"Energy" is the ability to do "work," a definition that relates pretty well to the world of practical experience. Everyone knows that "it takes energy to get the work done." Energy can take many forms. In addition to human "energy," there are mechanical energy, heat energy, chemical energy, electrical energy, nuclear energy, etc.
Although each of these forms of energy is in a form slightly different from the others, all share the basic feature: each provides the ability to do work.
To turn the idea around, work is the process of "spending" energy, usually in a way that we humans call "useful," although it isn't strictly necessary that the work be useful in our sense of the word. Since nature doesn't care about usefulness, expending energy in any form at all is properly called work.
Potential energy is energy accumulated in a useful form but not yet used. A relevant example is the chemical energy accumulated in a charged storage battery. Connecting the battery to a charger which, in turn, is connected via the power lines to a distant generating plant stores, in chemical form, a small part of the energy output of the generating plant. It makes no difference whether the energy is generated in a hydro-electric or a steam-electric plant: the same small part of the energy output of the plant is stored, as potential energy in chemical form in the battery.
When the battery is connected to the circuits of an electric fork lift truck, its chemical energy is converted into electrical energy and released, a little at a time, to the truck, which converts it into mechanical energy in the form of useful, measurable work: moving heavy coils of wire from one end of the plant to another, stacking loaded pallets, and so on. Each expenditure of energy reduces the potential stored in the battery, and so less is then available. When all of the usable energy stored in the battery has been "used up" the battery is discharged. To get more work out of it we must recharge it by restoring its supply of energy.
Whatever the source of the energy at the power lines, it takes work to generate it, accumulate it, and transmit it, and more work to convert it into chemical form in the recharged battery. For practical purposes, all of these processes from generating to charging are part of the energy cost and are basic components of every electric truck operator's utility bill.
Energy is the ability to do work and power is the rate at which the work is done. Lifting 55 pounds 10 feet in the air takes 550 foot-pounds of energy, and doing that much work in 1 second is 550 foot-pounds per second or 1 horsepower.
The term horsepower was invented in the 18th Century to create a practical unit for the rate of doing work. Presumably, it represents working "as fast as a horse," a rate we define as 550 foot-pounds per second. (Figure 1.)
Another unit used to represent the rate of doing work is the "watt." Because both the horsepower and the watt represent rates of doing work, they can be equated to one another, and it turns out that 1 horsepower is the same rate of work as approximately 750 watts*.
Where work and energy are concerned, only the total amount accumulated or spent matters ... not how quickly or slowly. Where power is concerned, however, the time factor enters. The faster a given amount of energy is spent, the higher the power rating; the slower the same amount of energy is spent, the lower the power rating.
*Here and elsewhere. we have deliberately rounded values to simplify arithmetic.
Figure 1: Horsepower
There are many ways to accumulate energy. For example, feeding our horse so that he can work that is, deliver horsepower is one way of accumulating energy, and so is charging a storage battery, which is actually called an accumulator in some other countries.
In accumulating energy, the important considerations are: the quantity of energy to be stored in the accumulator; the form in which the energy is accumulated, stored, and released for use; and the overall efficiency of the process of accumulating, storing, and releasing the energy.
In the lead acid storage battery, large quantities of energy are accumulated by chemical activity that is produced by the charging process. The energy is then released on demand in the convenient form of electric current.
With charging and discharging batteries, as with all energy transfer processes, energy losses occur. The inequality between what is put into a system and what is drained from it is the system's energy efficiency. Generally, the energy efficiency of lead acid batteries is about 76%, meaning that 76% of the energy that was put into the battery during charging is all that is available for release during discharge. The energy efficiency is given as an approximate number since discharge rates and temperature can affect it.
The battery charger, which interfaces the battery with a source of AC power, also has an efficiency rating and, thus, is considered when calculating the overall efficiency of a battery system. A good charger is about 85% efficient, making for a combined charger/battery efficiency of about 65%, meaning that 65% of the electricity from the AC line fed into the battery is available as DC energy to a machine's components, such as controllers, motors, drive trains, etc., which are also not 100% efficient in their use of energy.
Another factor affecting the battery's efficiency is how and when it is charged. For a further discussion of charging regimens and their effect on energy efficiency and economy, see Section 4, Optimizing Energy Usage.
As an interesting note: only about 7% of the potential energy put into the power plant is available for actual work; for example, the lifting and moving of pallets from one place to another in a warehouse, when using an electric fork lift truck.
Figure 2: Energy Efficiency from Electric Generation Plant to Work Done by Electric Fork Lift Truck
Our use of electrical energy is dependent on the "flow" of electrical energy, the magnitude of the "force" propelling it and the "resistance" to its flow that naturally occurs in all materials through which it flows. The materials and the circuits they make up through which electrical energy flows, in effect, convert the energy into other forms of energy, such as heat and light, or work, such as turning a motor.
When a battery is used to power a fork lift truck, it operates with a "force" that affects the "flow" of its stored energy depending on the "load" the power needs of the truck's network of "resistances."
The relationship between "flow", "force" and "resistance" is expressed mathematically in Ohm's Law, which was discovered by George Simon Ohm in the early 1800s. The Law states that E ("force" in volts) is equal to I ("flow" in amperes) times R ("resistance" in ohms). E = IR and its variants, I = E/R and R = E/l, comprise one of the basic tools electrical engineers have for designing electric powered machines.
In sections of this book, current is discussed in conventional terms as flowing from the positive to the negative; while, in other sections, it is discussed in the electrochemist's terms as flowing from the negative to the positive.
Figure 3: The Concept of Electrical Current
Current flow is the means by which the battery releases its energy in electrical form. Current is the flow of electric charge from the positive terminal of the battery through the "load" (motors, pumps, controllers, etc.) of the truck, and back to the negative terminal of the battery. (Figure 3.)
The flow of current from the battery depletes the battery's stored charge. The rate of that depletion or current drain is measured in amperes. An ampere is the flow of 1 coulomb* per second. It is also defined as the amount of current that can be forced through a resistance of 1 ohm by an electrical potential of 1 volt.
The volt is the unit of electrical potential, or pressure, that "forces" the current from the battery through the load and back to the battery. Since batteries are made up of many cells connected in series the total voltage of a battery, naturally, is the sum of the voltages of all its cells. A typical lead acid battery used in a fork lift truck may have 18 cells, nominally 2 volts each; the nominal battery voltage is therefore 36 volts.
The watt is the electrical unit that defines the rate at which work is done, or energy is spent. Mathematically, we can say that Watts = Volts Amperes.
The bigger the current (amperes) and/or the voltage (volts), the faster the stored energy can be converted into work.
If the watt is the rate of doing work, then the total amount of work actually done is the product of watts and hours: watt-hours. If a battery delivers 1000 amperes at 24 volts for 2 hours, the total amount of energy delivered is
1000 24 = 24,000 watts 2 hours = 48,000 watt-hours .
To keep the significant digits from attracting too many zeros, we use the prefix "kilo" (k), which means 1000, and, sometimes, "mega" (M) for millions. The energy delivered in this example is therefore 48 kilowatt-hours.
The coulomb is a unit of electrical energy that has been in use since before the discovery of the electron.
It is actually made up of 6,000,000,000,000.000.000 electrons.
A rating of 1000 watt-hours (1 kWh) is equivalent to 1 horse working 11/3 hours. Where batteries are concerned, the kilowatt-hour rating at a stated discharge current is an accurate description of exactly how much energy the battery can deliver before it is discharged. Since electric bills are rendered on the basis of the number of kilowatt-hours of usage, it is often useful to make calculations about energy usage and efficiency directly in kilowatt-hours.
An ampere-hour is the total amount of electrical charge transferred when a current of 1 ampere flows for 1 hour. Therefore, the total usable charge stored in a battery can be stated in terms of ampere-hours how long a current of a particular amperage can be drawn from the battery.
The ampere-hour rating accurately predicts the battery's capacity at a specified load current; batteries are therefore rated in ampere-hours at specified currents. A battery that can be discharged at 125 amperes for 6 hours before reaching its end-point voltage is rated at 125 amperes 6 hours = 750 ampere-hours. Its "capacity" is therefore stated as "750 ampere-hours at the 6-hour discharge rate (at + 25°C)."
The term battery capacity relates to the amount of usable electrical energy stored in the battery. It is important to keep in mind that a manufacturer's rated capacity is given for 100% discharge of the battery. The recommended usable capacity, however, is generally 80% of the rated capacity to insure maximum battery life. For practical purposes, battery capacity is usually stated in ampere-hours because a particular number of ampere-hours of capacity is equatable to operating a given vehicle for a given length of time before its output voltage reaches the end point. A battery rated at 1200 ampere-hours is, therefore, thought of as maximally having 960 ampere-hours of usable capacity. Capacity is also affected by discharge rate and other variables that are discussed more thoroughly in the following pages.
Most manufacturers also provide capacity ratings in terms of kilowatt-hour specification when describing their batteries. As with ampere-hour ratings, the conditions under which kilowatt-hour specifications are determined must be specifically stated to be meaningful.
Today's lead acid traction battery has a downward-curving discharge characteristic, meaning that the voltage of the battery decreases gradually as it is discharged. The end point voltage is that which determines that the battery is discharged. Defining an end point voltage is an attempt to provide users with a cut-off beyond which the battery should not be used or damage to it and the equipment it is powering may occur.
Depending on the rate of current drain and the equipment, end point voltage can vary. However, when a usage pattern of the battery and equipment are predictable, as is pretty much the case with fork lift trucks, an end point voltage is quite meaningful.
The end point voltage for electric fork lift truck applications is selected by general agreement among battery and truck manufacturers. Still, for a 2 volt cell, there are differing opinions on just where the end point should be set with a normal range lying between 1.65 and 1.75 volts per cell (with the battery "loaded"). In some cases, especially at high discharge rates, this range is extended as low as 1.2 volts per cell by some manufacturers.
Battery manufacturers may suggest that their batteries can safely be discharged beyond the accepted range of end point voltages. No significant loss of battery life will be caused by such operation, they say. Further, they may even maintain that such operation is cost-effective as far as battery life is concerned.
Truck manufacturers, however, may take a different position. They say that allowing the battery voltage to fall appreciably below the specified end point may do irreparable damage to electrical equipment installed on the truck. They point out that operating at undervoltage can, for example, overheat motors causing ultimate failure and/or burn relay contacts.
From the users' point of view, though, one kind of damage may be as bad as another. A damaged battery must be replaced; a damaged pump motor or other component must be repaired or replaced. In either case, loss of the use of the truck down time is a problem that may be more severe than the physical damage. Users, therefore, may have more of an interest in establishing and accurately detecting the end point voltage than either of the two suppliers.