Using speed to control battery temperature

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TexasLeaf

Well-known member
Joined
Mar 21, 2018
Messages
115
Location
Texas
I recently bought a 2018 Leaf SL. I have already had a couple of instances where the battery got hot and would not DCFC very fast. I'm hoping in the future to take some long distances trips in my Leaf so I'm thinking of ways to mitigate the hot battery and slow charging issues.

I made some long distance trips in my 1st generation Ford Focus Electric. One thing I found out driving an EV without fast charging is that the faster you drive the longer it takes to get where you want to go. It makes sense that the same principle could be applied to battery heating and slow charging.

Battery heating is caused by amperage to and from the battery. The Leaf thermal management system controls the amperage to the battery when charging but we control the amount of amperage from the battery with the speed we travel. We know that the battery has the ability to cool itself but the cooling system gets overloaded over long distances and with multiple fast charges.

There must be some speed at which the battery cools enough to allow full amperage during fast charging. But what is that speed? There are of course other factors such as accessory use and ambient temperature that would enfluence this threshold speed but when need to establish a baseline then work on the other stuff.

We can't changed the way the Leaf was designed but we can control the way we use it. Reading through this forum there appear to be some pretty knowledgble people that frequent here. I have found a lot of threads on the battery over heating issue but very little on how to manage the over heating.

I plan to perform some tests to see at what speed does the battery not get too hot to accept full amperage on fast charging. This is going to take some time, driving around in a loop at different speeds and stoping to fast charge when needed. Texas in summer would be a good place to perform these tests.

If anyone has performed these kinds of tests or knows of any good ways to manage battery temperature on long trips I would like to hear about it. Please don't just regergitate the infromation on battery temperature in the owners manual. I'm looking forward to hearing about your experiences, thoughts and ideas on this issue.
 
One issue I can see with trying that in Texas is that the ambient temp may often be too high to actually cool the pack if it's above 5 bars but below 8.
 
It looks like I'm going to ignore my own request and regurgitate some of the owners manual. Most things we do to increase power economy should also reduce battery heat. So things like using ECO mode and not using climate control will also reduce battery heating.

But some things we do for increasing power economy, like using regenerative braking and pre-conditioning the cabin, actually work against us when trying control battery heating. So basically anything we can do to reduce or eliminate amperage to and from the battery will help reduce battery heat.

One thing that is very import to this issue that's in the manual is that we should not fast charge when the battery temperature is near the red zone on the battery temperature gauge. What the heck does "near" mean? If we are able to control and even reduce the battery heating by managing the power consumption, just exactly where does the temperature on the battery temperature gauge need to be so that we know we will get full amperage during a fast charge?

But we are getting ahead of our selves. First we need to find out "if" we can control battery temperature on long trips. Once we answer the "if" question then we can answer the "how much" question we need answered for full amperage fast charge.
 
LeftieBiker said:
One issue I can see with trying that in Texas is that the ambient temp may often be too high to actually cool the pack if it's above 5 bars but below 8.


The 2018 Leaf doesn't have temperature bars, just a gauage 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.

Yesterday was just about as hot and I did a long trip in my Leaf. I didn't have any real trouble with battery temperature until my last charge after driving about 250 miles. If I can find a way to control battery temperature in Texas then we know we can control battery temperature almost anywhere.
 
Yes. There are ways to reduce heat buildup.
1) Do not accelerate hard, be very gentle. In any given moment, keep motor power below 20kW. Including uphill
In case of "I need to go faster right now" accelerate at power UP TO 30kW. This is important. Those seconds DO matter.
2) Do NOT regen hard. Keep regen up to 20kW. So slow down slower.

3) Start your journey at 100% charge state.
4) Visit quick chargers before you drop below 20%. This is important.
Rather than discharging below 20% on the next leg, consider charging more before departure.

So it would be easy to remember as 20-20-20 rule :)

Speed itself doesn't matter a lot.
Using AC is fine. It might even help to use mild AC while QC as outside fan is blowing while vehicle is stationary. That is good.
 
arnis said:
Yes. There are ways to reduce heat buildup.
1) Do not accelerate hard, be very gentle. In any given moment, keep motor power below 20kW. Including uphill
In case of "I need to go faster right now" accelerate at power UP TO 30kW. This is important. Those seconds DO matter.
2) Do NOT regen hard. Keep regen up to 20kW. So slow down slower.

Speed itself doesn't matter a lot.
I like this advice.
 
@TexasLeaf,
Are you able to monitor battery temps with LeafSpy ?

That would be really interesting data
 
SageBrush said:
@TexasLeaf,
Are you able to monitor battery temps with LeafSpy ?

That would be really interesting data

I don't have LeafSpy yet but this test would be a good reason to get it.
 
arnis said:
So it would be easy to remember as 20-20-20 rule :)

Speed itself doesn't matter a lot.
Using AC is fine. It might even help to use mild AC while QC as outside fan is blowing while vehicle is stationary. That is good.

I very much appreciate your suggestions but I am a little skeptical. As a rule I don't like rules-of-thumb. I prefer more explicit information like, "if your battery temperature is 110 degrees F or less you can get a full charge to 80% at full amperage".

Your comment about speed goes against the grain. For a change in speed you need a cubic change in power and amperage. But these are the kinds of issues I need testing for.

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.

I would also like to point out that I really don't like the idea of driving slow to reduce battery temperature. I had to do a lot of slow driving in my 1st gen Focus Electric for range on long trips. With my 2018 Leaf I was hoping to never have to drive an EV slow like that again.

I will probably just start out trying to reduce energy consumption by turning off e-Pedal and turning on ECO mode. If that doesn't work I will reluctantly try slower speeds. If I still don't feel comfortable with the driving slower speeds I might just bite the bullet and just deal with the longer charging sessions when they happen.
 
TexasLeaf said:
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.
 
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. ;-)
 
SageBrush said:
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.
 
TexasLeaf said:
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).
 
arnis said:
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 said:
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.
 
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.
 
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.
 
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.
 
LeftieBiker said:
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.
 
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