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I've been fooling with PHEV testing (sans PHEV) and I've been reading a lot about battery temps. I pulled up my buc (or buf) gauge and started reading battery temps. As stated in my other thread, I finally found out where the EV deny thermal threshold is. Anything above 45c will deny EV button requests. At one point (don't tell Wayne, but I was speeding) after a long highway exit and some really bad misses on lights, coupled with a balmy 103f ambient temperature reading, my battery fan came on. Freeked my out. I've never heard it before, and hope to never hear it again. I've been pretty surprised how hot they get actually and realized that having junk in my trunk was acting as a battery coozy.

Anyway I've moved my carseats around and started monitoring battery temp. I even have a meat thermometer in my trunk to measure ambient in the parking lot. I also have my passenger seat kicked back when no one is in the car all my AC vents aimed at the battery cooling intake. Last time my battery got near the EV deny thermal threshold, I kicked on the AC and aimed it at the intake. It slowly cooled and kept me in my grind.

At the last Hybrid Meet one of the members complained that his TCH mileage really took a steep dive this year. We went out to his car did a little inspection, and sure enough, he had a battery coozy too. Trunk full of Gym bags and boxes of electronics. All packed snugly against the battery. We cleared it all out and put the rear vents on full right to the battery intake, and he at least felt it was working better.

So please... Prius Pilots... Check those battery thermals!

11011011

I have been watching the battery temps this summer and last and it really doesn't take much to get them on the warm side. In the complete absence of shade to park under, one thing I have learned to do is keep my cargo cover deployed along with the obligatory windshield reflector. It seems to help vs. having the hatch area completely exposed.

The other thing I learned last summer is that running the A/C is a proverbial double-edged sword. If you get it running early in the commute to stabilize cabin air temp then you are probably OK. Running it on the highway seems to be OK any time. But if you are running around town (such that the ICE is shutting down frequently) and you notice that battery temps are getting high, turning on the A/C sometimes makes the situation worse! Sucking those amps out of the pack to run the compressor and then shoving them back in when the ICE lights increases battery temp a whole lot faster than the first dregs of cool air can cool it down. Now I either anticipate and get the A/C running when I first get in the car or, if I didn't do that and it looks like I'm in trouble, I just roll down the windows to get lots of fresh air.

Max temp for me has been 40-43C on the hottest of afternoons (approaching 100F) and running the A/C set to 80F from the get-go. Since my commute is 90% highway, failing to run the compressor only amounts to ~2C increase on average vs. what I would have expected otherwise.

Well, since you cross-posted, I guess I will too. :)

I'm not sure high battery temperature is an absolute "EV deny" criterion. Last summer I made a trip to the mountains of VA with a passenger aboard, doing a CAN-View play-by-play as we went along. Battery temperature peaked at 116F during the descent. I pointed this out and explained that EV mode (previously described to him) would not activate. I tried to demonstrate it -- and EV mode activated! SOC was at 82% at the time, so it seems at those levels the car has a greater desire to reduce SOC than to limit EV. I realize in Houston you likely won't see these conditions (unless you're force-charging prior to an MPG competition ;)), so for ordinary driving with typical SOC I can't dispute that 113F/45C is an upper limit of battery temperature at which EV is allowed.

Here's another interesting observation I made, presumably caused by a hot battery (though well below 113F): a spontaneously aborted S4 glide (http://priuschat.com/forums/gen-ii-prius-technical-discussion/36061-spontaneously-aborted-s4-glides-26-mph.html) at 26 MPH.

Ever since that event, I've watched battery temperature rather closely throughout our hot spells. Being the large thermal mass that it is, radiant and convective cooling is slow regardless of whether the AC is running. I am convinced that (pardon the cliché) prevention is the best medicine. I have window tinting, I keep my windows cracked (with the interior protected from a sudden rain shower with window deflectors), I use a windshield sunshade, and (perhaps most important) I always try to park in the shade during prolonged daytime periods. And of course your advice to keep the battery well-ventilated is on target as well.

Tim, I agree that AC is a double-edged sword. In the link above, I describe battery temperature increasing after turning AC on. Some suggest that Prius drivers always run the AC to keep the battery cool, but I have not seen any greater battery cooling ability with AC than with good air flow through the cabin with windows open (especially the right rear). I'll run the AC on my infrequent highway trips, but around town I'll do without if I (and any passengers I might have) can tolerate it.

Taking it a step further, heat is an enemy of battery life. It seems we're seeing a small but growing number of battery failure reports from owners in hot climates. So besides the immediate operational impact, these steps should help the battery live a long, happy life.

http://www.eaa-phev.org/wiki/Prius_OEM_fan_control
Use a thermostat (on above something like 80F-90F - a window A/C thermostat should work) to automatically turn on the battery cooler fan.

Or, you could keep your interior from getting that hot in the first place.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=250367409467&viewitem=&sspagename=STRK%3AMEWNX%3AIT

I got this a few weeks ago and have had great results. I havent had a chance to measure the difference in temperature, but it feels like only 5-10F higher than ambient. Not the normal 30-50F increase. It takes less than a minute to install, even less time to remove.

Or, you could keep your interior from getting that hot in the first place.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=250367409467&viewitem=&sspagename=STRK%3AMEWNX%3AIT

I got this a few weeks ago and have had great results. I havent had a chance to measure the difference in temperature, but it feels like only 5-10F higher than ambient. Not the normal 30-50F increase. It takes less than a minute to install, even less time to remove.Very Nice!

I've got some more data on battery fans and battery thresholds

Battery fan runs at low speed .|. > 95° F .|. > 35° C
Battery fan runs at medium speed .|. > 105° F .|. > 40° C
EV button requests are denied .|. > 113° F .|. > 45° C
Battery fan runs at full speed .|. > 122° F .|. > 50° C

11011011

I am further north. My problem is at the other end of temperature spectrum.

In the summer, I have no problem getting 52 to 62 MPG easily. My wife get similar FE without even trying to understand any of the tech terms we are discussing here.

But in the winter, our FE drops to 45MPG. Short trip local runs gave me only 29 MPG something. The battery temp is at 20Fs to 30F's. What can I do? Putting a heater blanket o the battery?

But in the winter, our FE drops to 45MPG. Short trip local runs gave me only 29 MPG something. The battery temp is at 20Fs to 30F's. What can I do? Putting a heater blanket o the battery?
FE drops in cold weather anyway for all cars. And for the Prius, a bigger issue in cold weather than battery temperature is ICE temperature. I highly recommend an engine block heater and grill blocking, especially for your short trips. Do a good search here and on PriusChat for details of each. And learn how to exploit the nuances of the Prius warmup sequence (http://www.techno-fandom.org/~hobbit/cars/five-stages.txt).

I wouldn't put a blanket on the battery. In mid-winter it might be OK, but on the occasional balmy January day, I'm guessing the battery could heat up too much. You certainly don't want it on there any other time of year. Heat is an enemy of battery life, and any marginal gain (if any) you might get with a blanket probably is not worth it.