Hot traction battery!

Here's something to work with. It's the plug-in model, so battery chemistry & operation is different. But it at least provides some basis of understanding the numbers...

 

In a hot humid climate, manually setting the ventilation higher will bring more humid air into the car .
This will increase the load on the air conditioning and thus the power draw on the battery
The increased load on the a/c may exceed the a/c's capacity to cool the air stream adequately and result in a higher RH in the car.
Higher RH in a hot car will make you feel hotter at any given temperature

More than minimum outdoor air into an air conditioned space is a good idea in many climates much of the time but hot humid conditions is not a good time for it.

 

Thank you but, no. I would like to see Toyota's expected normal operating temperature for the traction battery. I have not been able to find those numbers. Perhaps someone here knows where they might be published? A minimum and maximum temperature.

 

It would also be useful to know the design airflow for the traction battery fan under high battery temperature conditions.

 

Sure it would be great, but I'm not aware that Toyota has published any such info. I'd say the observation that the controller starts "protecting" the battery by restricting CDLs/CCLs at 116F/46C is a pretty good indicator of what they consider the max upper limit for normal operation. Seems pretty reasonable, as many batteries are spec'd to 50C max operating temp.

FWIW in my 9 y/o Gen 2 running through Phoenix summers where typical outside temps run ~110F, its still pretty rare for me to hear the battery fan running hard and I don't think I've ever experienced the ICE only mode with AC running. I did run into a few times last summer when my AC was not working.

Rob

 

BTW this is pretty old but interesting:
http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/2a_2002_01_1962.pdf

 

At 50C the traction battery is put in protection mode and ICE never stops. A common scenario is going up and down hills then parking it and turning it off immediatly. The battery fan doesn't run when the car is off so the HV battery continues to "cook" and for some hours the temperature increase instead of going down.
The optimal temperature is near 36C when the DCL hits its maximum.

 

This is true if re-circ is off. But think of the problem of person comfort in terms of evaporation capacity from the outside air. If you increase the pass-through 3x, the evap capacity increases by the same amount.

This might be a matter of personal preference, but as someone who hates humidity, I can say that a brisk wind is the only thing that keeps me alive in a hot & humid environment.

 

High humidity has no effect on A/C efficiency as long as you're not using a "swamp cooler", and you're not. It's funny how these "old wives tales" get propagated. In fact, high humidity can actually improve A/C performance because the air blowing over the condenser has more mass but the same temperature. It will therefore be able to absorb more energy.

Wind chill has no effect on cars, except to make them cool off faster. They experience the same temperature and will drop to the air temperature, not the "wind chill" temperature.
High humidity has no effect on cars, they see the same temperature, but the A/C system may be able to dump energy a little faster when it needs to. It is possible the high humidity may be able to "ice" the evaporator if the system is poorly designed.

 

Hi David,
So far as I know, humid air is less dense because H2O has a lower molecular weight than N2 or O2.

Regarding AC performance, I'm not sure about efficiency, and please correct me if I am wrong, AC has a lot more work to do in a humid environment if the AC is dehumidifing as well as cooling the air. This would be the heat of vaporization in reverse, no ?

 

Ambient temp was around 29 Centigrade (84 Fahrenheit), same for the car interior, just had the windows down. Moderately hot, not super hot I guess. My impression is what really drives the battery temp up is this sequence of events:

1. Warm/hot day (obviously)
2. Drive long enough that car's fully warmed up.
3. Park for a decent period, likely in full sun.
4. Second start up and drive.

That's when I've seen the highest temps, after a "hot soak".

 

Do you crack the windows for ventilation and use the tonneau cover ?

 

Assuming you mean during the parked period, no I didn't crack the windows, and not sure about the blah-blah cover, likely not. Good tips!

 

You're forgetting the fact that Prius generates electricity using the engine, by-passing the battery-pack entirely by using the second motor instead.

Not going to happen. Operating tolerance detail is something manufacturers will keep a close secret. The best you can expect to do is collect real-world data.

 

My battery temps have been as high as 130+F. Is that out of the operational specs? I guess the world will never know. Maybe they can tolerate 150F with no damage. So at what point should an owner worry about hot batteries and battery failure?

And no data from Toyota, or battery makers? Odd.

 

Apologies that this is a bit long. I figured that some more technical background might be helpful and it wasn't obvious to me how to make it more concise without leaving too much unexplained

More outdoor air helps, provided it is dry enough. Without a/c its the best you have.

With a/c however, excessive moisture is condensed from the air passing through the cooling coil and drained away from the occupied space. Because of this, for indoor spaces in hot humid climates, the indoor air is not just cooler but contains much less moisture than the outdoor air (can be less than half) . The effect on the a/c load under these conditions is that far less energy and cooling capacity are required to cool recirculated indoor air than to cool outdoor air.

For comparison purposes, at 1% cooling design conditions (temperature exceeded only 1% of the time and moisture content typical for those conditions)
Miami Beach has a temperature of 89F and the moisture contained in the outdoor air is 121gr/lb (grains / lb of dry air)
Denver has a temperature of 90F and the moisture contained in the outdoor air is 41 gr/lb
Victoria has a temperature of 75F and the moisture contained in the outdoor air is 62 gr/lb
Palm Springs has a temperature of 110F and the moisture contained in the outdoor air is 27 gr/lb

Air coming off the a/c cooling coil will generally be a bit below 100% RH if the coil is condensing moisture from the air. (It is below 100% due to bypass and remixing effects) If we assume the leaving air temperature is at 52F, the moisture content would be about 54 gr/lb (100% RH at 50F)

The 52F air rises to say 72F inside the car due to heat from occupants, sun and heat conduction from outside . The moisture content also rises due to moisture from the breath and skin of the occupants. Under condensing conditions, the net moisture content inside the car might be of the order of 59gr/lb (50% RH at 72F and low altitude).

Where the moisture content in the incoming air is low, the drier air passes through the cooling coil and the RH in the car is lower without condensing conditions having arisen.

At summer outdoor design conditions in the above cities you would have a lot of water condensed from the outdoor air in the hot humid climate of Miami (121 - 54 =67 gr/lb), some condensed in the cool humid climate of Victoria (62-54 = 8 gr/lb), and none in the drier climate of Denver or in the hot desert climate of Palm Springs.

Note that the 1% summer design conditions mean that the temperature will be exceeded for about 88 hrs pa.
There will also be other times when the moisture content of the air is very high and possibly higher than during standard summer design conditions. Even in Palm Springs they occasionally get days when moisture laden winds from Mexico bring hot humid conditions

Condensing large amounts of water puts a very heavy load on the a/c. The energy and capital cost of dealing with this is substantial so a/c designs are based on minimizing outdoor air intake under hot humid conditions. If operation is overidden to increase outdoor air intake under these conditions, the ability of the a/c system to control airborne moisture is compromised, the space humidity rises substantially and mould problems are more likely to occur

 

High humidity does not adversely affect the ability of car a/c to pump heat.

The problem is that if you bring much air that is both humid and hot into the car, you vastly increase the amount of heat that needs to be pumped. In hot humid climates, the load increase can easily overwhelm the a/c system.

From a cooling and humidity control point of view, it would be ideal to bring in no outdoor when conditions are hot and humid. Not a great solution however if you have people and a traction battery inside the car!

 

Weather Extremes! - Flash Floods and High humidity

I included Palm Springs in my previous post as we have a place in the area and mentioned occasional high temperature high humidity conditions

Conditions near our place right now are 92F, 60% humidity and there has been heavy heavy rain.

Moisture content in the air is 137 gr/lb. Compare this to the normal summer a/c design figure of 27 gr/lb and Miami's normal summer a/c design of 121 gr/lb!

This is an extreme example of the weather condition I mentioned in my previous post.

Fortunately our place is at a higher elevation but I hope that the roof has held up

Photos: Flash floods overtake cars, homes & close roads in Coachella Valley | News - Home

 

What about when the engine cycles off and the a/c runs on battery power?

Won't increased a/c draw mean increased battery draw while the engine is off and more frequent recharge cycles even if a/c power bypasses the battery while the engine is running?

 

The engine won't shut off. It will run continuously.

When the system exceeds normal operating threshold, it prevents the usual hybrid cycling.