Batteries come in a lot of different varieties, and many
years of work have been put into investigating different chemistries. The most
common are carbon-zinc, alkaline, lead acid, nickle metal hydride, nickle
cadmium and lithium ion. But there are many other battery chemistries, each
with their own advantages and disadvantages. Below we discuss different of the
battery designs currently used, some of the chemistry involved, and advantages
and disadvantages of each design. We have also included some useful definitions
and a list of parameters to guide you in matching your battery requirements to
a specific battery design.
Table of Contents
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- Anode: The electrode where oxidation (loss of
electrons) takes place. While discharging, it is the negative electrode; while
charging it becomes the positive electrode.
- Amps: Also known as Amperes. This is the
rate at which electrons flow in a wire. The units are coulombs per second, or
since an electron has a charge of 1.602 x 10-19 coulombs, an amp is
6.24 x 10+18 electrons per second. Think of marbles rolling through
a tube. If 6.24 x 10+18 pass by in 1 second you wuld have an amp of
- Amp hours: Also known as ampere hours.
This a measure of the amount of charge stored or used. For example if you had
an amp of marbles flowing out of your tube into a bucket for an hour, you would
have one amp-hour of marbles in the bucket ( 6.24 x 10+18 times 3600
seconds = 2.2 x 10+22 marbles. A 1 amp hour battery contains enough
charge to supply 1 amp for 1 hour, if you discharge at the same rate. Usually
if you discharge faster than the rate at which the the amp hours were specified
you will get fewer amp-hours out. You may notice that amp-hours and coulombs
measure the same quantity-charge. One amp-hour is 3600 coulombs, but amp hours
are easier to use in battery design. So remember, amps are flow ( "this motor
requires 2 amps to run at 1800 rpm.") Amp hours measure capacity, quantity, or
amount of charge ("this 100 amp-hour battery will supply 2 amps for 50 hours
before recharge." Amp-hours are amps times hours, not amps divided by
So Amp-Hours, (AH), or milliamp-Hours (mAH) is a measure
of the size of the battery a 10 mAH battery has half the capacity of a 20 mAH
battery, even though they may be in the same physical package.
- Batteries: Two or more electrochemical cells,
electrically interconnected, each of which contains two electrodes and an
electrolyte. The redox (oxidation-reduction) reactions that occur at these
electrodes convert electrochemical energy into electrical energy. In everyday
usage, 'battery' is also used to refer to a single cell.
- C:C represents the capacity of a battery divided
by 1 hour, its units are amps. It represents a 1 hour discharge rate using the
nominal capacity of the battery. So a discharge rate of 10C for a 5AH battery
would be 50 amps. The concept of "C" is also used for charge currents, since
both charge and discharge properties are proportional to the capacity of the
battery, so a 5C charge rate for a 5 AH battery would be 25 amps.
- Capacity: The total quantity of electricity or
total ampere-hours available from a fully charged cell or battery.
- Cathode: The electrode where reduction (gain of
electrons) takes place. When discharging, it is the positive electrode, when
charging, it becomes the negative electrode.
- Charge: The conversion of electrical energy,
provided in the form of current from an external source, into chemical energy
stored at the electrodes of a cell or battery.
- Discharge: The conversion of the chemical energy
of a cell into electrical energy, which can then be used to supply power to a
- Discharge curve: A plot of cell voltage over
time into the discharge, at a constant temperature and constant current
Each curve in this graph represents cell
performance at a different discharge rate. The farther right the curve ends,
the lower the discharge rate (Crompton 31.4).
- Dry cell: A
Leclanché cell, so called because of its non-fluid electrolyte (to
prevent spillage). This is achieved by adding an inert metal oxide so that the
electrolyte forms a gel or paste.
- Efficiency: For a secondary cell, the ratio of
the output on discharge to the input required to restore it to its initial
state of charge under specified conditions. Can be measured in ampere-hour,
voltage, and watt-hour efficiency.
- Electrolyte: The chemistry of a battery requires
a medium that provides the ion transport mechanism between the positive and
negative electrodes of a cell.
- Energy density (specific energy): These two
terms are often used interchangeably. Energy density refers mainly to
the ratio of a battery's available energy to its volume (watt hour/liter).
Specific energy refers to the ratio of energy to mass (watt hour/kg).
The energy is determined by the charge that can be stored and the cell voltage
- Fuel cell: A cell in which one or both of the
reactants are not permanently contained in the cell, but are continuously
supplied from a source external to the cell and the reaction products
continuously removed. Unlike the metal anodes typically used in batteries, the
fuels in a fuel cell are usually gas or liquid, with oxygen as the oxidant. The
hydrogen/oxygen fuel cell is the most common. In this fuel cell, hydrogen is
oxidized at the anode:
Hydrogen/oxygen fuel cell systems
work well in space travel applications because of their high efficiency, high
power-to-weight and volume ratios, and usable reaction product (water). They
can function for many months as long as fuel is supplied and therefore the
energy density cannot be measured.
||V vs SHE
|2H2 > 4H+ +
|4H+ + O2 +
4e- > 2H2O
- Half-reaction: Refers to the chemical processes
occurring at each electrode. The potential of the two half-reactions add to
give us the overall cell potential. We can see this in the zinc mercury cell,
||Zn + 2OH- > Zn(OH)2 +
||HgO +H2O + 2e- > Hg +
||Zn + HgO + H2O >
Zn(OH)2 + Hg
- Polarization: The voltage drop in a cell during
discharge due to the flow of an electrical current. The cell's internal
resistance increases with the buildup of a product of oxidation or a reduction
of an electrode, preventing further reaction.
- Power: Defined by voltage (V) and current (I),
Since V=IR, P=I2R and P=V2/R
Power also can be described by energy emitted per unit
of time: P=E/t.
- Power density (specific power): Power
density is the ratio of the power available from a battery to its volume
(watts/liter). Specific power generally refers to the ratio of power to
mass (watts/kg). Comparison of power to cell mass is more common.
- Primary cells: A cell that is not designed for
recharging and is discarded once it has produced all its electrical
- Prismatic: Just a word to say that the cells are
not cylindrical, as nature intended battery cells to be, but fit nicely into a
parallelepiped, rectangular or any other such flattened shape.
- Peukart Effect: When a battery is discharged
extremely quickly it will have less capacity than expected. This is the Peukart
effect, which is very strong for lead acid batteries and many primary cells,
but much less so for nickel cadmium, NiMH and lithium batteries. Peukert's
equation is In · t = C, where I is the discharge rate, t is
the discharge time and C is the capacity. The exponent "n" depends on the
battery chemistry and the temperature. A log-log plot of discharge time versus
discharge load will have a slope of "n"
- Reserve cell: A primary cell that may be kept
inactive and which is activated by adding an electrolyte or electrode, or
melting an electrolyte which is normally in a solid state.
- Secondary cells: A cell capable of repeated use.
Its charge may be fully restored by passing an electric current through the
cell in the opposite direction to that of discharge, thus reversing the redox
No one battery design is perfect for every application.
Choosing one requires compromise. That's why it's important to prioritize your
list of requirements. Decide which ones you absolutely must have and which you
can compromise on. Here are some of the parameters to consider:
- Voltage: Normal voltage during discharge,
maximum and minimum permissible voltages, discharge curve profile. Note that
secondary cells are characterized by their average voltage during a full
discharge, whereas primary cells ar characterized by their peak voltage. Thus
an alkaline cells is rated at 1.5V, though its average voltage during discharge
is 1.2V. A NiMH cell is rated at 1.2 volts, but its discharge range is 1.4 to 1
- Duty cycle: Conditions the battery experiences
during use. Type of discharge and current drain, e.g., continuous,
intermittent, continuous with pulses, etc. A 50% duty cycle means that the cell
is off for half the cycle, and on for the other half of the cycle.
- Temperature: In storage and in use. Temperatures
that are too high or too low can greatly reduce battery capacity.
- Shelf life: How rapidly the cell loses potential
- Service life: Defined either in calendar time
or, for secondary cells, possible number of discharge/charge cycles, depending
on the battery application. Service life depends on battery design and
operational conditions, i.e., the stress put on a battery. For stationary and
motive power application, the end of service life is defined as the point at
which a battery's capacity drops to 80% of its original capacity. Exceptions
would include car batteries where the service life ends when the capacity falls
- Physical restrictions: These include dimensions,
weight, terminals, etc.
- Maintenance and resupply: Ease of battery
acquisition, replacement, charging facilities, disposal.
- Safety and reliability: Failure rates, freedom
from outgassing or leakage; use of toxic components; operation under hazardous
conditions; environmentally safe
- Cost: Initial cost, operating cost, use of
- Internal resistance: Batteries capable of a
high-rate discharge must have a low internal resistance.
- Specific energy: As discussed in the definition
section, this is a measurement of possible stored energy per kilogram of mass.
This number is purely theoretical as it does not take into account the mass of
inactive materials, nor the variation in chemical reactions.
- Specific power: Also defined in the definitions
section, a P=E/t, so the specific power is discussed at a specific discharge
rate. It is possible for batteries with a high specific energy to have a low
power density if they experience large voltage drops at high discharge rates.
Specific power and specific energy can be compared in a Ragone plot .
- Unusual requirements: Very long-term or
extreme-temperature storage; very low failure rate; no voltage delay, etc.
Of course the ideal battery would perform well in all
these areas with a long shelf and service life, high specific energy and
specific power, low initial and maintenance costs, low environmental impact,
and good performance in a variety of conditions (temperatures, duty cycles,
etc.). When you find one that meets all these requirements, let us know! In the
meantime, we have to make do with batteries that work very well in specific
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York:: John Wiley & Sons, 1997.
- Crompton, T. R. Battery Reference Book. London:
- Linden, D. (Ed), Handbook of Batteries.
Maidenhead: McGrawHill, 1995.
- Linford, R. G. (Ed), Electrochemical Science and
Technology of Polymers. New York: Elsevier, 1990.
- Ovshinsky, S. R., Fetcenko, M. A., and Ross, J. A. "A
Nickel Metal Hydride Battery for Electric Vehicles", Science 260: 1993,
- Rechargeable Batteries Applications Handbook.
Stoneham: ButterworthHeinemann, 1992.
- Wells, A. F. Structural Inorganic Chemistry.
Oxford: Clarendon Press, 1975.