(January 29, 2009) At the debut of the Chevrolet Volt concept in January 2007 General Motors officials listed the lithium ion battery technology as the biggest technical hurdle to overcome on the road to production. Since that time, GM's engineering staff has become increasingly confident that the batteries will be able to meet the 10 year/150,000 mile durability target along with the 40 mile range.

In order achieve this the battery development team led by Denise Gray has developed advanced battery management systems that it believes are the key to maximizing battery life and performance. Recently Green Fuels Forecast spoke with Gray, GM director of hybrid energy storage systems and Bob Kruse, executive director of hybrid vehicle engineering about how the Volt battery will be managed.

Kruse explained that the knowledge that GM engineers and researchers have gained about batteries dating back to the EV1 program in the 1990s and continuing to this day will be a key competitive advantage for the company. For that reason, the company has decided that it will develop and produce the battery management systems for its hybrid and electric vehicles in-house. GM is working with several battery cell suppliers on the Volt as well the Saturn Vue plug-in hybrid programs.

At the recent North American International Auto Show, GM announced that LG Chem had been chosen to supply lithium ion cells for the initial production run of the Chevy Volt. GM will build a battery pack manufacturing facility to assemble the packs and management electronics.

Kruse explains that the key to the battery durability is conditioning of the pack. The batteries used in consumer electronics devices often begin to lose their ability to hold a charge after only 200-300 charge cycles. One of the reasons for this is that the electrodes develop cracks as a result of thermal stresses brought on by over-charging the cells. This is the result of attempting to use the full capacity of the battery pack in order to maximize its usefulness.

Like current hybrid vehicles, the battery for the Volt won't be using its full capacity. The pack will be charged up to about 80 percent and only depleted to about 30 percent charge. This will help avoid the heat build up that causes electrode cracking and reduced lifespan. GM is going well beyond this with the complete thermal management of the battery.

Determining the state of charge of advanced batteries is a complex problem because voltage and current behavior of different cell chemistries varies. General Motors engineers and scientists working on energy storage systems have been studying an array of different cell designs and developing parametric models of that behavior. Those models lie at the heart of the battery management system. The control system is structured in such a way that it a common system be used for a variety of different cell types and specifically calibrated for that type.

Another of the weaknesses of electro-chemical batteries is degraded performance when they are very cold. GM engineers have devised battery conditioning algorithms to help overcome this. The lithium ion pack for the Volt is liquid cooled to help keep the cells within the optimal temperature range during operation. While the vehicle is plugged in to charge, the battery inherently heats up and the cooling system will keep it at optimal temperature. If the ambient temperature is too cold, the battery will be pre-warmed in order to allow the car to operate on electricity as soon as it is unplugged.

The cell temperatures are constantly monitored during operation to ensure reliability and safety. If the car is parked unplugged in cold weather and the driver starts it up, the range extender may start up immediately even if the battery has a high state of charge. Once the pack warms up sufficiently, the range extender can be shut down and electric operation can resume.

"Right now if you're not plugged in, if you're plugged in it (the battery) will start immediately because we've conditioned the battery. If you're not plugged in and the battery is not conditioned and we've got to deal with the elements, right now we're thinking 0-10°C we won't use the battery. The more we can use it the better but we've got that area of refinement we'll have to do as we get more of the engines, more of the vehicles, more of the batteries and tune it all up" Gray tells Green Fuels Forecast.

With GM now building the packs for the Volt and other Voltec architecture vehicles, cell formats become an issue. GM is continuing to work with a number of cell suppliers for its plug-in programs and a standard physical format that could be plugged into existing pack designs would allow changes without having to re-tool the pack or production line. Different vehicles using the same pack architecture could have the balance between power and range tuned for the specific application by using different cell chemistry.

According to Gray, "A lot of that discussion happens through USABC (United States Advanced Battery Consortium) because we've got Ford, Chrysler and GM working together on what the requirements of HEV cells are as well as PHEV cells. A lot of that work on commonality, requirements and then move into common formats and footprints, so a lot of that work has been talked about for the past couple of years. The other activity we've had is a lot dialogue with our hybrid collaboration (on the Two-Mode hybrid system) with BMW and DCX (formerly DaimlerChrysler), we've talked through those kind of things as well."

"So the answer is yes, we're always talking about common and what's the least common denominator as all these things get packaged in different cars." For the foreseeable future different vehicles will continue to use different pack formats which will to some degree dictate the cell format. Gray tells GFF "The space they (the designers and engineers) give us in the vehicle dictates how tall (and wide) a cell can be in order to fit. The sensitivities are very large when it comes to location within the vehicle itself."

For hybrids, most designs are converging on putting the battery under the load floor in the rear but BEV and ER-EV vehicles remain more problematic due to the larger pack sizes needed. With the smaller hybrid packs "if it can help dictate a certain format (for the cells) we can build on that, you can add more cells, take them away," says Gray.

The battery pack for the two-mode hybrid full-size SUVs and pickup trucks from GM and Chrysler are supplied by Panasonic EV Energy, the joint venture of Toyota and Matsushita. "That's a shared cell and module with the Prius" explains Gray. "We're partnering with Cobasys and a number of us are buying our batteries from them. Again that's a common cell design." GM uses batteries from Cobasys for it's mild-hybrid system and Mercedes-Benz will use Cobasys batteries for its Two-Mode hybrid ML450 later in 2009.

Moving forward General Motors plans to refine the management system and the battery models as it gathers more knowledge about different battery designs. One of the means to doing this will be a new larger battery development and test lab at the General Motors Technical Center in Warren MI. The new lab which is claimed to be the largest of its kind in the world will provide access to all battery manufacturers. GM will provide testing services to any battery maker free of charge. General Motors gets access to the performance data and will share it with the manufacturer. That data will be used to expand the automaker's internal knowledge base about battery performance hopefully leading to lower-cost, longer life electrical energy storage solutions.