by Arthur Ed LeBouthillier

This article appeared in the July 1999 issue of The Robot Builder.

In developing  mobile robots, one is often faced with severe limitations on the options available for power sources. Sure, one can consider internal-combustion engines, steam power, solar power and a host of other options, but each of these has unique
problems that limits their use to particular environments or to particularly large robots.

Batteries are about the only real option available for mobile robots especially with robots for in-the-house usage. Additionally, rechargeable batteries are the only real economical option for hobbyists. It helps to understand how to determine the proper
batteries for your application.

Battery Basics

Batteries use dissimilar materials to create electrical energy from chemical energy. There are two major kinds of batteries: primary and rechargeable. Primary batteries refer to batteries based on non-reversible chemical processes. Rechargeable
batteries use reversible chemical processes allowing you to use them time and time again.

There are two major factors to consider when choosing a battery: the voltage and the current rating. The voltage rating is fairly straightforward; a battery is rated as providing a particular voltage for the duration of its charge. In reality, a battery never
supplies precisely its rated voltage; rather, it supplies around the rated voltage and the voltage specification is an average over its charge period. At full charge, a battery generally has more than its rated voltage. A 12 Volt lead-acid battery, for
example, can supply over 13.8 volts. As it reaches the end of its charge, a battery also supplies less than its rated voltage. Again, for a lead-acid battery it could go below 10.1 volts. However, between its high charge value and its low charge value, most
batteries supply a fairly linear drop in voltage over their discharge period. Figure 1 shows a discharge cycle for a typical battery.

So how much time does a battery take to discharge? That is specified by the Amp-Hour rating. What that means is that the battery will put out its specified voltage (on average) for one hour at the specified amps.

 Therefore a 12 Volt 4 Amp-Hour could theoretically supply 4 amps for one hour; likewise, it
could supply one amp for four hours. In reality, you are likely to see less than the maximum rating when you draw higher currents. The specification for a battery is measured over a period of 10 hours and scaled to be equivalent for 1 hour. When measuring a battery designated as 12 Volt 4 Amp-Hour, it was tested by drawing 0.4 Amps for 10 hours. Drawing at higher amperage causes greater losses due to heating and internal resistances. Therefore, to see the rated charge duration for a battery, you shouldn’t draw current at rates which are too high.

Again, the way to figure out “too high” is to realize that the specification calls for drawing the power over a ten hour period and as you draw current at a rate that would lead to a battery discharging too soon before that, you are approaching “too high” of a current draw and will see less overall power from the battery.

Power and Energy Density

Another real factor in choosing a battery for a mobile robot is how much power or energy it can
supply per pound. As a quick refresher, Watt determined that power (in units of Watts) is equal to
the voltage (in units volts) times the current (in units of amps).

The Energy Density refers to the number of watt-hours that a battery can supply per pound.
Table 1 shows a list of different battery technologies and their energy densities.

As can be seen, lead-acid batteries (Pb-acid) have an energy density of 14-20 Watt-hours per pound. Nickel-Cadmium (Ni-Cd) can have a slightly higher energy density than lead-acid batteries and Nickel-Metal Hydride (NiMH) batteries can supply even more energy per pound. Conversely, these other battery technologies may not be able to supply as high a current for each pound that lead-acid can. Listed are also a number of other battery technologies which may be very difficult to use (because of temperature requirements) or expensive. Nonetheless, it is interesting to rate the batteries you might actually plan on using against some alternative “exotic” battery technologies.

One of the more interesting technologies of the exotic battery technologies is the lithium ion (Li+)
battery because it provides very high energy and power densities and is reasonably easy to use
(although it requires a high-quality charger). A Li+ battery can hold almost twice the energy of lead-acid batteries for the same weight. Even more exciting than the Li+ battery is the
aluminum-air battery (Al-air). It provides extremely high energy density as well as high power density.

Yardney, a manufacturer of these batteries, has one model which is only about 13 pounds but which is able to supply 3.0 kW-h at 12 volts. This is equivalent to having a 250 amp-hour battery at 12 volts! Of course, such exotic battery technology costs much more than the equivalent lead-acid battery but for some applications, this cost can be justified. One disadvantage of the Al-Air batteries is that they are not electrically rechargeable, because the anode (the aluminum plates) must be replaced to perform the recharge function. Therefore, they are more like primary cells with replaceable plates.

Summary

A battery is a chemical device able to supply a certain amount of current at a voltage for a
duration. One can determine his battery needs by finding out the required operational parameters and then choosing a battery type that will fulfill this need.

Source

Dowling, Kevin, Power Sources for Small Robots, CMU-RI-TR-97-02