How Cheap Beat Cool in the Chevy Volt

Friday, May 28th, 2010

John Petersen explains how cheap beat cool in the Chevy Volt:

Battery Cost The Chevy Volt will use Li-polymer batteries manufactured by Korea’s LG Chem. While Li-polymer batteries have had a spotty safety record in cell phones and laptops and do not begin to approach the extreme cycle-life of Li-phosphate and Li-titanate chemistries, they are far and away the cheapest variety of Li-ion batteries with prices in the $600 to $700 per kWh range as opposed to the $1,300 to $2,000 per kWh range.

Passenger Safety To resolve the basic safety issues associated with Li-ion batteries, GM has designed a T-shaped battery pack that sits in front of the rear axle and runs forward through the space that used to be taken up by the driveshaft. At first glance, the battery pack looks like it comes out of a battle tank instead of a passenger car.

The topside of the battery pack looks far stronger than the bottom side of the battery pack and it’s clear that the basic geometry has been designed to deflect the potentially explosive force of a battery failure down and away from the passenger compartment. The absence of any visible deformation in the 35 mph crash test photos of the battery pack confirm that GM thinks armor plate is more cost-effective than exotic chemistry. Overall, GM’s battery pack design is a cheap but effective way to avoid potential personal injury risks.

Cycle Life Performance Li-polymer batteries are not renowned for the extreme cycle-life of their more glamorous and expensive cousins like Li-phosphate and Li-titanate. To optimize the cycle life of the batteries in the Volt, GM has chosen to install a 16 kWh battery pack in the Volt but only use 55% of the theoretical capacity to power the car. By limiting the maximum state of charge to 85% and switching to internal combustion when the state of charge falls to 30%, GM believes it can get a 10-year life out of batteries that would die much more quickly with a wider cycling range. Once again, GM has chosen a cheap but cost-effective way to balance battery capacity and cycle life.

Weight and Energy Density The final weight of the Volt battery pack is about 175 Kg. This works out to an energy density of roughly 50 Wh/Kg for useful battery capacity, about the same value as a high quality lead-acid battery.

Recycling While the Chevy Volt battery pack will be built to European recycling standards, those standards only relate to safe disposal of potentially toxic materials and do not get into issues like recovering materials of sufficient purity that they can be used to make new batteries. This is good from a pure disposal perspective, but suboptimal if one’s environmental sensitivities extend beyond landfills to include the environmental damage caused by mining and other resource extraction activities.

In the Chevy Volt, cheap has already beaten cool like I predicted it would. Since GM has established battery cost reduction as a primary goal for future generations of PHEVs, I would not be at all surprised to see GM and other auto makers paying particular attention to advanced lead-acid and lead-carbon chemistries over the next few years because the widely heralded energy density and size advantages of Li-ion chemistry evaporate when the technology is reduced to safe commercial practice.

Leave a Reply