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Eric, does discharge rate have any effect on degradation?
Both charge and discharge rate affect battery degradation. I've heard varying information on this (and I'm sure it's also chemistry dependent), but a .2 C charging rate (about 12 kW for the Bolt EV) seems to be the ideal. Anything more or less actually has a slightly increased impact on battery life. As for discharge rates, I'd have to find the spec sheets for the Bolt EV's cells, but I believe they are rated for a 5 C discharge rate (about 300 kW for the Bolt EV). The closer you get to that, yes, the greater the impact on battery life. In the case of the Bolt EV, though, GM has restricted the battery output to 160 kW (about 2.7 C discharge), so it's really hard to discharge the battery fast enough to significantly increase degradation.
 

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Discussion Starter #22
a .2 C charging rate (about 12 kW for the Bolt EV) seems to be the ideal. Anything more or less actually has a slightly increased impact on battery life.
Now that's really surprising. It means that the highest L2 charging at 32 amps is not as good as slow DC charging.

Something else surprising just happened to me. I took a short trip of about 10 miles in a temperature in the upper 80's. When I got home, I left the car on and plugged it in, and waited to see if a cooling cycle would begin. It did not. So I turned the car off, and a cooling cycle began immediately. Which seems to contradict both you and Sean.
 

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There are many different things going on that make it very difficult to second guess exactly what is going on in the decision tree. Using L1/L2 I have noticed that I see a battery heating cycle start after the battery has reached it's target charge level. It would appear that, at that point, it is looking to improve current delivery assuming I leave when I have told it I will. The amount of heating was not huge. It was only enough to raise it from the low 50's to about 60F.
 

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Eric, does discharge rate have any effect on degradation?
By discharge rate, you mean rapid acceleration? What I have seen in various articles is fast charge and discharge, in hot temps (and maybe cold too) can stress the batteries.
 

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Now that's really surprising. It means that the highest L2 charging at 32 amps is not as good as slow DC charging.
Yes, but from GM's perspective, it's a cost-benefit trade off. A larger charger would weigh more, cost more, and take up more space, all for some small fraction of an improvement in battery life. The better reason for those additional costs would be the benefit of recharging in less time.

Again, though, that .2 C charging rate is an ideal based on chemistry. I've heard it applied to LiFePO4 cells, so it might not translate the same to the NCM chemistry used it the Bolt EV. It's possible that the ideal charge rate for the Bolt EV's cell chemistry is .1 C, which would mean the Bolt EV's L2 charging is very close to the ideal charging speed for maximum battery life.

Something else surprising just happened to me. I took a short trip of about 10 miles in a temperature in the upper 80's. When I got home, I left the car on and plugged it in, and waited to see if a cooling cycle would begin. It did not. So I turned the car off, and a cooling cycle began immediately. Which seems to contradict both you and Sean.
What I was referencing were DC fast charging patterns that I've observed, so my observations might or might not also apply to L2 AC charging, which is very different. In fact, what you heard might have been the cooling system engaging for the AC charger itself rather than the battery. It's very unlikely that a 10 mile drive in upper 80 F temperatures would warm the battery enough to require cooling.
 

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Discussion Starter #27
It's possible that the ideal charge rate for the Bolt EV's cell chemistry is .1 C, which would mean the Bolt EV's L2 charging is very close to the ideal charging speed for maximum battery life.
Early on I thought that the slower the battery is charged, the better for the battery. But I've seen a graph from some fleet data analysis that indeed shows L2 is better for the battery than L1.
In fact, what you heard might have been the cooling system engaging for the AC charger itself rather than the battery. It's very unlikely that a 10 mile drive in upper 80 F temperatures would warm the battery enough to require cooling.
I don't understand. Why would the AC charger require cooling?
 

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What I was referring to was "spiking" the charge rate. What I have observed happen is that when you are running climate control after the first charging rate step down, if you shut off climate control, the battery charging rate will spike for a short period of time before it settles back to the typical max rate. For instance, if my Bolt EV is at 60% battery and charging at 38 kW, I can turn on climate control to max (drawing 42 kW to 45 kW from the charger). If I then turn off climate control, the Bolt EV's battery charging rate will spike to over 40 kW and slowly reduce to 38 kW. Now that I'm thinking of it, I don't recall that happening when running AC. It's primarily been when running the heater, so there might be a difference there too.
Interesting. Because I'm somewhat of a nerd, I'd sit there and constantly turn the climate control on and off to make my car charge faster. :unsure:
 

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I don't understand. Why would the AC charger require cooling?
The on-board charger absolutely does require cooling. Try charging for an hour or two at least then open the hood and feel what's warm. The same thing is true for '13+ Leafs (where the OBC is under the hood).

The power electronics simply produce waste heat. Surely you've noticed AC adapters including power bricks for laptops and other electronics get warm when they're actively being used/charging.
 

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Yes, but from GM's perspective, it's a cost-benefit trade off. A larger charger would weigh more, cost more, and take up more space, all for some small fraction of an improvement in battery life. The better reason for those additional costs would be the benefit of recharging in less time.

Again, though, that .2 C charging rate is an ideal based on chemistry. I've heard it applied to LiFePO4 cells, so it might not translate the same to the NCM chemistry used it the Bolt EV. It's possible that the ideal charge rate for the Bolt EV's cell chemistry is .1 C, which would mean the Bolt EV's L2 charging is very close to the ideal charging speed for maximum battery life.



What I was referencing were DC fast charging patterns that I've observed, so my observations might or might not also apply to L2 AC charging, which is very different. In fact, what you heard might have been the cooling system engaging for the AC charger itself rather than the battery. It's very unlikely that a 10 mile drive in upper 80 F temperatures would warm the battery enough to require cooling.
Generally a low rate like .1C is the specified ideal rate based on charge acceptance. In other words, the battery is most efficient at storing power at this low rate. This does not imply that there is any deleterious effect on battery longevity from charging slower. If the rate gets low enough such that the amount of power needed to control charging is approached, the then charge "acceptance rate" may tend toward 0 since less of the power is actually going toward stored energy. But L1 should be completely safe and offer super long life.
 

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Discussion Starter #34
Power electronics heat up very quickly. That's why they have heat sinks and coolant systems. In the computer world, people have burned out CPUs simply by powering them up without a heatsink attached.
Are you saying that the cooling cycle I heard and saw was for the electronics other than the AC charger and battery?
 

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No, I'm saying the cooling was most likely for the AC charger, and it would activate almost immediately.
Then I still don't understand. Do you understand that I had not done any charging before I plugged in?
 

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`I am not so sure turning on the AC in the cabin also cools the batteries. Are we guessing here or do we know this factually? Isn't there a three-way valve that directs the coolant to the batteries and/or the cabin but will only go to the batteries if a sensor calls for it? That's how I would set it up (30 years HVAC technician).
 

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Then I still don't understand. Do you understand that I had not done any charging before I plugged in?
Yes. The Bolt EV's power electronics require cooling when they are powered on. When you plug in, you are activating the Bolt EV's 240 V onboard charger, which requires cooling. It really doesn't matter whether you had been charging previously. That coolant loop is designed to protect the electronics from overheating.
 

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`I am not so sure turning on the AC in the cabin also cools the batteries. Are we guessing here or do we know this factually? Isn't there a three-way valve that directs the coolant to the batteries and/or the cabin but will only go to the batteries if a sensor calls for it? That's how I would set it up (30 years HVAC technician).
Yes, that is my understanding as well. They are a shared system, but one doesn't necessarily impact the other.

In case you were referring to something I posted, that was not what I was attempting to communicate. When I've referred to battery conditioning and cabin heating/cooling in this thread, I'm specifically speaking in terms of additional power draws (beyond the max charging rate for the battery). The recent discussion is about why the fans and cooling system appeared to activate when the OP turned off the Bolt EV and plugged into a L2 AC charger. In that regard, I'm simply stating that the cooling fans are most likely for the charger and not the battery.
 

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`I am not so sure turning on the AC in the cabin also cools the batteries. Are we guessing here or do we know this factually? Isn't there a three-way valve that directs the coolant to the batteries and/or the cabin but will only go to the batteries if a sensor calls for it? That's how I would set it up (30 years HVAC technician).
You do know that there are 3 coolant tanks in the Bolt, right? Read...

 
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