CM0256 Pervasive Computing Lecture 8 – Power Sources Tom Goodale
[email protected] Lecture Outline In this lecture we: Examine power sources for mobile devices What power sources are there ? What are their characteristics ? What are their limitations ? Power Sources Mechanical e.g. clockwork Electrical mains power solar power batteries fuel cell Radiation ? Batteries A battery is a device to store energy and make it available in electrical form. Electricity is produced by a chemical reaction, producing electricity, and some by-products, such as heat or gases. In rechargable batteries this chemical reaction is reversible. Battery Arrangements Strictly, an electrical “battery” is an interconnected array of one or more similar “cells”. In current English usage, however, it is more common to call a single cell used on its own a battery than a cell. For example, a hand lamp (flashlight) (torch) is said to take one or more "batteries" even though they may be D cells. A car battery is a true "battery" because it uses multiple cells. Multiple batteries or cells may also be refered to as a battery pack, such as a set of multi-cell 12 V batteries in an electric vehicle. Battery Arrangements In almost any device that uses batteries, you do not use just one cell at a time. You normally group them together serially to form higher voltages, or in parallel to form higher currents. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents add up. The upper arrangement is called a parallel arrangement. If you assume that each cell produces 1.5 volts, then four batteries in parallel will also produce 1.5 volts, but the current supplied will be four times that of a single cell.