is 47C too hot for HV?

I have seen the 116F value a few times when doing start/stop errands with time in between stops and starts. I see it mostly with temps in the high 80's and 90's, especially after driving through town on the way home from work and then going out to dinner about an hour later. When stopped and the fan not running the temperature will continue to rise.

There was a research paper on here at one time that showed the battery in the 125F range while doing their test cycles and the paper indicated that this was within the battery's temperature ratings.

I read on a post on here somewhere that the computer starts limiting battery usage at 116F. I did not notice a change in usage when I have seen 116F.

 

Were you using your AC at the time when the battery temps went up?

 

When testing with AC, the Recirc vs Outside setting may also be relevant. The Outside air setting should be forcing more cooled cabin air through the battery than would the default Recirc setting, provided that all the windows are closed.

 

I would expect the battery to be hot, but I couldn't say how hot. Just compare to AA NiMH cells. They get quite hot when used at higher currents.

The information that was posted on battery temp was for the GII. The GIII uses the battery in a different way, perhaps more, than the GII. One of the suspected reasons some reports of failures in Taxi service are more frequent.

 

Since the battery uses cabin air for cooling, I do not see how the outside temperature would be a factor. Not using A/C would seem to make things worse.

 

Outside temperature will be relevant to the battery temperature at the start of the trip.

Having the cabin air source set to Recirculate or Fresh/Outside will change the amount of cabin air forced through the battery. If all windows are closed, outside air source will tend to pressurize the cabin and force more air through the cabin exhaust venting, which also happens to be the battery cooling path.

 

What you said above seems logical but it is not what I see on my Gen III monitoring battery temps. Monitoring the Battery Cooling Fan Inlet Temperature, BtT, the inlet air temperature remains just below battery temps until the fan comes on. This does not matter if I have AC in Recirc or Outside selected or whether the windows are opened or closed. You would think that there would be natural convection through the HV battery enclosure but according to the inlet air temp this is not the case. Air flow does not begin until the fan comes on, or at least the inlet temp does not begin to decrease until the fan comes on.

Once the fan comes on the inlet temp will drop to just above the Cabin temp. Once the fan is on I cannot see a change in inlet temp if I change to Outside air versus Recirc. Since we do not have an air flow measurerment I do not see another way to determine if the air flow increases between Recirc and Outside Air selection.

Another way to look at this is if you select Outside Air you are bringing in more hot outside air that will take longer to cool down the cabin. I have found that if you leave it in Auto and Recirc it will cool the cabin down quicker and allow the AC unit to begin slowing down and using less Watts and less impact on mpg. Also the quicker the cabin cools down the quicker the Inlet Air temp cools down.

 

This is correct. The thermostat closes around 115F, at which point cabin air is used to cool the battery. It will eventually come to an equilibrium somewhat higher than the cabin air temp. The recirc/outside setting will have an effect only to the extent it changes the cabin air temp. The battery is cooled using a fan, not internal air pressure- which would be very hard to do in any case.

 

Going downhill I have loged temps over 48°, from there it won't go higher.

click on a chart sheet:
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Actually, it is four temperature sensors. The battery cooling fan is variable speed and is controlled by the ECU, which monitors the battery temp via the sensors. It also watches the intake air temp. Using the input, it determines the necessary blower speed.

 

The blower fan comes on at 97F not at 115F.

 

Great data. I guess this shows the upper limit to the HV battery temp is around 118F. Any clue as to when Battery Fan Mode (BFM) 6 kicks in?

 

Keep in mind the temp sensors are not -in- the cells. It takes time for them to match the -actual- cell temp. I suspect the cells temp is pretty close cell to cell, as the heating is internal, not caused by external energy. There is no reason for differential heating, as the battery current (the thing that heats them up) is the same for all cells. Now if the cells were poorly matched some would start to heat up faster than others. The ones with the lowest capacity will start to heat up faster than the others, as NiMH cells become very inefficient at charging once they reach about 80% charge (much more of the charging energy is converted to heat). If some cells were, say, 20% low on capacity, they would start to really heat up even though the majority of the cells were still at, say, 70% charge.
This is why it's not a good idea to run the battery charge down as low as possible or up as high as possible, and is why the Prius tries to run them between about 30% and 80% charge. I say "run the battery charge" as in the driver is trying to force the system to operate to the extremes. If you just let it do its' thing you are probably fine.

As for the temp still rising after the car is off, this is just the energy slowly reaching the sensors and heating -them- up, not the cells continuing to heat up. A better way of judging cell temp would be to monitor the sensor temp and the battery voltage. You could come up with a formula to more accurately figure the cell temp this way, as the voltage is affected by the temp. But it would be complex and probably unnecessary, except for those of us interested in an accurate representation of cell temp.

 

So just out of curiosity, why are we second guessing those who designed the thing?

Don't get me wrong, I love reading threads like this because I always learn a lot. But it seems like people read something and then come up with their own better ideas that could easily have been incorporated in the factory design if they were worthwhile. Such as the talk of recirculating air affecting battery cooling and being disappointed in cell temps hours after shutting it off.

I mean seriously...don't you guys think if this were an issue they would have made changes to the design long before now? There are lots of ways to cool something much more effectively than the current system, so if it were necessary don't you think the system would be completely different? If battery temp was as big of an issue as some of us are making it to be, wouldn't we have seen mass battery failures on the Gen 2s by now?

This reminds me of another forum when people were getting freaked over their transmissions hitting 190°. Then an engineer comes along who worked on the transmission asserting that the specifications permit continuous operation all day long at 220°, and up to 250° for up to a half hour. People were getting worked up without even knowing what the system was designed to tolerate.

Of course this wasn't on a Prius forum, but without knowing the design limitations what are we getting worried about?

 

I believe that every design is a compromise, but without knowing the design specs who is to say what's too hot? What data supports the fact that warmer temperatures have a meaningful effect on battery life? That's a serious question, I'm relatively new to all of this.

Not at all, just that I think the problem should be identified before trying to fix it.

That's not true at all. In fact most modern transmissions have a thermostat in the cooling circuit to control the transmission's temperature. Most of my experience is with pickup trucks, and my last few Fords have had transmission thermostats that didn't open until above 180 degrees farenheit. In fact my last one, a 2013 F150 would run 195 degrees all day long, whether it was 0 or 100 degrees out. There are a few reasons for that, but I'm getting a bit off topic so I'll shut up now.