Controlling/Monitoring battery temperature.

[Malarkey]

Has anyone put effort into the effects of precisely controlling the temperature in the battery compartment?

I wonder if there's a tighter sweet spot than just human-comfort zones.

Would be interesting to find out if, say, 70C makes it happier than 65C or 75C. Would also be interesting to see how broad the beneficial effect is. If it's marginally better at 70C but the effort required to keep it there consumes more gas than it would consume due to inefficiencies at 85C then hypermilers may let it run warmer or colder.

[NoMoShocks]

<div class='quotetop'>QUOTE(Malarkey @ Jul 25 2007, 07:59 PM) [snapback]485225[/snapback]</div>

Has anyone put effort into the effects of precisely controlling the temperature in the battery compartment?

I wonder if there's a tighter sweet spot than just human-comfort zones.

Would be interesting to find out if, say, 70C makes it happier than 65C or 75C. Would also be interesting to see how broad the beneficial effect is. If it's marginally better at 70C but the effort required to keep it there consumes more gas than it would consume due to inefficiencies at 85C then hypermilers may let it run warmer or colder.
[/b]

Interresting question! I have often wondered myself if I am shooting myself in the foot by not running my Air Conditioning to save feul as I sit there sweating in stop and go traffic. I have not heard the battery fan coming on, but I listen to the radio, and the windows are down, so I don't know if I would hear it. I guess it would be a great experiment to buy a Radio Shack remote temperature sensor and maybe an idiot light that comes on when the battery cooling fan comes on and gather some data.

What will make this hard is all the other variables. Here in Portland area, one day is 85 and the next is 60. Traffic is stop and go one day, and slow and go the next.

[Danny Hamilton]

I'm hoping that hypermilers around here will correct me if I'm wrong, but it is my understanding that hypermilers attempt to use the battery as little as possible, so they aren't likely to get a significant benifit from carefully controlling the battery temperature.

Driving with the windows down at any speed above a crawl will certainly reduce your MPG. If you enjoy driving with the windows down, then simply accept the reduced efficiency as the cost of the pleasure. On the other hand, if you are sweating, and keeping the windows down in an attempt to improve MPG, you are probably making a mistake, especially after the 2004 model year.

I wouldn't think that cooling off the batteries with the A/C would make a huge difference in your MPG unless you have driving conditions that require extensive use of the battery (regen, stealth, etc). However I'd expect that reducing the aerodynamic drag by putting up the windows would have a noticable affect.

[NoMoShocks]

Yaah, this is what I have been wondering. I have no doubt that cruzing along at 35 or more MPH, that using the very efficeint Prius electrically driven AC would be much better than adding the drag of the open windows, but I have heard some say that in stop and go type traffic conditions, the AC really hurts the MPG. And when I am alternating between 0-15 MPH, that is when I think the open windows are better.

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 25 2007, 08:33 PM) [snapback]485244[/snapback]</div>

I'm hoping that hypermilers around here will correct me if I'm wrong, but it is my understanding that hypermilers attempt to use the battery as little as possible, so they aren't likely to get a significant benifit from carefully controlling the battery temperature.

Driving with the windows down at any speed above a crawl will certainly reduce your MPG. If you enjoy driving with the windows down, then simply accept the reduced efficiency as the cost of the pleasure. On the other hand, if you are sweating, and keeping the windows down in an attempt to improve MPG, you are probably making a mistake, especially after the 2004 model year.

I wouldn't think that cooling off the batteries with the A/C would make a huge difference in your MPG unless you have driving conditions that require extensive use of the battery (regen, stealth, etc). However I'd expect that reducing the aerodynamic drag by putting up the windows would have a noticable affect.
[/b]

[catgic]

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[richard schumacher]

<div class='quotetop'>QUOTE(NoMoShocks @ Jul 25 2007, 10:19 PM) [snapback]485236[/snapback]</div>

I have often wondered myself if I am shooting myself in the foot by not running my Air Conditioning to save feul as I sit there sweating in stop and go traffic. I have not heard the battery fan coming on, but I listen to the radio, and the windows are down, so I don't know if I would hear it.[/b]

You indeed might not hear the battery fan straining to keep the system cool under those conditions, and there's a good chance that you *are* reducing your battery system efficiency. There's a reason why the battery gets its cooling air from the cabin and not directly from the outside.

[Danny Hamilton]

[quote=NoMoShocks @ Jul 26 2007, 02:30 AM) [snapback]485314[/snapback]</div>

Yaah, this is what I have been wondering. I have no doubt that cruzing along at 35 or more MPH, that using the very efficeint Prius electrically driven AC would be much better than adding the drag of the open windows, but I have heard some say that in stop and go type traffic conditions, the AC really hurts the MPG. And when I am alternating between 0-15 MPH, that is when I think the open windows are better.
[/b]

In my very unscientific test of the fuel efficiency of the Prius A/C I have on occasion (once accidentally, and 4 times now intentionally) left the Prius in the following condition:

parked
turned on
radio off
MFD off
All lights off
Nothing plugged into any of the 12V outlets
A/C set to 76F

The ambient outdoor temperature on these occasions varied between 85F and 95F and it was parked in full sun mid-day.

My calculations using the drop in MPG for the tank on the MFD and the number of miles on the tank found that in each of these occasions the A/C used about 0.1 gal of fuel per half hour +/- 5% (YMMV).

Here is an example calculation:

Prior to "experiment" MFD indicates 371mi] 57.4mpg
After 2 hour "experiment" MFD indicates 371mi @ 54.4mpg

371 / 57.4 = 6.4634 gal
371 / 54.2 = 6.8450 gal

6.8450 - 6.4634 = 0.3816 gal / 2 hours = .1908 gal/hr or .0954 gal / halfhour

Obviously the amount of fuel that is "wasted" by the A/C depends on important factors such as outside temp, , cloudiness, and time of day but in my area of the country this is probably a worst case senario for all but the most extreme few days of the year.

I would think this amount of fuel would be used to run the A/C under these conditions whether the car is standing still or moving at 55mph. The difference is that the number of miles changes faster when the car moves faster (obviously), so the MPG doesn't take as big of a hit from the fuel used by the A/C.

If the temp outside is comfortable at all, I just shut off the compressor and use the fan to vent fresh air into the car to keep it at a reasonable temp. If the temp outside is hot, I figure planet be darned I'm willing to shell out the $0.60 per hour to be comfortable whether I'm standing still or moving.

[1fixitman]

[quote=Danny Hamilton @ Jul 26 2007, 11:30 AM) [snapback]485430[/snapback]</div>

In my very unscientific test of the fuel efficiency of the Prius A/C I have on occasion (once accidentally, and 4 times now intentionally) left the Prius in the following condition:

parked
turned on
radio off
MFD off
All lights off
Nothing plugged into any of the 12V outlets
A/C set to 76F

The ambient outdoor temperature on these occasions varied between 85F and 95F and it was parked in full sun mid-day.

My calculations using the drop in MPG for the tank on the MFD and the number of miles on the tank found that in each of these occasions the A/C used about 0.1 gal of fuel per half hour +/- 5% (YMMV).

Here is an example calculation:

Prior to "experiment" MFD indicates 371mi] 57.4mpg
After 2 hour "experiment" MFD indicates 371mi @ 54.4mpg

371 / 57.4 = 6.4634 gal
371 / 54.2 = 6.8450 gal

6.8450 - 6.4634 = 0.3816 gal / 2 hours = .1908 gal/hr or .0954 gal / halfhour

Obviously the amount of fuel that is "wasted" by the A/C depends on important factors such as outside temp, , cloudiness, and time of day but in my area of the country this is probably a worst case senario for all but the most extreme few days of the year.

I would think this amount of fuel would be used to run the A/C under these conditions whether the car is standing still or moving at 55mph. The difference is that the number of miles changes faster when the car moves faster (obviously), so the MPG doesn't take as big of a hit from the fuel used by the A/C.

If the temp outside is comfortable at all, I just shut off the compressor and use the fan to vent fresh air into the car to keep it at a reasonable temp. If the temp outside is hot, I figure planet be darned I'm willing to shell out the $0.60 per hour to be comfortable whether I'm standing still or moving.
[/b]

This is great data. There is an increase in the used amount of fuel while moving. I will explain. When your car is sitting still the air outside the vehicle becomes stagnant and acts as an insulator of sorts. There is only so much heat transfer with low or slow moving air. While the car is moving there is a greater heat transfer rate from the outside to inside as the air is moving or exchanging rapidly outisde the vehicle. The best way to get a really good number would be to drive the car on a hot day at a circular paved race track. The test would need to be conducted for 10 laps with the AC and for 10 laps without the AC within a 2 hour period between 11am and 1pm at the same speed on cruise control. Not stopping inbetween runs. If it clouds up during either run then it needs to be restarted. I will take the sitting still numbers but would like to see moving mumbers too. I do not live near a good circular paved race track.
D Rock

[Danny Hamilton]

[quote=D Rock @ Jul 26 2007, 01:22 PM) [snapback]485579[/snapback]</div>

This is great data. There is an increase in the used amount of fuel while moving. I will explain. When your car is sitting still the air outside the vehicle becomes stagnant and acts as an insulator of sorts. There is only so much heat transfer with low or slow moving air. While the car is moving there is a greater heat transfer rate from the outside to inside as the air is moving or exchanging rapidly outisde the vehicle. The best way to get a really good number would be to drive the car on a hot day at a circular paved race track. The test would need to be conducted for 10 laps with the AC and for 10 laps without the AC within a 2 hour period between 11am and 1pm at the same speed on cruise control. Not stopping inbetween runs. If it clouds up during either run then it needs to be restarted. I will take the sitting still numbers but would like to see moving mumbers too. I do not live near a good circular paved race track.
D Rock
[/b]

Actually without A/C the interior of the car gets a lot hotter than ambient exterior temp in full sun at noon on an 85F day. A significant amount of the interior temp comes from radiant heat rather than conductive or convection. For that matter, the hotter temp of the vehicle surface should create an updraft which would keep the air near the car from becoming "stagnant".

Shortly after I posted my comments it dawned on me that I haven't left the car "On" with the A/C off in the same conditions. I suppose there is some "base metabolic rate" for lack of a better term that would cause the vehicle to use some amount of fuel over time to keep the battery charged. Therefore, my estimate of $0.60 per hour to run the A/C is probably high. And the estimate of how much fuel is "wasted" when the car is not moving (or is crawling along slowly in traffic) as compared to putting the windows down is probably a bit high for the same reason.

BTW, I just noticed a typo in my earlier comments.

The post "experiment" MFD numbers were 371mi] 54.2mpg (not 54.4 mpg).

[David Beale]

The car will use fuel to keep the cat warm when in "ready". The engine must also be kept warm, but the cat requirement, in my experience, keeps the engine warm. Probably will run for 30 sec. every 10 min. or so to keep the cat up to temp. though I haven't tested this myself. I would expect the A/C would be first in fuel use requirement, then cat, then engine.