Authored by Dr. James F. Young, Electrical & Computer Engineering Department, Rice University.
(Originally published here. See copyright notice at end of article.)
There are several ways to power a robot. Some large robots use internal combustion engines to generate electricity or to power hydraulic or pneumatic actuators.
For a small robot, however, battery power offers a number of advantages, since batteries are cheap, relatively safe, small, and easy to use. Also, motors convert electrical power into mechanical power with relatively high efficiency. Many different types of batteries exist, each with its own tradeoffs. This chapter discusses a variety of batteries, standard ways of rating them, rechargeability and the design of battery chargers.
Typical DC motors may operate on as few as 1.5 Volts, or up to 100 Volts or more. Roboticists often use motors that operate on 6, 12, or 24 volts because batteries are typically available with these values. The terms battery and cell are often used interchangeably, but they have different meanings. Technically, a cell is a unit housing a single chemical reaction that produces electricity, while a battery is a bank or collection of cells connected together.
The voltage produced by the cells' chemical reaction depends on the materials used in the reaction. This voltage is called the nominal cell voltage and differs among various battery technologies. For example, a standard flashlight cell uses a carbon-zinc reaction and has a cell voltage of 1.5 Volts. Most car batteries have six lead-acid cells, each with a cell voltage of 2.0 Volts (yielding the 12 volt battery).
In general, the larger the cell, the more current and energy it can supply. The cell capacity is measured in ampere-hours, which is the number of hours the cell can supply a certain amount of current before its voltage drops below a predetermined threshold value. For example, 9 Volt alkaline batteries (which consist internally of six 1.5 volt alkaline cells) are generally rated at about 1 ampere-hour, meaning the battery can continuously supply one ampere of current for one hour before "dying." In the capacity measurement, the 9 Volt alkaline battery is declared non-functional when the battery voltage drops below 5.4 Volts (a rather arbitrary number determined by some committee).
However, battery capacity depends on how the battery is used. The standard amp-hour measurement usually assumes a twenty hour discharge time. That is, the 9 Volt battery would need to be tested by having it supply 1/20th of its rated capacity -- this would be 50 milliamps -- for twenty hours. If it were drained faster, as in a one-hour test or in a high-current application, the effective capacity would be much less. The voltage would drop below the threshold before the product of current and time reached 1 ampere-hour.
In many applications the most important rating of a cell is the capacity per unit weight or power density; power density differs widely across battery types. Inexpensive carbon-zinc cells have the lowest power density of all cell types. Alkaline cells have about ten times the power density of carbon-zinc cells, and nickel-cadmium cells have less power density than alkalines, but they are rechargeable.
The voltage of a cell drops over the course of its life as it discharges. The characteristic discharge curve varies considerably over different types of cell. For example, alkaline cells have a fairly linear drop from full cell voltage to zero volts. Since the drop is gradual, it is usually easy to tell when the battery capacity is used up and the battery needs replacement. Nickel cadmium cells have a slower linear voltage drop region up to a certain point after which the voltage then falls off sharply. The more constant voltage is important for some applications, but it also means that nickel cadmium cells will appear to suddenly "die" with no warning.
A cell can be modeled as a perfect voltage source in series with a resistor. This important cell characteristic, called the internal resistance, determines the maximum rate at which power can be drawn out of the cell, along with the maximum voltage and current.
Lead acid cells have very low internal resistance, which makes them well-suited for use as a car battery, because very high current can be drawn from the cells to operate the car's starter motor. During the recycle time of a standard camera flash unit, the cells must supply charge as quickly as they can. Again, the rate is limited largely by the cells' internal resistance. Alkaline cells have higher internal resistance than nickel-cadmium cells; therefore a flash unit takes longer to recycle when alkaline cells are used.
Cells with low internal resistance (in particular, lead acid and nickel cadmium cells) can be dangerous to work with, because extremely large currents can flow if the cell is shorted externally, since the internal resistance is so low. These currents will heat the metal they are flowing through, easily melting the insulation. The cells will also become very hot and may possibly explode. For this reason it is very important not to short a lead acid or nickel cadmium cell. Alkaline cells and carbon zinc cells, should also not be shorted, but their higher internal resistance will tend to limit currents. Still, they can get hot and produce burns.
Another important characteristic of a cell is whether or not it is rechargeable, and if so, how many times it can be recharged. Recharging is cheaper than replacement and often more convenient. The chemicals in cells are quite toxic to the environment, making rechargeability all the more relevant. Unfortunately, the cells with the highest power densities -- alkaline and lithium -- are not rechargeable, although the characteristics of rechargeable cells are improving.
The Memory Effect
The term memory effect refers to a phenomenon observed in rechargeable nickel cadmium cells. If such cells are only partially discharged before recharging, they tend to "remember" the level of discharge, and, over time, become usable only to that discharge level. There is disagreement among cell manufacturers as to whether or not the memory effect actually exists, but most concur that nickel cadmium cells should be discharged fully before being recharged. Some cell technologies, such as lead acid cells and the new nickel hydride cells, do not exhibit this effect. Lead acid cells typically last for several hundred cycles of full discharge, and a thousand cycles of partial discharge.
Last but not least is cost: like many things in life (but not all), cells having high performance cost a lot more than the cells that perform poorly. For consumer purposes, it is generally agreed that nickel cadmium cells, which cost several times as much as alkaline cells, are far less expensive over the cells' lifetimes. Nickel cadmium cells can be recharged several hundred times while alkaline cells are disposed of after one use. On the other hand, nickel cadmium cells exhibit the "sudden death" property mentioned earlier.
Figure 1 summarizes the characteristics of commonly available cell technologies. The advent of laptop computers and the need for convenient electric cars have created a real market demand for improved batteries. New battery types include nickel metal hydride, lithium polymer, sodium sulfur, zinc-air, and zinc bromine. The very high capacity, rechargeable, nickel hydride cells are very expensive, but offer twice the capacity of either lead acid or nickel cadmium cells. The problem with newer batteries is that none of them offer all the features of an ideal battery: inexpensive, long-lived, high power density, and safety.
|Figure 1. Table of Cell Characteristics.|
There are two ways that cells may be combined to make batteries: series connections and parallel connections. When cells are connected in series, their voltages add but their ampere-hour capacity does not. Series batteries should be composed of cells with equal capacities. When cells are connected in parallel, the total voltage remains the same, but cell capacities add.
|Figure 2. A Battery Charger Schematic Diagram.|
The rule of thumb for charging batteries is to charge them at a rate equal to one-tenth of the battery's amp-hour capacity. For example, if a battery is rated for 1.3 ampere-hours, then it should be charged at a rate of about 130 mA. However, some manufacturers specify a particular recommended charging rate for their batteries.
A simple battery charger, shown in Fig. 2, has been designed to charge the battery at a current of up to 520 mA, as recommended by the manufacturer. This charger has also been designed to prevent the battery from over-charging, and to indicate when the battery is fully charged. These tasks are accomplished by using a transistor in the circuit as a switch to control the current to the battery, and a zener diode to sense when the battery is charged. The zener diode requires 12 volts across it before any current will flow, so if the battery voltage is below this value the resistor biasing circuit will turn the transistor on (conducting) and all the current from the DC supply will flow through the battery, charging it. As the battery charges, its voltage rises, and eventually the zener diode will begin to conduct current, drawing current away from the transistor base, turning it off. The transistor will turn off completely when the battery voltage reaches about 13 Volts: 12 Volts across the zener, plus about a 1.7 Volt drop across the indicator LED, minus the 0.7 Volt forward bias voltage of the transistor. At this point, the current from the DC supply is flowing through the zener diode and the LED. The resistors in the circuit limit the maximum charging current to the battery, and control the current going into the base of the transistor.
Because of the voltage sensing circuit, this charger will not over charge the battery at the fastest recommended rate, yet can be left on indefinitely without damaging the battery. The LED provides a convenient indication of full (or at least the same) charge.
First through sixth editions, copyright © 1994-1999 by John K. Bennett;
Seventh edition, August 2000, copyright © 2000 by James F. Young;
Rice University, Houston, Texas
Permission is hereby granted to reproduce and to distribute verbatim copies of this document in whole or in part, provided that no fee is collected for its distribution (other than reasonable reproduction costs) and provided that this copyright notice is included.
Other than verbatim copies with copyright notice intact, no part of this document may be reproduced in any form without written permission of the author. For example, the right to make derivative works based on this document is not granted, and requires written consent.
Substantial portions of this document are derived from The 6.270 Robot Builder's Guide, Copyright © 1992 by Fred G. Martin, with the permission of the author. Please see the Acknowledgements for additional contributors and information.
The M.I.T. Department of Electrical Engineering and Computer Science and the M.I.T. Media Laboratory, who sponsored the development of the original 6.270 class technology, have agreed to unrestricted and free distribution of the robotics technology described in the course documentation for the 6.270 class, including their printed circuit board artwork and software programming environment. The Department of Electrical Engineering and the George R. Brown School of Engineering at Rice University have agreed to similar unrestricted distribution of the ELEC 201 class technology developed at Rice. While the material has not been placed in the public domain (it is still copyrighted), this means that any individual or organization can use the material for whatever purposes they desire. Our mutual hope is that people will use this work in the spirit of GNU software, which is distributed freely and is supported largely by a community of interested users. Corrections and suggestions for improvement are most welcome.
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