How does A/C work on the Prius?

Maybe you should try monitoring your battery and interior temps for awhile. I took my last Prius to over 158,000 miles with no observable degradation of the battery or fuel economy. My current Prius seems to do just fine without the A/C. Again, I think you need to collect more data and then maybe we can have useful dialog. I would love to see more information on this subject. Thus far only a few of us have anything to show for it and all that data looks the same.

As for the mpg difference between A/C and no A/C lets just say that when temps get high enough to be harmful to the HV battery then using it will cost you more than 2-3mpg unless you are only looking at a single very long trip.

 

I generally don't do that sort of driving, however, if you are stuck in that kind of traffic for a long enough period of time then I agree that A/C should be used. I disagree that it should be used on very short trips in similar conditions because you are not in the car long enough for the A/C to have a truly beneficial affect. It's use may cause temps to spike even higher due to the A/C consuming 1500-1800w of power.

 

If the A/C is in good health it should be able to lower the temperature of the incoming air by 15 degrees. I don’t mean the outside air temp, by the way. This is why the A/C has to recirculate the cabin air, so that as the temperature of the cabin air comes down, the A/C should be able to keep lowering it by 15 degrees with each recirculation, until the set temperature is reached. With outside temperatures here in the mid-90s, my A/C unit is easily able to bring the cabin temp down to 75, within about 10 minutes or so. You’re A/C unit may have a problem, I would have it checked. I am not an HVAC guy per say, but as a broadcast engineer, I have had to design the cooling systems for new transmitter sites and studios, so I do have some experience with A/C equipment and heat loads.

I am very fortunate that I am able to park my car in the shaded areas of the company parking garage, on most days, which is probably the best way to keep the car cool.

 

I actually feel that the exact opposite argument can be made. When it is hot out, I simply start the A/C in auto mode, as soon as the engine comes on for its warm up sequence, that way the initial current demand surge can be supplied by the engine, instead of the HV Battery. I have noticed no ill effects from doing this. Once I buy my Scangauge I will be able to research it a little bit more in depth.

 

How are you monitoring cabin temp? After 30miles (98F ambient) with my A/C set on auto 76F the cabin temp still read 86F+- according to the cabin sensor. My A/C works fine. Maybe it is a difference between how you and I are reading the temperature? I was using Torque to get my reading. Thanks for reminding me to get my damn SGII upgraded to the new version.

 

I think your lack of I'll effects has to do with you starting off with a cool car. I observe the same thing when I start my car with a cool cabin temp. It's like turning on the A/C when it's 75F outside and setting your A/C to 72F. The power draw is very minimal compared to the same setting but 100F ambient temps.

 

I was using a simple approach. I assume when the air-conditioner throttles back the fan and air temp to the output needed to maintain the temperature, that it is measuring the air temp at the set point. Without any other way to measure the temp, I had to go with that. I do have a non-contact thermometer at work that I can borrow. I will use that and take some measurements of temperatures around the cabin and see what I come up with. I will be curious to see what the temperature is at the air intake for the Traction battery, compared to the temperatures up front.

 

I have the same results when doing this starting from my driveway. There is no shade there and lots of sun.

 

I understand what you are saying about AC on short trips and it not having time to have much affect. If I were more concerned with mpg then I may not run mine for short trips. In 90F weather I am more concerned with keeping myself and the cabin cool so I keep mine on. I have not seen any temp spikes due to the AC that I can recall.

As for your earlier comment about your car not being below mid-80's after 30 minutes of driving, try taking your car out of ECO. I had similar results while in ECO. I switched to Normal with the higher temps and in my 20 miles of highway driving my cabin is in the upper-70's by the time I get to town.

As for mpg on the highway I did not notice an mpg hit switching to Normal. I would think that had to do with the cabin cooling down quicker and reducing the AC demand quicker.

It would be nice to have a trending device to be able to truly compare.

 

Aye, I thought about that after I left the house in 93F heat yesterday on my way to the Bay Area so I unplugged the SGII and started up Torque to monitor temps. With the car in Normal Mode the did indeed get cooler once I lowered the A/C setting below 76F. The problem is my mpg started dropping from 63mpg (180+miles) to 62.1F before I got the cabin cooled down to 80F or so. This after I had been driving on the freeway for 20miles so the car was warmed up and cross vent cooling had done all it could do for me up to that point. The battery temps without A/C hovered around 96.8F and reached a peak of 98.6F without the A/C. With the A/C set at 72F the battery temp quickly dropped to 84F where it stuck. I even parked the car and let it idle with the A/C for about 15min or so and the temps stayed the same. So I'm sure you can get the cabin cooler but not without really dinging fuel economy which I probably should have cleared up in my first post. However, you guys are most definitely correct in that on longer trips the battery will enjoy the 14F+ temp drop with the A/C on. On longer trips you spread out the mpg hit too so it's not so bad.

 

We just all need to be aware of the affects of AC (or lack of) on the HV battery temps. Then we can make informed decisions as to how much we want to press for higher mpg's while understanding the afffects on the battery temps.

 

The choice is, how much is that 1.4% drop in MPGs worth, compared to the heat exposure of your HV Battery. I have some information on recommended heat exposure levels for different battery types around here somewhere. I’ll post it when I find it.
I agree with jdcollins5, its about having the information, to make informed decisions.

 

Again, the drop can be quite a bit more than that. I've tested it and many others have observed it. I posted the government testing on the GenI battery and the battery was fine up to 113F according to them. At 122F was were battery longevity could be negatively affected. Like I said, get your gauges in and collect the data. It would interesting to see how a milder climate affects fuel economy with A/C use. In hot climates the effect is much higher than 1.4%.

 

I did find some of the information that I pulled for NiMh batteries when I was working out cooling requirements for UPS power supplies

Ambient temperature:
Because the rated capacity of a battery is based on an ambient temperature of 25°C (77°F), any variation can affect performance and reduce battery life. For every 8.3°C (15°F) average annual temperature above 25°C (77°F), the life of the battery is reduced by 50%.

Remember this is average temperature so it is not as bad as it sounds, but it is still sobering. So anything that reduces the average exposure temperatures will extend the battery’s life span.

Storage:

Store in a cool place, but prevent condensation on cell battery terminals. Elevated temperatures may result in reduced battery life. Optimum storage temperatures are between -31 degrees F and +95 degrees F.

This comes from OSHA MSDS sheets for NiMh batteries. Note this tops out at +95 degrees F, not +113 degrees F. Another point made here is condensation; so if you are in a very humid area, the A/C will also reduce the moisture content of the air cooling your battery, which is a good thing.

Charging:

NiMH cells are also exothermic during charging and as they approach full charge, the cell temperature can rise dramatically. Consequently, chargers for NiMH cells must be designed to sense this temperature rise and cut off the charger to prevent damage to the cells.

This is the danger zone for the Prius HV Battery. This is where maximum cooling is needed, during the charge cycle

Thermal Runaway:

External Thermal Effects
The thermal condition of the battery is also dependent on its environment. If its temperature is above the ambient temperature it will lose heat through conduction, convection and radiation. If the ambient temperature is higher, the battery will gain heat from its surroundings. When the ambient temperature is very high the thermal management system has to work very hard to keep the temperature under control. A single cell may work very well at room temperature on its own, but if it is part of a battery pack surrounded by similar cells all generating heat, even if it is carrying the same load, it could well exceed its temperature limits.

Temperature - The Accelerator
The net result of the thermo-electrical and thermo-chemical effects possibly augmented by the environmental conditions is usually a rise in temperature and as we noted above this will cause an exponential increase in the rate at which a chemical reaction proceeds. We also know that if the temperature rise is excessive a lot of nasty things can happen
o The active chemicals expand causing the cell to swell
o Mechanical distortion of the cell components may result in short circuits or open circuits
o Irreversible chemical reactions can occur which cause a permanent reduction in the active chemicals and hence the capacity of the cell
o Prolonged operation at high temperature can cause cracking in plastic parts of the cell
o The temperature rise causes the chemical reaction to speed up increasing the temperature even more and could lead to thermal runaway
o Gases may be given off
o Pressure builds up inside the cell
o The cell may eventually rupture or explode
o Toxic or inflammable chemicals may be released

Thermal Capacity - The Conflict
It is ironic that as battery engineers strive to cram more and more energy into ever smaller volumes, the applications engineer has increasing difficulty to get it out again. The great strength of new technology batteries is unfortunately also the source of their greatest weakness.
The thermal capacity of an object defines its ability to absorb heat. In simple terms for a given amount of heat, the bigger and heavier the object is, the smaller will be the temperature rise caused by the heat.
For many years lead acid batteries have been one of the few power sources available for high power applications. Because of their bulk and weight, temperature rise during operation has not been a major problem. But in the quest for smaller, lighter batteries with higher power and energy densities, the unavoidable consequence is that the thermal capacity of the battery will be decreased. This in turn means that for a given power output, the temperature rise will be higher.
(This assumes a similar internal impedance and similar thermochemical properties which might not necessarily be the case.) The result is that heat dissipation is a major engineering challenge for high energy density batteries used in high power applications. Cell designers have developed innovative cell construction techniques to get the heat out of the cell. Battery pack designers must find equally innovative solutions to get the heat out of the pack.

EV and HEV Battery Thermal Considerations
Similar conflicts occur with EV and HEV batteries. The EV battery is large with good heat disipation possibilities by convection and conduction and subject to a low temperature rise due to its high thermal capacity. On the other hand the HEV battery which must handle the same power is less than one tenth of the size with a low thermal capacity and low heat dissipation properties which means it will be subject to a much higher temperature rise.

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Taking into account the need to keep the cells operating within their allowable temperature range, the EV battery is more likely to encounter problems to keep it warm at the low end of the temperature range while the HEV battery is more likely to have overheating problems in high temperature environments even though they both dissipate the same amount of heat.
In the case of the EV, at very low ambient temperatures, self heating (I2R heating) by the current flow during operation will most likely be insufficient to raise the temperature to the desired operating levels because of the battery's bulk and external heaters may be required to raise the temperature. This could be provided by diverting some of the battery capacity for heating purposes. On the other hand, the same heat generation in the HEV battery working in high temperature environments could send it into thermal runaway and forced cooling must be provided.

See also EV, HEV and PHEV Specifications in the Traction Battery section

Thermal Runaway
The operating temperature which is reached in a battery is the result of the ambient temperature augmented by the heat generated by the battery. If a battery is subject to excessive currents the possibility of thermal runaway arises resulting in catastrophic destruction of the battery. This occurs when the rate of heat generation within the battery exceeds its heat dissipation capacity. There are several conditions which can bring this about:
1 Initially the thermal I2R losses of the charging current flowing through the cell heat up the electrolyte, but the resistance of the electrolyte decreases with temperature, so this will in turn result in a higher current driving the temperature still higher, reinforcing the reaction till a runaway condition is reached.
2 During charging the charging current induces an exothermic chemical reaction of the chemicals in the cell which reinforces the heat generated by the charging current.
3 Or during discharging the heat produced by the exothermic chemical action generating the current reinforces the resistive heating due to the current flow within the cell.
4 The ambient temperature is excessive.
5 Inadequate cooling

Unless some protective measures are in place the consequences of the thermal runaway could be meltdown of the cell or a build up of pressure resulting an explosion or fire depending on the cell chemistry and construction.

The thermal management system must keep all of these factors under control.

I look forward to getting my gauges and testing all this out.

 

Here are tests specific to the Prius GenI. You are correct, it was 40C not 45C. Eek! I'll read through your post more thoroughly tonight. Thanks for posting it.

http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/2a_2002_01_1962.pdf