Page 39 - Lindens Handbook of Batteries

1.14 PRINCIPLES OF OPERATION

1.4.4 Theoretical Energy*
The capacity of a cell can also be considered on an energy (watthour) basis by taking both the volt-
age and the quantity of electricity into consideration. This theoretical energy value is the maximum
value that can be delivered by a specific electrochemical system:
Watthour (Wh) = voltage (V) × ampere-hour (Ah)
In the Zn/Cl cell example, if the standard potential is taken as 2.12 V, the theoretical watthour capac-
2
ity per gram of active material (theoretical gravimetric specific energy or theoretical gravimetric
energy density) is
Specific energy (watthours/gram) = 2.12 V × 0.394 Ah/g = 0.835 Wh/g or 835 Wh/kg

Table 1.2 also lists the theoretical specific energy of various electrochemical systems.

1.5 SPECIFIC ENERGY AND ENERGY DENSITY OF
PRACTICAL BATTERIES

The theoretical electrical properties of cells and batteries are discussed in Sec. 1.4. In summary,
the maximum energy that can be delivered by an electrochemical system is based on the types of
active materials that are used (this determines the voltage) and on the amount of the active materials
that are used (this determines ampere-hour capacity). In practice, only a fraction of the theoretical
energy of the battery is realized. This is due to the need for electrolyte and nonreactive components
(containers, separators, electrodes) that add to the weight and volume of the battery, as illustrated in
Fig. 1.3. Another contributing factor is that the battery does not discharge at the theoretical voltage
(thus lowering the average voltage), nor is it discharged completely to zero volts (thus reducing the
delivered ampere-hours) (also see Sec. 3.2.1). Further, the active materials in a practical battery are
usually not stoichiometrically balanced. This reduces the specific energy because an excess amount
of one of the active materials is used.

FIGURE 1.3 Components of a cell.

*The energy output of a cell or battery is often expressed as a ratio of its weight or size. The preferred terminology for this
ratio on a weight basis, e.g., watthours/kilogram (Wh/kg), is “specific energy;” on a volume basis, e.g., watthours/liter (Wh/L),
it is “energy density.” Commonly, the term “energy density” may be used to refer to either ratio.