What is battery?
By definition, a battery consists of a combination of two or more
similar cells, a cell being the fundamental source of electrical energy
developed through the conversion of chemical or solar energy.
All cells can be divided into the primary or secondary types. The secondary is
rechargeable, whereas the primary is not. That is, the chemical reaction
of the secondary cell can be reversed to restore its capacity.
The two most common rechargeable batteries are the lead-acid unit (used primarily
in automobiles) and the nickel-metal hydride (NiMH) battery (used
in calculators, tools, photoflash units, shavers, and so on). The obvious
advantages of rechargeable units are the savings in time and money of
not continually replacing discharged primary cells. The battery is the most common of the dc sources.
Primary Cells (Nonrechargeable)
The popular alkaline primary battery uses a powdered zinc anode (+); a potassium (alkali metal)
hydroxide electrolyte; and a manganese dioxide/carbon cathode (-) as shown in Fig.No.1. Note that for the cylindrical types
(AAA, AA, C, and D), the voltage is the same for each, but the amperehour
(Ah) rating increases significantly with size. The ampere-hour rating
is an indication of the level of current that the battery can provide for
a specified period of time. Another type of popular primary cell is the lithium battery, shown in
Fig.no.1: Alkaline primary cells.
Fig.no.2: Lithium primary batteries.
Secondary Cells (Rechargeable)
: The 12 V lead-acid battery as shown in Fig.No.3 typically used in automobiles,
has an electrolyte of sulfuric acid and electrodes of spongy lead (Pb)
and lead peroxide (PbO2). When a load is applied to the battery terminals,
there is a transfer of electrons from the spongy lead electrode to
the lead peroxide electrode through the load.
Fig.no.3: Lead acid battery.
Fig.No.4: Lead acid battery six cells in series.
This transfer of electrons will continue until the battery is completely discharged. The discharge time is determined by how diluted the acid has become and how heavy
the coating of lead sulfate is on each plate. The state of discharge of a
lead storage cell can be determined by measuring the specific gravity
of the electrolyte with a hydrometer. Since the lead storage cell is a secondary cell, it can be recharged at
any point during the discharge phase simply by applying an external dc
current source across the cell that passes current through the cell in a
direction opposite to that in which the cell supplied current to the load.
This removes the lead sulfate from the plates and restores the concentration
of sulfuric acid.
The output of a lead storage cell over most of the discharge phase is
about 2.1 V. In the commercial lead storage batteries used in automobiles,
12.6 V can be produced by six cells in series, as shown in Fig.No.4. In general, lead-acid storage batteries are used in situations where
a high current is required for relatively short periods of time.
Nickel-Metal Hydride (NiMH):
The nickel-metal hydride
rechargeable battery has been receiving enormous interest and development
in recent years. The Toyota Prius and two other hybrids would use
NiMH batteries rather than the lead-acid variety. For applications such
as flashlights, shavers, portable televisions, power drills, and so on,
rechargeable batteries such as the nickel-metal hydride (NiMH) batteries
shown in Fig.No.5 are often the secondary batteries of choice. These
batteries are so well made that they can survive over 1000 charge/discharge
cycles over a period of time and can last for years.
Fig.No.5: nickel-metal hydride rechargeable battery.
Shown in Fig.No.6 these batteries carries more
energy in a smaller space than either the lead-acid or NiMH rechargeable
batteries. Its range of applications includes computers, a host of consumer
products, power tools, and recently the sleek Tesla roadster as shown in Fig.No.7 with
its battery pack composed of more than 6800 3.7 V Li-ion cells the size
of a typical AA battery. It can travel some 265 miles but the battery pack
costs between 10,000 and 15,000 dollars. Another problem is shelf life. Once
manufactured, these batteries begin to slowly die even though they may
go through normal charge/discharge cycles, which makes them similar
to a normal primary cell, so lifetime is a major concern.
Fig.No.6: Lithium-ion (Li-ion) battery.
Fig.No.7: Sleek Tesla roadster.