Direct Energy, 2018a

216 9.4 Measures of Batteries and Fuel Cells
other directly. A fuel cell can continue to operate as long as the fuel and
oxidizer are added and the oxidation products are removed.
9.4 Measures of Batteries and Fuel Cells
9.4.1 Cell Voltage, Specic Energy, and Related Measures
Just as chemists have multiple measures of the ability of charges to ow,
they have multiple measures of energy or charge stored in a device. In this
section, the following measures of batteries and fuel cells are dened:
• Cell voltage in volts
• Specic energy in J g
or W·h
kg
• Energy density in m J or W·h
3 L
• Capacity in mA·h orC
• Specic capacity in mA·h g
• Charge density in mA·h
L
or C kg
or C L
Denitions throughout this section follow references [128, ch. 1] and [140].
If these measures are calculated using knowledge of chemical reactions
and quantities found in the periodic table, they are called theoretical values.
If these quantities are experimentally measured, they are called practical
values. Practical values are necessarily less because no energy conversion
device is ever completely ecient. Measures preceded by the word specic
are given per unit mass. Measures followed by the word density are give
per unit volume. For example, specic energy is measured in the SI units
of joules per gram and energy density is measured in the SI units of joules
per meter cubed. However, these rules are not closely followed, so the term
energy density is sometimes used to mean energy per unit weight instead
of per unit volume. It is safest to explicitly specify the units of measure to
avoid this confusion.
Theoretical cell voltage, V cell measured in volts, is the voltage between
the anode and the cathode in a battery or fuel cell. It is the sum of the
redox potential for the half reaction at the anode and the redox potential
for the half reaction at the cathode. It represents the voltage between the
terminals of a completely charged battery or fuel cell. Many authors call
this measure theoretical cell potential instead of cell voltage, and symbols