Environmental Engineering Reference
In-Depth Information
Fig. 5.14 Ragone plot for
different types of electric
energy storage systems
10000
Super capacitors and
Flywheels
1000
Pb/acid
Ni/cd
Li
Na
100
Zn/air
10
Al/air
1
1
10
100
1000
Specific energy [ Wh/kg ]
the amperhour efficiency could be very close to 100% for a state of charge ranging
from about 20 to 80%, but the efficiency might fall down greatly for values of the
charge under 20%, depending on the type of battery.
Energy efficiency is defined as the ratio of electrical energy supplied by a
battery to the electrical energy required to give it back to the state before dis-
charge. The energy efficiency is strongly dependent on how the battery is used, in
particular the energy efficiency can decrease considerably when the battery is
charged or discharged very rapidly.
5.3.1.5 Battery Lifetime
The number of cycles of a battery is a parameter representative of the battery
lifetime of the battery and it affects the vehicle maintenance costs. The number of
cycles of a battery represents the number times the battery can be completely
charged and then discharged, without compromising its properties to store energy.
Hundreds of cycles are guaranteed by many battery companies, although this
number depends on the battery type.
Many types of batteries loss a certain amount of the initial charge when they are
unused, and this phenomenon is called self-discharge. For that reason, some
batteries need to be periodically recharged, if they are left unused for long periods
of time. Self-discharge is related to kinetics of chemical reactions occurring inside
each type of battery, and then it is strongly dependent on temperature.
Several types of storage batteries have been studied and tested for many years
to be used on electric vehicles and other of them are presently under development.
The key characteristics of the most used batteries to supply electric cars and the
more promising ones are presented and analyzed below.
