Battery Modeling for Energy-Aware System Design

M any features of modern portable electronic devices—such as high-speed processors, colorful displays, opti-cal/magnetic storage drives, and wireless network interfaces—carry a significant energy cost. However, advances in battery technology have not kept pace with rapidly growing energy demands. Most laptops, handheld PCs, and cell phones use rechargeable electrochemical batteries—typically, lithium-ion batteries—as their portable energy source. These batteries take anywhere from 1.5 to 4 hours to fully charge, but they can run on this charge for only a few hours or, in the case of some newer pocket PCs, up to about 14 hours. The battery has thus emerged as a key parameter to control in the energy management of porta-bles. 3-8 To meet the stringent power budget of these devices, researchers have explored various architectural , hardware, software, and system-level optimizations to minimize the energy consumed per useful computation. Maximizing the number of useful computations is effectively a problem of maximizing battery lifetime subject to system performance constraints. Given a load applied to a battery over a certain period, information about when the battery fails as well as its state of charge, or remaining capacity, at any time can be used to trade off system performance for battery lifetime at both the design stage and runtime, possibly with the user's active participation. For example, an energy-aware picture phone could let a user trade off image quality with talk time and the number of photos the phone could take using the remaining battery capacity. Incorporating battery-state information into a lifetime optimization strategy requires a mathematical model that captures battery nonlinearities. Accurate low-level models 9-11 based on the differential equations that describe the complex phenomena occurring in an electrochemical cell have been around for about a decade, but solving these equations can take days. In recent years, however, researchers have developed high-level battery models 4,5,7,12-15 that reduce simulation time while predicting relevant variables with acceptable accuracy. Because the energy drawn from a battery is not always equivalent to the energy consumed in device circuits, understanding discharge behavior is essential for optimal system design. Batteries consist of cells arranged in series, parallel , or a combination of both. Two electrodes—an anode and a cathode, separated by an electrolyte— constitute each cell's active material. When the cell is connected to a load, a reduction-oxidation reaction transfers electrons from the anode to the cathode. This transfer converts the chemical energy stored in the active material …