A battery converts chemical energy into electrical energy. Batteries work because they contain three things: an anode, a cathode, and an electrolyte. When the electrodes are connected to a circuit, electrons flow from the negative to positive electrode, powering flashlights, cell phones, or motors (and, as part of the battery's chemical reaction, positive ions migrate through the battery's electrolyte).
Figure 1. A battery cell in a circuit with a light bulb.
The magic behind batteries is a series of chemical reactions called redox reactions. A redox reaction involves the transfer of electrons between different types of metals or other materials that make up the electrodes. Batteries work because the materials making up the electrodes have different oxidation states. When the positive cathode and negative anode are connected by a circuit, electrons will flow from the anode to the cathode, producing an electrical current.
Redox reaction is short for oxidation-reduction reactions. Oxidation-reduction reactions always happen together: if something is reduced, something else is oxidized. Whatever is reduced gains electrons (often becoming less positive) and whatever is oxidized loses electrons. In the case of a battery, when the battery is connected in a circuit to a load (a light bulb, a motor, or a radio) the anode is oxidized and the cathode is reduced. As electrons move through the circuit, positive ions migrate through the electrolyte to balance the movement of electrons through the external circuit.
Figure 2. A battery cell powering a light bulb, showing the flow of electrons from anode to cathode.
Most importantly, every battery has an anode and cathode, and it is the chemical reaction at each that generates the electrical current that makes batteries useful.
The best way to learn about the chemical reactions important to a battery is to build one. This is surprisingly easy to do.
The film canister battery is a simple example, but it illustrates the relationship between the basic components that allow all batteries to generate electricity when connected to a circuit: the anode, cathode, and electrolyte.
Electricity is carried by a stream of electrons: it can be a burst of electricity between cloud and earth as in a lightning strike or it can be a flow of electricity through a circuit connected to the terminals of a battery.
A useful analogy for thinking about electricity is the flow of water. Just as a flow of water can turn a water wheel, as the water moves from a high point to a low point, electrons can power a flashlight as they move from the anode to the cathode of a battery.
Figure 3. A battery can be thought of as a water tank, defined by the height of the tank (voltage), the size of the tap (current), and its capacity (number of electrons available to move through the circuit).
It is important to remember that a flashlight doesn't use up any electrons, just as the waterwheel doesn't use up water. In both cases, what is illuminating the flashlight or turning the water wheel is the energy carried by the electrons or the water.
Some basic units are important to understanding how batteries perform.
Current is the flow of electrons per unit time. In the case of a water wheel, we would measure current as the amount of water that flows through the wheel every second. In the case of a battery, the current is measured in Amps. An Amp is defined as 1 Coulomb (6.24E18 electrons) per second. On an atomic level, that means 6.24E18 electrons flow past every second.
The voltage measures how much force is pushing the electrons. In the case of a water wheel, the greater the fall of the water, the more power the water imparts to the wheel. Similarly, the greater the difference in the potential energy available between the anode and the cathode, the more power each electron can provide to the flashlight. In the case of the battery, volts is the measure of the energy available, or the electrical potential, between the anode and the cathode.
How much power is a battery providing? To answer that question, we need to know how many electrons it is providing every second (the current) and the force behind those electrons (the voltage). If we think of the battery as a water tank, we are asking how wide open is the tap (the current) and how high up is the water tank (the voltage)? A watt, which is a measure of power, is the product of both of these factors: current and the voltage.
To measure the maximum power a battery can deliver, we need to know how wide open the tap is and the force behind those electrons (the voltage). Different batteries can be discharged at different rates, depending upon their size, chemistry, and design.
What most people care about, however, is the storage capacity of the battery: How long can it power a flashlight, a cell phone, or a car? To answer this question, we need to know not just how much power a battery can generate at a particular moment, but how long it can sustain that level of power.
One common measurement of battery capacity is the Amp-hour. A 1 Amp-hour battery can deliver a current of 1 Amp for an hour. But the Amp-hour measurement does not account for the battery's voltage. Watt-hours are the product of the battery capacity and its voltage.
Watt-hours are a measure of the total energy a battery can store. (Watt-hours are also used to measure how much energy a light, an air conditioner, or a car consumes). How much power can a battery supply for one hour? To use the water tank metaphor, if we know the height of the water tank and how wide the tap is open, we want to know how long the battery can keep supplying electrons
A 10 watt-hour battery can supply 10 watts for one hour. Alternatively, it could supply 1 watt for 10 hours or 20 watts for half an hour. Although we won't get into the details, it is important to note that the capacity of a battery does depend upon the current it is delivering. Capacity will increase if the current is low and it will decrease if the current is increased. That is to say if a 10 watt-hour battery supplied 20 watts, it might not be able to do so for a full 30 minutes. Likewise, if the current draw is only 1 watt, it might actually last longer than 10 hours.
To summarize, there are two main types of capacity we care about: what is the maximum power a battery can deliver (which is important if you want to know how fast your electric car will accelerate) and what is the energy capacity a battery can deliver (which is important if you want to know how far you can drive that car on a single charge).
Let's compare three of the most common types of batteries to illustrate their respective qualities: alkaline-manganese, lead-acid, and lithium-ion batteries. These batteries have one major distinguishing feature: the alkaline-manganese battery is a primary battery, meaning it is a single-use battery that is disposed of or recycled after being depleted. The lead-acid and lithium-ion batteries are both secondary batteries, meaning that they can be depleted and recharged hundreds of times during their useful lives.
|Applications||Remote controls, flashlights, toys||Starting card, electric golf carts, fork lifts||Cell phones, electric cars, power back-up|
|Cathode||Manganese||Lead Dioxide||Lithium Cathode|
Battery performance is determined by the electrochemistry of the battery: the combination of materials that make up the cathode, anode, and electrolyte determine battery performance. If you want to know how wide you can open the tap, how high up the tank is located, and just how many electrons that tank can supply, the answers are all determined by the specific chemical properties of the anode, cathode, and other materials that make up a battery.
Lithium-ion batteries offer twice the voltage of lead-acid or alkaline-manganese batteries, meaning that fewer batteries are needed to provide the voltage necessary to run a power tool or an electric car. In many applications, individual battery cells are combined into more complex battery packs with higher voltages. For instance, the typical car battery is comprised of 6 lead-acid battery cells with a total of 12 volts. See more about this in the applications page.
Alkaline-manganese batteries excel at applications that require low currents, such as remote controls and fire alarms - they have a relatively small tap, to use the water tank analogy. Although lead-acid batteries designed for automobiles can generate high currents for very short periods of time (long enough to start your car), they can't match the ability of lithium-ion batteries to supply high currents over a sustained period of time.
When thinking about the capacity of a battery, one critical detail that is not captured by our comparisons to water tanks is important: electrons cannot simply be stored by themselves, like water in a tank. Instead, each stored electron comes with additional baggage, such as the atoms of lead, lithium, or zinc, that make up the batteries' anode. Since electrons are what we want, the less extra baggage they come with the better.
The amount of extra baggage differs greatly across battery chemistries. For instance, a lithium atom supplies three electrons, but it comes packaged with 3 protons and 4 neutrons, for a total atomic weight of 6.941. A lead atom supplies two electrons, but those electrons come packaged with 82 protons and 125 neutrons for a total atomic weight of 207.2.
Energy density measures the capacity of a battery per unit weight. In practice, after factoring in other parts of the battery, a lead-acid battery is approximately 4 times as heavy as a lithium-ion battery with the same capacity. Although an alkaline-manganese battery can almost match a rechargeable lithium-ion battery's energy density, it can only be used once.
Considering that lithium-ion batteries are rechargeable and can supply high voltage, high currents, and high energy density, why isn't everything powered by lithium-based batteries? There are a variety of reasons. For some applications, such as remote controls, inexpensive alkaline batteries with long shelf lives are preferable. Sometimes, the shelf life of alkaline batteries, which can retain a charge for years, is important. For other applications, such as starter batteries for automobiles, the weight of the battery is relatively unimportant relative to the size of the vehicle, and lead-acid batteries are a long-time standard, reliable, and less expensive. And for stationary applications, such as back-up power supplies for internet servers or cell phone towers, the lighter weight of lithium-ion batteries is not as important, and lead-acid batteries are frequently used in those applications.
But the primary disadvantage of lithium-ion based batteries is the cost. The upfront cost of a rechargeable lithium-ion battery is about six times that of a disposable alkaline-manganese battery or a lead-acid battery. As prices continue to fall, however, the market share of lithium-ion batteries is likely to continue to expand.
As a battery discharges, the battery changes chemically as the redox reactions take place in the anode and the cathode. As those reactions proceed, the voltage begins to decline. The rate at which the voltage fades depends upon the battery chemistry, how much current it is providing, and how continuous the drain is. Eventually, the voltage drops below the level needed to power the circuit, and the device turns off.
For instance, in the zinc-copper film canister battery we built above, the zinc anode is oxidized to become zinc oxide and the copper ions in the electrolyte are reduced as electrons flow from the anode to the cathode. As the amount of zinc and copper left to react declines, the voltage declines. Since the current draw is low, even our homemade battery has a long life: it powered a small LED light for over two weeks!
What makes a rechargeable battery special is that if you connect the battery to a power source, the chemical reactions are reversed. The anode becomes the cathode, the cathode the anode, and electrons shuttle backwards as the battery regains its capacity and voltage. In effect, charging serves as a pump to refill the water tank, pumping water from below the water wheel back into the tank, and restoring the battery's capacity.
Each time a battery is cycled, however, the anode and cathode deteriorate slightly. As a result, the capacity of rechargeable batteries degrades with each cycle (of use and then recharging). Eventually even rechargeable batteries will fail.