ACtion Magazine - April 2013 - (Page 30)
Hybrid evolution continues
by Andrew Ross
As automakers learn more about what works in the real world and
refine their systems, and create systems that rely more heavily on the
electric component to provide motive power, the approaches have also
diversified: from the original mild hybrids and micro hybrids at one end of
the spectrum, to strong hybrids at the other where vehicles like GM’s Volt
aren’t even called “hybrids” by their makers.
“Gradually all the extra space and weight that used to be taken up by
batteries is being reduced,” said Martin. “Batteries are the key to hybrids,
and the move to lithium-ion (Li-ion) batteries from nickel metal hydride
(NiMH) is a key to the changes in hybrid packaging.”
The main difference is that the nominal voltage of a Li-ion cell is 3.6V,
versus the 1.2V for NiMH. But, Martin points out the primary advantage
“is something called energy density; the more energy we can pack into
The Chevy Volt battery pack has coolant passages between
each battery plate.
a smaller volume, the more attractive it is for car designers,” and also for
consumers, he added.
The downside is that Li-ion is finicky. The operating conditions and
charging parameters need to be tightly controlled lest they suffer what
has been termed “thermal runaway.” That term describes the condition
of a battery overheating, releasing oxygen into the highly flammable
electrolyte, which then explodes. As these battery packs are really made
up of many small tightly packed individual cells, that condition can spread
to the next cell and so on. One has only to think of the ongoing bad press
plaguing the Boeing 787 Dreamliner battery packs to understand the
Not that all of the thermal runaway examples have been airborne. The
web is littered with images of the charred remains of early Tesla models
that suffered such an ignominious fate. (Interestingly, Tesla’s Elon Musk
ACTION • April 2013
has very recently offered advice on battery pack design to Boeing.)
Martin’s presentation outlined how the specific chemistry of a Li-ion
cell can determine both its viability as a power source and the risk of
thermal runaway, and even heat production. To understand the impact
of the specific chemistry on the importance for cooling, you need only
compare the Nissan Leaf and the aforementioned Chevy Volt.
The Nissan Leaf, which uses a Li-ion technology devoid of cobalt,
contains air-cooled, stacked-laminated battery cells with lithium
manganate cathodes. That’s right: air cooled. “The Leaf has liquid cooling
on the electronics, but not on the battery packs,” said Martin. “When
they first released the design, people said that if they don’t have active
cooling on the battery pack, they were going to have trouble. But SAE
papers show that they did a tremendous job without liquid cooling.”
The Leaf does employ a heating system for the batteries as they can be
damaged if they operate in extremely cold temperatures (anything below
minus 20 C), but rather than worrying about battery cooling, when
battery temperatures rise, the system simply reduces the current draw.
“Power to the traction motor is reduced. They don’t go after it with a
cooling system, they actually change the way the car drives.”
“This is completely different from the Volt.” The Volt has a very
carefully planned and closely monitored liquid cooling system, and
the plate-style batteries have alternating cooling fins throughout the
battery pack; like a sliced loaf of bread located in what would pass for a
transmission tunnel in a conventional front engine-rear wheel drive car.
There is a good animated video about the Volt battery pack on
YouTube that reveals the layout of the battery and its cooling system
(search for “Chevy Volt battery animation”). It’s apparent that there is
more attention paid to controlling the temperature of the battery pack,
and to keeping the coolant clean and serviceable, than is normally paid to
the engine’s cooling system.
There are in fact four cooling systems on the Volt—engine, power
electronics, battery, and electric drive unit--and GM goes to great pains
to emphasize that only Dex-Cool 50/50 pre-mix is used in all but the
electric drive unit cooling, which cools using its ATF.
Overall, Martin says, the hybrid evolution has created a market that will
demand much of the professional technician, a job made no easier by the
proliferation of systems currently on the roads.
he move from mild to strong hybrids has been unabated,
says presenter Tony Martin, and that has added a great
deal of complexity to the hybrid landscape.
The Nissan Leaf battery pack is air cooled.
Table of Contents for the Digital Edition of ACtion Magazine - April 2013
ACtion Magazine - April 2013
Under the Southern Cross
News & Updates
State of the Industry
R-1234YF Design and Service Considerations
Heavy Duty/Off Road Technical Session
Hybrid Evolution Continues
Modern Automotive HVAC Systems
Old-Timers, Team Players, Slackers and Kids: Do Your Employees ‘play’ Well Together?
Cheap Isn’t Always Best
A/C Season Check List: Is Your Shop Ready?
MACS 2013 Training Event Social Wrap-Up
Wallace Talks Up Big Brother
New Products & Services
ACtion Magazine - April 2013