems. If anything it helped keep the car cooler than it would have been in full sun.
I hope that's the situation. I had never looked under the hood after we bought the car so did not notice the level of the coolant tank prior to the incident. I will keep an eye on it from now on. Thanks.
 

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You should have it checked out by the dealer under warranty.

There's no situation in which the battery coolant would be circulating under the conditions you describe, with the battery at 40%.
Even if it were, there's no reason for that circuit to be pressurized while cooling. The battery temperature will typically be maintained to be closer to body temperature than boiling. The "hot side" of the chiller operation would be in the refrigerant loop at the condenser.
 

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You should have it checked out by the dealer under warranty.

There's no situation in which the battery coolant would be circulating under the conditions you describe, with the battery at 40%.
Even if it were, there's no reason for that circuit to be pressurized while cooling. The battery temperature will typically be maintained to be closer to body temperature than boiling. The "hot side" of the chiller operation would be in the refrigerant loop at the condenser.
I think this is right. I have monitored coolant loop temps using a phone app. It keeps the battery pretty tight on temperatures, around 45-80 something. So pressure wouldn't be from being hot, but might be from a pump being on, and I agree, the pump should not have been on in this case. So it might just be that it was over filled from the start, and all is OK. But being under warranty, seems like your first stop might have been to visit the dealer before opening the cap. The battery coolant cap on the Gen 1 volt is special one that you need a special tool to open. Is that not the case on the 2019's?
 

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I think this is right. I have monitored coolant loop temps using a phone app. It keeps the battery pretty tight on temperatures, around 45-80 something. So pressure wouldn't be from being hot, but might be from a pump being on, and I agree, the pump should not have been on in this case. So it might just be that it was over filled from the start, and all is OK. But being under warranty, seems like your first stop might have been to visit the dealer before opening the cap. The battery coolant cap on the Gen 1 volt is special one that you need a special tool to open. Is that not the case on the 2019's?
On my 2019, it was a 'squeeze and turn' cap. I will contact the dealer today and post back with their response. Thanks
 

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The pressure in the coolant tank could have been caused by elevation gain. You said you drove up 1300 feet before parking.
 

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I've got a 2018 LT. Both my electronics and battery coolant reservoirs were "overfull" when I took possession of the car 27k miles ago. Not full to the cap, but well over the arrow indicators on the coolant overflow tanks. I never bothered with them, and they are at the same level today that they were nearly two years ago. I've operated the car in highway temps as high as 113f. And as low as 10f. Never a problem, never any overflow.
 

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I did not know the battery coolant loop was sealed, under pressure. The ICE coolant loop will potentially reach temperatures above 212F, so would start to boil off if not sealed. The battery coolant probably never gets hotter than somewhere around ~125F, else the Volt would sense an overheated battery condition and probably shut down the car before the battery continued to overheat. I agree it could have been cause by overfilling the battery coolant overflow tank possibly combined with air bubble(s) trapped in the battery pack. Definitely something for the dealer to evaluate.
 

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If you decide to have a dealer look at the issue, I suggest you take it in as soon as possible. The weather will start to cool off soon and that will make it much harder to reproduce the problem (the TMS will rarely run). I would think that hooking up the vacuum filler device to it might resolve any remaining air pockets in the system.
 

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The pressure in the coolant tank could have been caused by elevation gain. You said you drove up 1300 feet before parking.
It won't be that much. This sounds more like it was boiling or way over pressured. You can drive from sea level to where I live (5,700 ft) and not have a coolant surge tank do this. They'll his as you open them but not spray out.
 

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I did not know the battery coolant loop was sealed, under pressure. The ICE coolant loop will potentially reach temperatures above 212F, so would start to boil off if not sealed. The battery coolant probably never gets hotter than somewhere around ~125F, else the Volt would sense an overheated battery condition and probably shut down the car before the battery continued to overheat. I agree it could have been cause by overfilling the battery coolant overflow tank possibly combined with air bubble(s) trapped in the battery pack. Definitely something for the dealer to evaluate.
It has a 5 PSI pressure relief cap. The electronics coolant loop also has a 5 PSI pressure cap.
 

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It has a 5 PSI pressure relief cap. The electronics coolant loop also has a 5 PSI pressure cap.
Good to know. So unlike a traditional engine coolant system that may be pressurized to 14 - 15 PSI (1 atmosphere) the Volt's battery and electronics coolant loops are low pressure. If the coolant overflow tank cap is removed, other than from movement of the vehicle, is there any chance of coolant escaping from the battery and electronics coolant loop overflow tank? I.e. is the cap really there to keep contaminants from entering the overflow tank and the coolant from sloshing out?
 

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Good to know. So unlike a traditional engine coolant system that may be pressurized to 14 - 15 PSI (1 atmosphere) the Volt's battery and electronics coolant loops are low pressure. If the coolant overflow tank cap is removed, other than from movement of the vehicle, is there any chance of coolant escaping from the battery and electronics coolant loop overflow tank? I.e. is the cap really there to keep contaminants from entering the overflow tank and the coolant from sloshing out?
Both to keep the coolant in and to keep contanaments out. The 5 PSI caps tell me these loops aren't supposed to get that hot and so far the hotest I've seen my Volt in EV mode is about 105F.
 

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It sounds like this is a battery coolant loop leak, not caused by overheating or overfilling.
 

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It sounds like this is a battery coolant loop leak, not caused by overheating or overfilling.
How do you explain the apparent pressurization? Pump activity does not, by itself, result in pressurization. Leak events tend to reduce pressure.
 
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