LeftieBiker
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Sun Apr 01, 2018 8:55 pm

Joe6pack wrote:I am a mechanical engineer, so I get how the heat is generated in the battery and how it is dissipated. Assuming no solar loading, the battery would be at ambient temperature. Once current begins flowing into or out of the battery, it begins to heat up as a function of the internal resistance (as lorenfb pointed out) and the amount of current flowing. Of course, under normal driving conditions, there shouldn't that much current. On the interstate, where you may see higher current draws, you are also going to have a lot of air flow around the battery pack.

I live in Georgia and even on the hottest days, I don't think I've seen the 24 kWh battery hit 115 degrees F just driving around - even on the interstate. If we knew all the variables, we could probably rough something out, but it's probably easier just to take some readings using LEAFSpy.
The 24kwh packs have passive air cooling that dissipates heat reasonably well while driving, as long as the air temp isn't too high. The problem here is that Nissan seems to have reallocated space in the pack previously used for airflow to more battery mass. If not even driving in cool air can cool the core of the pack, it's going to degrade.
Last edited by LeftieBiker on Sun Apr 01, 2018 11:34 pm, edited 1 time in total.
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dhanson865
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Sun Apr 01, 2018 10:16 pm

DaveinOlyWA wrote:How do we determine internal resistance from LEAF Spy? If you are talking about HX, well my internal resistance is super high. I just checked its 115.75%
resistance goes up as a battery degrades, HX goes down as the battery degrades.

It might be that HX is the inverse of resistance but I haven't seen any documentation saying what HX is and I have no reason to say it is or isn't anything else.
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SageBrush
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Sun Apr 01, 2018 10:54 pm

LeftieBiker wrote:
Joe6pack wrote:I am a mechanical engineer, so I get how the heat is generated in the battery and how it is dissipated. Assuming no solar loading, the battery would be at ambient temperature. Once current begins flowing into or out of the battery, it begins to heat up as a function of the internal resistance (as lorenfb pointed out) and the amount of current flowing. Of course, under normal driving conditions, there shouldn't that much current. On the interstate, where you may see higher current draws, you are also going to have a lot of air flow around the battery pack.

I live in Georgia and even on the hottest days, I don't think I've seen the 24 kWh battery hit 115 degrees F just driving around - even on the interstate. If we knew all the variables, we could probably rough something out, but it's probably easier just to take some readings using LEAFSpy.
The 24kwh packs have passive air cooling that dissipates heat reasonably well while, driving, as long as the air temp isn't too high. The problem here is that Nissan seems to have reallocated space in the pack previously used for airflow to more battery mass. If not even driving in cool air can cool the core of the pack, it's going to degrade.
Ambient air does not circulate in the pack. As for the repackaging and its effect on heat dissipation, ask Nissan. Back in the days of the 24 kWh pack Nissan said that they left space to aid heat dissipation. Today that space does not exist so they either have worse heat dissipation or Nissan found another way to preserve the battery cooling capacity.

Joe: as an engineer, tell us how much summer temps will decrease heat dissipation for a constant battery temperature of 115F. Say ambient in winter is 35F and summer is 95F.

There is an angle of this QC overheating story that I find helpful: reducing power during charging will presumably (?) reduce resistance heat proportional to the square of current. That data says that at 22 kW, 35F ambient and 115F in the battery, heat generation = heat dissipation since the battery temperature plateaus.

The implication then is that the decrease in heat dissipation that occurs with increasing ambient temperatures will have to be offset by reduced power flow to keep battery temperature constant (though high.) Which brings us back to Newton. Uncertain for now is the improved cooling while driving but it is not difficult to figure that out from the year-round battery experience.
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Joe6pack
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Mon Apr 02, 2018 5:24 am

SageBrush wrote:
The implication then is that the decrease in heat dissipation that occurs with increasing ambient temperatures will have to be offset by reduced power flow to keep battery temperature constant (though high.) Which brings us back to Newton. Uncertain for now is the improved cooling while driving but it is not difficult to figure that out from the year-round battery experience.
Heat transfer is a function of the difference in temperature between the sink (ambient air) and the source (the battery) and the thermal conductivity of the medium through which the heat is being transferred (for conduction). The greater the difference in temperature between the sink and the source, the more heat will be dissipated

The heat is taken away from the battery ultimately by convection. But first the heat has to be conducted away from the source through conduction to the outside of the battery case. We don't know the thermal conductivity of the battery. But, I suppose if we new for a fact that 22kW charging at 35 degrees F resulted in a steady state battery temperature of 115 degrees F, then we could derive the thermal conductivity of the system (convection and conduction). This could then be used to calculate a theoretical steady state temperature for a 95 degree F ambient temperature. This presumes of course that the thermal conductivity doesn't change as a function of temperature as well.

This still wouldn't be an equivalent situation to driving as the convection present when driving would not be present while charging.
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SageBrush
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Mon Apr 02, 2018 6:06 am

Joe6pack wrote:
SageBrush wrote:
The implication then is that the decrease in heat dissipation that occurs with increasing ambient temperatures will have to be offset by reduced power flow to keep battery temperature constant (though high.) Which brings us back to Newton. Uncertain for now is the improved cooling while driving but it is not difficult to figure that out from the year-round battery experience.
Heat transfer is a function of the difference in temperature between the sink (ambient air) and the source (the battery) and the thermal conductivity of the medium through which the heat is being transferred (for conduction). The greater the difference in temperature between the sink and the source, the more heat will be dissipated

The heat is taken away from the battery ultimately by convection. But first the heat has to be conducted away from the source through conduction to the outside of the battery case. We don't know the thermal conductivity of the battery. But, I suppose if we new for a fact that 22kW charging at 35 degrees F resulted in a steady state battery temperature of 115 degrees F, then we could derive the thermal conductivity of the system (convection and conduction). This could then be used to calculate a theoretical steady state temperature for a 95 degree F ambient temperature. This presumes of course that the thermal conductivity doesn't change as a function of temperature as well.

This still wouldn't be an equivalent situation to driving as the convection present when driving would not be present while charging.
Agree with everything you wrote.

I'm hoping for a fraction though, comparing 95F to 35F for a 115F battery. What is the fractional conduction ?
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Two years in Colorado, now in NM
03/2018: 58 Ahr, 28k miles
11/2018: 56.16 Ahr, 30k miles
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lorenfb
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Mon Apr 02, 2018 7:54 am

To fully understand what the actual battery cell temperature is under various conditions, e.g. driving or charging, one needs
to know a cell's thermal resistance (degrees C per watt) to ambient to determine its actual temperature (ambient + temp rise).
In the case of the Leaf's battery, there're are three resistances; cell to its case, case to the vehicle's chassis, and chassis to
ambient. If one knows the cell's temperature at a certain power level (watts) and the ambient temp, the overall thermal
resistance can be determined whether the vehicle is moving or charging. So then for each addition watt of power consumed
by a cell, there's an additional rise in its temperature. One can assume that the chassis temp is at ambient for charging.
Furthermore, there's an issue of there being a thermal time constant, i.e. a step function in battery power doesn't immediately
dissipate based on the thermal resistance calculation only.
#1 Leaf SL MY 9/13: 76K miles, 47 Ahrs, 5.0 miles/kWh (average), Hx=70, SOH=73, L2 - 100% > 1000, temp < 95F, (DOD) > 20 Ahrs
#2 Leaf SL MY 12/18: 10.3K miles, SOH 109Ahrs/115Ahrs, 5.2 miles/kWh (average), DOD > 20%, temp < 105F

DaveinOlyWA
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Mon Apr 02, 2018 10:23 am

Joe6pack wrote:I am a mechanical engineer, so I get how the heat is generated in the battery and how it is dissipated. Assuming no solar loading, the battery would be at ambient temperature. Once current begins flowing into or out of the battery, it begins to heat up as a function of the internal resistance (as lorenfb pointed out) and the amount of current flowing. Of course, under normal driving conditions, there shouldn't that much current. On the interstate, where you may see higher current draws, you are also going to have a lot of air flow around the battery pack.

I live in Georgia and even on the hottest days, I don't think I've seen the 24 kWh battery hit 115 degrees F just driving around - even on the interstate. If we knew all the variables, we could probably rough something out, but it's probably easier just to take some readings using LEAFSpy.
I have seen my batt temps hit 110º just sitting in a parking lot on an 85º day so 115º is hardly a stretch.

We are spending too much time pigeon holed on temperature. In the absence of high SOC, temperature is only a minor player in the degradation game.
2011 SL; 44,598 mi, 87% SOH. 2013 S; 44,840 mi, 91% SOH. 2016 S30; 29,413 mi, 99% SOH. 2018 S; 25,185 mi, SOH 92.23%. 2019 S Plus; 11,333.1 mi, 93.73% SOH
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LeftieBiker
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Mon Apr 02, 2018 12:34 pm

In the absence of high SOC, temperature is only a minor player in the degradation game.
That may be true of the 24kwh packs, but we don't know if it's true of the 40kwh, or even the 30kwh units.
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lorenfb
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Mon Apr 02, 2018 12:53 pm

dhanson865 wrote:
DaveinOlyWA wrote:How do we determine internal resistance from LEAF Spy? If you are talking about HX, well my internal resistance is super high. I just checked its 115.75%
resistance goes up as a battery degrades, HX goes down as the battery degrades.

It might be that HX is the inverse of resistance but I haven't seen any documentation saying what HX is and I have no reason to say it is or isn't anything else.
Today I ran a resistance test using LeafDD:
Initial Conditions;
Temp = 18C, SOC = 94%, Hx = 71, Miles = 62K

After test:

SOC = 94%, Hx = 71, 18C, internal battery resistance = .110 (110 mohms)

If Hx is the average value of the battery resistance (mohms) over time and NOT the present value for the battery resistance,
(battery resistance changes with temp and over time), then Hx could possibly represent the average battery resistance
over time.

Hopefully, someone will do a simple test for battery resistance on a 30/40 kWh battery;
1. No B-mode or Eco
2. Use LeafSpy on the screen that displays battery voltage and current.
3. Note the battery voltage when stopped.
4. When safe, do a hard acceleration and monitor the battery voltage and current after a few seconds.
5. Stop the acceleration, find a safe parking spot.
6. Take the difference in battery voltage (steps 3 & 4) and divide it by the peak current measured in step 4.

Example:
Voltage change - 10 volts, Current change 100 amps, then battery resistance is .10 ohms (100 mohms)

Hopefully, I don't have to rent/lease a '18 Leaf to obtain the data.
#1 Leaf SL MY 9/13: 76K miles, 47 Ahrs, 5.0 miles/kWh (average), Hx=70, SOH=73, L2 - 100% > 1000, temp < 95F, (DOD) > 20 Ahrs
#2 Leaf SL MY 12/18: 10.3K miles, SOH 109Ahrs/115Ahrs, 5.2 miles/kWh (average), DOD > 20%, temp < 105F

kennethbokor
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Re: Possible Widespread 2018 Traction Battery Quick Charge Problems

Mon Apr 02, 2018 4:00 pm

Hello all interested, here is an official response from Nissan Canada regarding #rapidgate:

"The 2018 Nissan LEAF has charging safeguards to protect the battery during repeated fast charging sessions in a short period of time. While the safeguards may increase charging times after multiple fast charging sessions, they are important to maintaining battery life over an extended time period."

I’m not surprised at the response as I believe like many others do, that Nissan has targeted the new Leaf for a use case and market that supports a very high percentage of using the Leaf in no more than 1 or 2 DCFC’s in a successive timeframe. So, I don’t believe any “fix” is going to come from this, just some new “spin” wording from Nissan to point out this use case to prospective buyers and that it is a “feature”. It’s not a negative if you don’t mind longer travel times, so really this #rapidgate is rather subjective.

As long as prospective buyers are aware of this “safeguard”, then they can choose to buy a new Leaf or not. At least they will know up front and if this #rapidgate has done anything, it’s at least empowered prospective buyers with more information so they can make a more informed decision.

This does not sour me at all in my decision to purchase a new Leaf and I am sure this really won’t affect global sales – they will still be over 60,000 units, probably pushing north of 90,000 this year.
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