Batteries

In science and technology, a battery is a device that stores chemical energy and makes it available in an electrical form. Batteries consist of electrochemical devices such as one or more galvanic cells, fuel cells or flow cells. The earliest known artifacts that may have been batteries are the Baghdad Batteries, from some time between 250 BCE and 640 CE. The modern development of batteries started with the Voltaic pile developed by the Italian physicist Alessandro Volta in 1800. The worldwide battery industry generates US$48 billion in sales annually

Cell vs. battery

Strictly, an electrical "battery" is an interconnected array of one or more similar voltaic cells ("cells"). That distinction, however, is considered pedantic in most contexts (other than the expression dry cell), and in current English usage 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 -- here, six 2 V lead-acid cells -- in series. Multiple batteries or cells may also be referred to as a battery pack, such as a set of multi-cell 12 V batteries in an electric vehicle.

Electrical component
 
Circuit symbol for a battery; simplified electrical model; and more complex but still incomplete model (the series capacitor has an extremely large value and, as it charges, simulates the discharge of the battery).The cells in a battery can be connected in parallel, series, or in both. A parallel combination of cells has the same voltage as a single cell, but can supply a higher current (the sum of the currents from all the cells). A series combination has the same current rating as a single cell but its voltage is the sum of the voltages of all the cells. Most practical electrochemical batteries, such as 9 volt flashlight (torch) batteries and 12 V automobile (car) batteries, have a series structure. Parallel arrangements suffer from the problem that, if one cell discharges faster than its neighbour, current will flow from the full cell to the empty cell, wasting power and possibly causing overheating. Even worse, if one cell becomes short-circuited due to an internal fault, its neighbour will be forced to discharge its maximum current into the faulty cell, leading to overheating and possibly explosion. Cells in parallel are therefore usually fitted with an electronic circuit to protect them against these problems. In both series and parallel types, the energy stored in the battery is equal to the sum of the energies stored in all the cells.

A battery can be simply modelled as a perfect voltage source (i.e. one with zero internal resistance) in series with a resistor. The voltage source depends mainly on the chemistry of the battery, not on whether it is empty or full. When a battery runs down, its internal resistance increases. When the battery is connected to a load (e.g. a light bulb), which has its own resistance, the resulting voltage across the load depends on the ratio of the battery's internal resistance to the resistance of the load. When the battery is fresh, its internal resistance is low, so the voltage across the load is almost equal to that of the battery's internal voltage source. As the battery runs down and its internal resistance increases, the voltage drop across its internal resistance increases, so the voltage at its terminals decreases, and the battery's ability to deliver power to the load decreases.

Battery concepts

Here is some heavy-duty information about voltaic cells, the building blocks of batteries. In the figure (above, on the left) the battery consists of two voltaic cells in series. The positive (negative) terminals (electrodes) are the longer (shorter) lines. Real voltaic cells have ion-carrying electrolyte, made of solid or liquid, separating their terminals. Thus their terminals are not in direct electrical contact. The figure above shows no line connecting the negative terminal of the top cell to the positive terminal of the bottom cell, but in a real cell they would be in direct electrical contact.

The electrolyte contains ions that can react with chemicals in the electrode. Chemical energy is converted into electrical energy by chemical reactions that transfer charge between the electrode and the electrolyte at their interface. Such reactions are called Faradaic, and are responsible for current flow through the cell. Ordinary, non-charge-transferring (non-Faradaic) reactions also occur at the electrode-electrolyte interfaces. Non-faradaic reactions are one reason that voltaic cells (particularly the lead-acid cell of ordinary car batteries) "run down" when sitting unused.

Around 1800 Volta studied, for many different types of voltaic cell, the effect of different electrodes on the net electromotive force (emf) of the cell, Ε. (Emf is equivalent to what was called the internal voltage source in the previous section.) He showed that Ε is the difference of the emfs Ε1 and Ε2 associated with the two electrolyte-electrode interfaces. Hence identical electrodes yield Ε=0 (zero emf). Volta did not appreciate that the emf was due to chemical reactions. He thought that his cells were an inexhaustible source of energy, and that the associated chemical effects (e.g., corrosion) were a mere nuisance -- rather than, as Faraday showed around 1830, an unavoidable by-product of their operation.

Electromotive force (emf) is measured in units of volts; therefore the word "force" is a misnomer. Voltaic cells, and batteries of voltaic cells, are normally rated in terms of volts. The voltage across the terminals of a battery is known as the terminal voltage. The terminal voltage of a battery that is not discharging equals its emf. The terminal voltage of a battery that is discharging (charging) is less than (greater than) the emf.

Most voltaic cells are only rated at 1.5 or so volts because of the limitations to how much electrical energy the chemical reactions can provide. Because of the relatively large energy release of Li compounds, Li cells can provide as many as 3 or more volts. This large energy release can be a hazard.

The conventional model for a voltaic cell, as drawn above, has the internal resistance drawn outside the cell. This is a correct Thevenin equivalent for circuit applications, but it oversimplifies the chemistry and physics. In a more accurate (and more complex) model, a voltaic cell can be thought of as two electrical pumps, one at each terminal (the faradaic reactions at the corresponding electrode-electrolyte interfaces), separated by an internal resistance largely due to the electrolyte. Even this is an oversimplification, since it cannot explain why the behavior of a voltaic cell depends strongly on its rate of discharge. For example, it is well-known that a cell that is discharged rapidly (but incompletely) will recover spontaneously after a waiting time, but a cell that is discharged slowly (but completely) will not recover spontaneously.

The simplest characterization of a battery would give its emf (voltage), its internal resistance, and its "charge", or capacity. In principle, the energy stored by a battery equals the product of its emf and its capacity.

Battery capacity

The capacity of a battery to store charge is often expressed in ampere hours (1 A·h = 3600 coulombs). If a battery can provide one ampere (1 A) of current (flow) for one hour, it has a real-world capacity of 1 A·h. If it can provide 1 A for 100 hours, its capacity is 100 A·h. The more electrolyte and electrode material in the cell, the greater the capacity of the cell. Thus a tiny AAA cell has much less capacity than a much larger D cell, even if both rely on the same chemical reactions (e.g. alkaline cells), which produce the same terminal voltage. Because of the chemical reactions within the cells, the capacity of a battery depends on the discharge conditions such as the magnitude of the current, the duration of the current, the allowable terminal voltage of the battery, temperature, and other factors.

Battery manufacturers use a standard method to determine how to rate their batteries. The battery is discharged at a constant rate of current over a fixed period of time, such as 10 hours or 20 hours, down to a set terminal voltage per cell. So a 100 ampere-hour battery is rated to provide 5 A for 20 hours at room temperature. The efficiency of a battery is different at different discharge rates. When discharging at low rate, the battery's energy is delivered more efficiently than at higher discharge rates. This is Peukert's Law.

Battery lifetime

Disposable alkaline batteries are designed to be used only once. Even if never taken out of the original package, disposable (or "primary") batteries can lose two to twenty-five percent of their original charge every year. This rate depending significantly on temperature, since typically chemical reactions proceed more rapidly as the temperature is raised. This is known as the "self discharge" rate and is due to non-faradaic (non-current-producing) chemical reactions, which occur within the cell even if no load is applied to it.

Until relatively recently, storing batteries at cool temperatures (such as in the refrigerator) could significantly reduce the rate of these side (non-faradaic) reactions and thus extend the storage life of the battery. However, these side reactions have now been reduced to a level where modern batteries need only be stored in a dry place and at normal room temperatures. Some brands of batteries (like Duracell or Energizer) will provide dependable long life even after 5 years of storage under these conditions.

Extreme temperatures also reduce battery performance.

Some information on caring and disposing of alkaline batteries can be found here and here.

Rechargeable batteries self-discharge more rapidly than disposable alkaline batteries. In fact, they can self-discharge up to three percent a day (again, depending on temperature). Due to their poor shelf life, they shouldn't be left in a drawer and then relied upon to power a flashlight or a small radio in an emergency. For this reason, it’s a good idea to keep a few alkaline batteries on hand. Ni-Cd Batteries are almost always "dead" when you get them, and must be charged before first use.

Most Ni-MH batteries can be recharged 500-1000 times whereas Ni-Cd batteries can only be recharged about 400 times.

Special "reserve" batteries intended for long storage in emergency equipment or munitions keep the electrolyte of the battery separate from the plates until the battery is activated, allowing the cells to be filled with the electrolyte. Shelf times for such batteries can be years or decades. However, their construction is more expensive than more common forms.

Rechargeable Batteries

Rechargeable batteries are batteries that can be restored to full charge by the application of electrical energy. They come in many different designs using different chemistry. They are also called storage battery or secondary cell. Attempting to recharge non-rechargeable batteries may lead to a battery explosion. Some types of rechargeable batteries are susceptible to damage due to reverse charging if they are fully discharged; other types need to be fully discharged occasionally in order to maintain the capacity for deep discharge. There exist fully integrated battery chargers that optimize the charging current.

Energy to weight ratios

In the order of improving energy per weight ratios there are:

Gel battery
Lead-acid battery
Nickel-cadmium battery
Nickel metal hydride battery
Lithium ion battery
Lithium polymer battery

Recharging
 
Battery chargerThe energy used to recharge rechargeable batteries mostly comes from mains electricity using an adapter unit. Recharging from solar panels is also attractive. Recharging from the 12V battery of a car is also possible. Use of a hand generator is also possible, but it is not clear if such devices are commercially made.

For uses like radios and torches, rechargeable batteries may be replaced by clockwork mechanisms or dynamos.