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Fall and Spring, prime Prius weather - Page 3

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Posted by bwilson4web on September 19, 2011, 12:44 pm

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:

So if one is driving an NHW11 up tall hills at speeds above 60-65 mph,
the traction battery is at the edge of heat-pumping. All it takes is
to avoid this is to drive slower up the hills. Fortunately, many of
these grades have truck climbing lanes and no one blinks an eye if
you're in the climbing lane at 55 mph.

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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