Cool Power: Chevy Volt’s Complex Systems

Hybrids, as have so many vehicles have come to be termed these days, employ a variety of designs that are sure to drive technicians to distraction for years to come.

From the original mild hybrids to micro hybrids at one end of the spectrum and strong hybrids at the other where vehicles like GM’s Volt aren’t event called “hybrids” by their makers.

What they all share is the need to cool the batteries that provide motive power to the electric drive. (There is one exception, the Nissan Leaf, which relies on passive cooling and reducing the load—and output—of the battery packs should temperatures rise above the optimal range).

Hybrid designs on the market today employ a variety of battery cooling strategies with forced air and liquid cooling all finding their place. But none have taken cooling to the level of obsession the way the Chevy Volt has.

Taking a step back for a moment, it is important to recognize that a key reason cooling is so important is the move to Lithium Ion batteries from nickel metal hydride (NiMH).

That shift has made packing more desirable as the Li-ion pack more punch. The nominal voltage of a Li-ion cell is 3.6V, versus the 1.2V for NiMH, but the Li-ion option takes up less space for a given power output.

The downside is that Li-ion technology is that their operating conditions and charging parameters need to be tightly controlled lest they suffer what has been termed “thermal runaway.” Thermal runaway 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 then spread to the next cell and so on.

One has only to think of the ongoing bad press plaguing the Boeing Dreamliner battery packs to understand the implications. And, if you want some lurid pics, the web is littered with images of the charred remains of early Tesla models that suffered such an ignominious fate.

Okay, so back to the Volt.

Which leads to the reason why 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.

All four systems utilize their own separate radiator (or rad-partition) for heat exchange, and are sandwiched together and mounted in the traditional location at the front of the engine compartment.

These radiators (and internally routed coolants) are primarily cooled by undercar airflow directed by an air-dam, through the radiators. Airflow is augmented by a pair of variable speed, electrically powered (12V) cooling fans controlled by the Engine Control Module (ECM).

The most complex of these is the system used to control the temperature of the high voltage battery pack.

The Volt has a very carefully planned and well monitored cooling system-battery arrangement, with plate-style batteries alternating with 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.

The cooling system actually contains loops for coolant and a secondary cooling loop using refrigerant and a coolant-to-refrigerant heat exchanger (evaporator).

The plates are tightly packed, hence the automaker tells service personnel to be extremely diligent in using the 50/50 Dexcool or or other GM6277M compliant coolant premix to keep contamination dangers—and blockages to a minimum.

The Volt’s T-shaped battery is equipped with a pair of quick-disconnect fittings which create the coolant in/out connections to the high voltage battery housing. The coolant inlet to the battery housing includes a debris filter, and a variable high voltage heating element that operates directly off the 360V Li-Ion battery.

The battery cooling system shares a radiator assembly and the twin 12-volt variable speed cooling fans with the power electronics cooling system. The lower section of the dual radiator is used for battery system cooling. The battery cooling system has its own 12-volt coolant pump, a refrigerant-to-coolant heat exchanger and a three-way coolant flow control valve to route coolant through the radiator, the heat exchanger, or go to bypass mode. There is also an air separator and surge tank that is integrated with the electronics reservoir/tank (a single housing divided internally into two separate tanks).

Controls are through the Hybrid Powertrain Control Module 2. This and other networked modules monitor ambient conditions, the battery in/out coolant temperatures, various Li-Ion cell temperature probes, as well as refrigerant temperatures and pressures to establish battery heating or cooling requirements.

The module can selectively turn the coolant pump on or off, positions the coolant flow control valve, and depending on whether cooling or heating is required, request either the a/c loop compressor to operate (cooling), or turn on the battery coolant heating element, if temperature drop too low. The battery cooling/heating system can be activated when the vehicle is on and during charging operations if necessary.

When battery heating is required the three-way coolant flow control valve will be moved to position “A” to permit fast heating of the battery cells in cold weather.

The three-way valve will be set to Position “B” whenever the Li-Ion battery cells are too hot. This routes coolant thought the loop containing the refrigerant heat exchanger; refrigerant flow is controlled through a thermal expansion valve (TXV) ahead of this coolant to refrigerant heat exchanger (evaporator). (There is a second TXV and evaporator for the passenger compartment.)

During more temperature stable operating conditions, battery coolant would be circulating out to the battery cooling radiator and back to the pump with the valve in “Position C.” This route permits temperature stability by controlling cell temperatures through pump control.

The key goal for this sophisticated system is to ensure that the battery provides a long, useful life.

For the automotive aftermarket professionals, it is important to recognize that due to the temperature sensitivity of the battery pack it is critical to use of only approved fluids in any service occasion—including the use of 50/50 Pre-Mix requirement to ensure that contamination does not damage the battery cooling system.

It is also important for you and your customers to know where to go for service information as these vehicles will eventually find their way into customer service bays.

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SIDEBAR

Work Safely

As has been the practices for some time, whenever hybrid systems are discussed, it is important to emphasize the following the proper safety practices, whether conducing inspection or performing maintenance of a hybrid vehicle.

Wear approved safety glasses

Remove anything that can make you a conductor, Rings, watches, belt buckles.

Wear rubber soled shoes

Use proper, good condition, high voltage gloves. Minimum Class 0, 1000 volt AC. (They can be damaged by petroleum solvents and UV light.)

Also remember that even when it is off, and the systems should be de-energized, things can go wrong so should be treated as possibly energized.

Remember: the vehicle is in your shop because something is wrong with it. You shouldn’t assume that its systems are operating properly.