The dual system can be less expensive because it can use less expensive battery chemistry for some of its capacity without having to boost battery size to get enough energy storage.
A proprietary computerized energy management system splits up the vehicle power needs between the two batteries. The lithium-ion battery comes from battery maker A123, Watertown, Mass., a partner on this project. The lithium-ion batteries avoid the traditional positive electrode materials used in most laptop and cell-phone lithium batteries which can become unstable if overcharged or overheated as when the battery develops an internal short. The unstable materials release oxygen, oxidizing other materials in the battery, which in turn produces more heat. The cycle can continue in thermal runaway.
In contrast, the A123 battery replaces cobalt oxide electrodes with iron phosphate, a much more stable material. The electrodes are also thin, which makes for a fast discharge but limits the overall storage capacity.
The NaMxTM battery used for bulk energy storage contains a beta alumina electrolyte, which conducts sodium ions, but which is also an electrical insulator. In the charged state, this alumina tube separates liquid sodium in the anode from nickel chloride in the cathode. Battery internal temperature must stay at 300°C to keep the sodium metal in liquid phase and also provide fast ion transport through the beta alumina during discharge.
The GE research is being done as part of a $13 million project with the Federal Transit Administration and Northeast Advanced Vehicle Consortium, funded under the National Fuel Cell Bus Program.
More info: http://www.businesswire.com/news/home/20101202005928/en/GE-Demonstrates-%E2%80%9CElectric-Bus-Future%E2%80%9D
