"Dana's proprietary process is cleaner than competitive flux-brazing processes, which could react with the long-life engine coolant that is used in the battery cooling system," Ted Zielinski, Director of Advanced Thermal Engineering for the Dana Power Technologies Group, told AEI.

As the thinnest aluminum plates that Dana has ever produced for a production application, the cooling plates have 144 fins to distribute the coolant. An electric pump can be used to circulate coolant through the cooling plates during battery charging and discharging cycles.

According to Lance Turner, development engineer for General Motors' VOLTec rechargeable energy storage system, each fin addresses the face of two cells, providing a thermal bridge between the common cells in parallel, as well as the neighboring series cells.

"Multiple paths are provided to pass coolant across the face of each prismatic cell, maximizing coolant surface exposure," said Turner.

Sixteen thermal sensors in the T-shaped battery pack transmit data to the Volt's thermal management system. The system is programmed to stay within 3.6°F (2°C) of the pack's optimal temperature, which depending on usage conditions falls in a range between 50 and 85°F (10 and 30°C).

If the batteries are too hot, a shorter life span is possible. Batteries that are too cold can lose power output and/or energy capacity.

Three different systems regulate the Volt's coolant temperature. An electric heater located at the battery pack's inlet warms the coolant when the Volt is plugged in and charging during cold weather. During normal vehicle operating conditions, coolant passes through a heat exchanger. And when the battery temperature is excessive, a chiller in the air-conditioning circuit dissipates battery heat.

"The vehicle manages a closed-loop operation of its thermal system with multiple controllers rendering diagnostics and correlations for proper operation," said Turner.

Battery cooling for the plug-in Volt is vastly different from what was used in the EV1, an all-electric two-door coupe that GM produced from 1996 to 1999.

According to Turner, the EV1 began its development with lead-acid batteries using a pack configuration that was biased toward 12-V bricks—essentially mono-blocks connected in series with an upper and lower level.

EV1's evolution to a nickel metal-hydride (NiMH) pack also meant using air to cool the batteries. "There was a carryover issue because of the single air inlet point, which meant adjacent batteries shared the same warm air with batteries placed downwind," said Turner.

The higher internal resistance of the EV1's NiMH batteries meant that, under normal operating conditions, the batteries needed higher conditioning.

In contrast, the Volt's thermal system was designed for temperatures associated to cell behavior akin to battery end-life. "The Volt's integral cooling plates means the system is managing the fluid at the cell level as opposed to the mono-block level," said Turner.

Volt engineers strived to optimize the nine-module battery pack's energy density, so "arranging the cells in a horizontal stack—similar to a fuel cell—made more sense for manufacturing and packaging. In addition, the thermal system was designed in tandem with the cells," Turner said.