Test The Limits

Thermal battery work is a hot issue

By Jaehoon Han

March 25 2015

Lithium-ion batteries are like humans – they don’t like to be too cold and they don’t like to be too hot. Achieving this equilibrium creates a problem for manufacturers working on the latest hybrid and battery electric vehicles, particularly as conventional internal combustion engines are pretty robust. In contrast, they are able to deal with very high or very low temperatures pretty effectively.

The same can’t necessarily be said of lithium-ion batteries, which produce little energy when too cold and can emit combustible gases if too hot. Battery lifespan is also reduced at high temperatures, leading to potential warranty issues.. Engineers face the challenge of making sure batteries are always operating at just the right temperature.

Exa already models the heat rejection of the internal combustion engine to allow our customers to do thermal work. Now we’ve added a similar capability for electric vehicles, the system able to accommodate the heat source of battery cells.

Manufacturers have to make a decision on whether they want the lithium-ion battery to be liquid- or air-cooled. In general, the bigger the battery and the tighter the packaging, the more benefit there is to be gained from liquid cooling.

For instance, one popular electric vehicle model available in the market packaged all its batteries within a narrow gap between the passenger floor and the underside of the car with a heating and cooling circuit around each cell. Because the batteries themselves need adequate cooling and heating (plus the need for the passenger compartment), the vehicle is fitted with an HVAC (heating, ventilation and air conditioning) unit which is three times the size of that required in a normal passenger car.

Using Exa’s simulation, engineers can study how the battery pack behaves thermally outside the car, examining, for example, how an air intake fan system moves air through the package and cools the device.

When running the battery pack through its normal operating cycle, the cooling system draws heat away from the batteries, limiting the build-up of excessive temperatures in any locations. Being able to simulate the battery pack under these circumstances is become increasingly important as the number of battery-powered vehicles grows. The battery may work perfectly under normal operating conditions, but too much heat can potentially damage the battery pack – an expensive game, mitigated by running a simulation of the system before further physical development takes place.

Ultimately, engineers have choices as to where they position batteries – the difficult part is working out how they work thermally once in place.

But thermal is a pass or fail scenario for new car development, and either the battery pack meets specifications around temperatures or it doesn’t. Simulation is the only way to place it more quickly in the pass category.