Fall and Spring, prime Prius weather

Let's be realistic: most people are going to run their A/C long before it hits 85F. In fact, you are *supposed* to do so, to keep the battery pack cool. I've found that using the A/C in Eco mode (it cycles less often and produces a more "cool," less "cold" air stream) makes little difference in MPG, however. Above about 83F we can get 55MPG average. below that it's 53-54. Below 50 the mileage starts to take a real hit.

ever run several weeks at 10 degrees F? With high winds?

What's the MPG hit in the northern winter? Are they still skittish in cross-winds?

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There is an excellent paper on traction battery cooling, NREL/ CP-540-31306, that covers traction battery temperature during the emissions tests. Also, 202-01-1962, "Thermal Evaluation of Toyota Prius Battery Pack." But a really important paper, "Thermal Behavior of Small Nickel/Metal Hydride Battery during Rapid Charge and DIscharge Cycles, Takuto Araki, Masato Nakayama, Kenichi Fukuda, and Kazuo Onda, describes 'heat pumping.'

This last paper confirms what I found in a series of tests. How we drive, especially if we are doing a lot of discharge-charge cycles, can heat-pump our traction batteries. Charging is exothermic but discharging also provides some ohmic heating. For example, I was surprised to find a 'forced charge' can increase the traction battery temperature by nearly 8-9C:

The other hill trick is to use "B" when descending a long, steep hill. Often I'll just moderate the speed with the shifter and descend with a minimum heating of the traction battery. But it took a while and instrumentation to get metrics about this effect.

As for AC, I use it to stay comfortable. If it is dry and the sun doesn't shine directly on me, up to the low 90s is OK. But some humid days in the sun, even the upper 70s is too hot. But then I also drive conservatively and pay attention to heat management. For example, parking in shade instead of direct sun, using sun shields, and rain guards on the windows.

A simple trick is to monitor the energy flows. If you are driving so there is a lot of traction battery discharge and charge activity, you're starting to 'heat pump' the traction battery. If you can change to more modest, engine driven acceleration and hill climbs, the traction battery will run cooler.

One of the reasons I upgraded my pack last year was for improved thermal management. The modules after 2004 have much lower internal resistance which reduces ohmic heating. They also have stronger terminals, more resistant to internal pressure that increases with heat. Heat is the enemy of long battery health.

Bob Wilson

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Here is my graph of MPG vs temperature:

I understand there is stiffening plate that can be bought for the NHW20s that improves handling. I has not been a problem with our NHW11, the sedan body, nor the 1.8L ZVW30. But then my driving tends to be relaxed, less stressful.

Bob Wilson

Dunno about the 2010 on up models, but the 04-09 models are a bit skittish. OTOH, I've learned to deal with it to the degree that I don't notice it anymore.

Trust me, 10 deg F and high winds to drive against will pull that baby down to between 30 and 35 mpg in the blink of an eye.

What's the MPG hit in the northern winter? Are they still skittish in cross-winds?

I've experience a mileage hit of about 5% at temperatures as low as -30F. No surprise considering the energy required to deal with fluids and cabin heat at such extremes. Skittish in cross-winds? Nothing compared to the SUV and Mini-Van I previously owned. As far as wind is concerned, the Prius does well at minimizing the effect of a headwind and maximizing the effect of a tail wind. You should drive one some time.

Dunno about the 2010 on up models, but the 04-09 models are a bit skittish. OTOH, I've learned to deal with it to the degree that I don't notice it anymore.

Trust me, 10 deg F and high winds to drive against will pull that baby down to between 30 and 35 mpg in the blink of an eye.

Now on my second Prius and living in an area with both wind and temperature extremes (western Minnesota) my worst tank ever was 48 mpg. Yes, one can achieve lower instantaneous readings. Perhaps this is what you mean by "in the blink of an eye."

No, I meant tank average.

Driving on the freeway against headwinds at 10 deg F, with the cabin heat on becuase *I* want to be warm, is not the Prius's strong suit.

No, I meant tank average.

Driving on the freeway against headwinds at 10 deg F, with the cabin heat on becuase *I* want to be warm, is not the Prius's strong suit.

I've never consumed an entire tank at high speed in cold weather against a strong headwind. I once consumed an entire tank at high speed in warm weather with a strong tailwind and averaged 70 mpg. I suppose the average of our experiences would be 52.5 which is very close to the lifetime mileage (only 20K miles) on my 2010.