SageBrush
Posts: 2795
Joined: Sun Mar 06, 2011 2:28 am
Delivery Date: 13 Feb 2017
Location: Colorado

Re: Using speed to control battery temperature

Tue May 29, 2018 4:43 pm

TexasLeaf wrote: I prefer more explicit information


Then how about a little physics:

Power is proportional to current
Heat generation is proportional to the square of current
Therefore heat generation is proportional to the square of power

Put another way: go as fast as you want down a long hill, but take it easy otherwise.
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Car is now enjoying an easy life in Colorado
3/2018: 58 Ahr, 28k miles
-----
2018 Tesla Model 3 LR, Delivered 6/2018

LeftieBiker
Posts: 9395
Joined: Wed May 22, 2013 3:17 am
Delivery Date: 31 May 2013
Location: Upstate New York, US

Re: Using speed to control battery temperature

Tue May 29, 2018 7:41 pm

The 2018 Leaf doesn't have temperature bars, just a gauge with blue, normal and red ranges. Right now it's 96 degrees F outside, my Leaf has been sitting in the sun all day and the battery temperature is just a little on the red side of middle normal. So there is a little room between Texas ambient temperatures and the red zone.


Oops!!! I now drive a 2018, but I'm still thinking in terms of bars. Anyway, my suggested approach to reading and describing the new temp gauge is to read it left to right as a percentage, with very cold temps reading about 10-20% on the gauge, and max temp in the Red Zone being 100%. My car usually stays between 40% and 60%, for example. And again: oops. ;-)
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.

goldbrick
Posts: 228
Joined: Tue Aug 01, 2017 3:33 pm
Delivery Date: 01 Aug 2017
Leaf Number: 311806
Location: Colorado front range

Re: Using speed to control battery temperature

Wed May 30, 2018 9:42 am

SageBrush wrote:Power is proportional to current
Heat generation is proportional to the square of current
Therefore heat generation is proportional to the square of power


Not quite. Heat generation is proportional to power (I^2 * R).

Heat is just a form of power and getting x^2 units of heat from x units of power would enable perpetual motion.

arnis
Posts: 900
Joined: Sat Jan 23, 2016 3:21 pm
Delivery Date: 23 Jul 2014
Leaf Number: 015896
Location: Estonia, Europe

Re: Using speed to control battery temperature

Wed May 30, 2018 2:26 pm

TexasLeaf wrote:I would also have to do some testing to see if the running the AC when charging helps or hurts. Sure the condenser fan will blow air across the batteries but it's hot air coming off the condenser. If you have any empirical data to share I would like to see it.



Normally, AC compressor load is 200-300W in warm weather and normal cabin temperature.
That means less than 1000W of heat is being generated at the exterior condenser.
There are two MASSIVE fans outside. Air temperature rises less than half a degree.
Which is much less than battery temperature above ambient(+0.5 degrees).
Short range EVs <30kWh -- Medium range: 30-60kWh -- Long range: >60kWh
Charging: Trickle <3kW -- Normal 3-22kW -- Fast 50-100kW -- Supercharging >100kW

TexasLeaf
Posts: 70
Joined: Wed Mar 21, 2018 5:37 am
Delivery Date: 21 Mar 2018
Leaf Number: 303111

Re: Using speed to control battery temperature

Thu May 31, 2018 3:41 am

arnis wrote:Normally, AC compressor load is 200-300W in warm weather and normal cabin temperature.
That means less than 1000W of heat is being generated at the exterior condenser.
There are two MASSIVE fans outside. Air temperature rises less than half a degree.
Which is much less than battery temperature above ambient(+0.5 degrees).



The 1000W of heat rejection sounds about right but not the 0.5 degrees of temperature increase. Using the simple HVAC equation of Q=1.08*V*DT, you would need about 7,000 CFM of air to maintain only a 0.5 temperature increase. You would need a fan as big as the entire car to push this much air.

It's more realistic to assume a 10 degree F temperature increase, this would require an air flow of about 350 CFM. Looking at the size of the HVAC fans on the Leaf, 300 CFM sounds about right. So driving with the air conditioner heats your battery two ways, one with the current that it draws and two with the hot air it blows over the battery.

I do appreciate you comments though and you did get me thinking. The Leaf has a heat pump. If you get about 1000W of heating then you should be able to get about 800W of cooling.

So it makes sense to keep your cabin temperature as high as tolerable on long trips with multiple charges where your are worried about battery heat build up. I might also help to run the heater with the windows rolled down when charging. You just have to make sure that it's the heat pump doing the heating and not the backup electric heater.

I'm don't know how much running the heat pump during heating is going to help cool the battery. I know that the Leaf motor compartment is not design very well designed for extracting heat and heat builds up in there. It might do more good just to open up the hood and let the motor compartment vent during charging.

TexasLeaf
Posts: 70
Joined: Wed Mar 21, 2018 5:37 am
Delivery Date: 21 Mar 2018
Leaf Number: 303111

Re: Using speed to control battery temperature

Thu May 31, 2018 7:16 pm

TexasLeaf wrote:The 2018 Leaf doesn't have temperature bars, just a gauage with blue, normal and red ranges.


Actually it appears that I was wrong. The temperature gauge of the 2018 Leaf is broken up into the same twelve segments or "bars" as the temperature gauge of the older Leafs; two bars for the blue zone, eight bars for the normal zone and two bars for the red zone. Except there is no gap between the bars so the gauge looks linear instead of the true step gauge that it is.

I ran a little test on my way to and from work today, a 13 mile trip each way. On my way into work, I drove at 70 mph, I turn the heat pump on and rolled down the windows but on my way home I drove at 60 mph and I turn on the air conditioning as usual. On the morning drive it was about 77 degrees F outside but on the evening drive it was about 96 degrees F.

I started both trips with the battery temperature gauge at six bars. When I finished the morning trip the battery temperature gauge was still at six bars and the engine compartment was cool. When I finished the evening trip the battery temperature was at seven bars and I could feel quite a bit of heat rising from the motor compartment when I opened the hood.

I know the range between bars is so great that comparing bars is hardly a reliable test. But the motor compartment being cool at the end of my morning commute indicates to me that the heat pump removes a significant amount of heat from the engine compartment and has to be helping to cool the battery. But the thermostat only goes up the 90 degrees F, so if the ambient temperature is hotter than 90 degrees the heat pump won't come on and remove heat from the motor compartment and battery.

Whenever I have had trouble with high battery temperature and CHAdeMO charging speeds the battery temperature gauge was reading ten or eleven bars. I am fairly confident that ten bars causes the battery thermal management system to restrict CHAdeMO current flow. I am also fairly confident that if you start your CHAdeMO charge with the battery temperature at eight bars or less you should be able to fully charge without restricted current flow.

So the object of my tests will be to manage the vehicle operation to maintain the battery temperature at eight bars or less on very long trips with many CHAdeMO charging stops. I understand that CHAdeMO charging is probably going to raise the battery temperature up to ten bars. The vehicle operation is going to have to be able to not only maintain the battery temperature at eight bars but also bring down the battery temperature form nine or ten bars after charging.

I envision starting out a long trip in the morning driving normally at the speed limit. As the trip progress and the battery heats up, the heat pump would need to be turned and vehicle speed reduced as necessary to keep the battery temperature to eight bars. Once the ambient temperature rises above 90 degrees F, only vehicle speed would be effective in controlling battery temperature.

It's going to be a few weeks before I have enough free time to perform these test. By then I also hope have LeafSpy so I can have a more accurate way of telling what the battery temperature is. In the meantime any feedback would be welcome.

SageBrush
Posts: 2795
Joined: Sun Mar 06, 2011 2:28 am
Delivery Date: 13 Feb 2017
Location: Colorado

Re: Using speed to control battery temperature

Thu May 31, 2018 7:30 pm

What do you mean by heat pump use ? So far as I know, it can transfer heat in either direction and either function as "AC" (cooling) or as a cabin heater.

I'm skeptical of your ability to cool down the battery during driving on a hot day but I'll be glad to be wrong.
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Car is now enjoying an easy life in Colorado
3/2018: 58 Ahr, 28k miles
-----
2018 Tesla Model 3 LR, Delivered 6/2018

LeftieBiker
Posts: 9395
Joined: Wed May 22, 2013 3:17 am
Delivery Date: 31 May 2013
Location: Upstate New York, US

Re: Using speed to control battery temperature

Thu May 31, 2018 10:14 pm

So you're proposing that we blast the (heat pump) heat in hot weather, with the windows open to stop us from frying, to cool the pack? Yikes. I remember when my family was driving to Central NY when I was a kid, to drag race (my father ran a 150HP Volvo 122S in various high-letter NHRA classes), and the family station wagon started to overheat from the full load, trailer in tow, highway upgrade, and hot weather. Dad had to blast the heater to keep the wagon's engine from overheating, and it was...not fun. Let me propose a small modification to that:

Ideally, you'd want to have a ventilation duct connected to a plastic exhaust hose running outside of the car. Failing that, you could try closing all vents save the two upper front ones you can aim at the passenger side window. Then you run the heat through those two ducts only, Recirculate off, aimed out the open window. (I'd recommend against having passenger in that spot, except maybe for a heat-loving elderly person.) It would still get hot as blazes in the car, but not quite as hot. I can tell you how to build a portable freezer block A/C unit for the car interior, if you like.
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.

TexasLeaf
Posts: 70
Joined: Wed Mar 21, 2018 5:37 am
Delivery Date: 21 Mar 2018
Leaf Number: 303111

Re: Using speed to control battery temperature

Sat Jun 02, 2018 6:43 am

I wanted to add a few notes on things that might reduce battery temperature when CHAdeMO fast charging. If the heat pump does effectively help remove heat from the battery (I'm not at all sure it will) and the ambient temperature is less then 90 degrees F (the maximum cabin temperature set-point) then you could run the heat pump while charging with the windows open or the doors open. If the ambient temperature is above 90 degrees F or if you just don't want to run the heat pump, then you could open the hood to prevent heat from getting trapped inside the motor compartment which would help draw heat away from the battery pack.

Things like charging in the shade or in a cool parking garage also might help. These are all things I plan to test when I can free up some time. Again, any additional suggestions you can think of for cooling the battery on long trips with multiple CHAdeMO charges would be appreciated.

SageBrush
Posts: 2795
Joined: Sun Mar 06, 2011 2:28 am
Delivery Date: 13 Feb 2017
Location: Colorado

Re: Using speed to control battery temperature

Sat Jun 02, 2018 8:26 am

LeftieBiker wrote:So you're proposing that we blast the (heat pump) heat in hot weather, with the windows open to stop us from frying, to cool the pack? Yikes. I remember when my family was driving to Central NY when I was a kid, to drag race (my father ran a 150HP Volvo 122S in various high-letter NHRA classes), and the family station wagon started to overheat from the full load, trailer in tow, highway upgrade, and hot weather. Dad had to blast the heater to keep the wagon's engine from overheating, and it was...not fun..

That works because the radiator pumps air cooled fluid through the hot engine and discharges the heat into the cabin

An EV is a different animal entirely. Heating the cabin is via battery energy, albeit at a COP > 1.0.
If the intent is cool off the battery, nothing good will come of this maneuver.
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Car is now enjoying an easy life in Colorado
3/2018: 58 Ahr, 28k miles
-----
2018 Tesla Model 3 LR, Delivered 6/2018

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