Environmental Engineering Reference
In-Depth Information
for example a battery with a capacity C = 52 Ah, can be discharged at a current of
2C or 0.4C, to mean that the discharging current is 104 or 20.8 A, respectively.
5.3.1.3 Battery Energy and Power
The SI unit for energy is the Joule, but for batteries the Watthour (Wh) is preferred
to Joule, which is a very small unit. 1 Wh is equal to 3600 J. The energy stored in
a battery (E
batt
) depends directly on the charge (C) and voltage at its terminals
(V
batt
), as expressed by the following relation:
E
batt
ð
Wh
Þ¼
V
batt
ð
V)
C
ð
Ah)
:
ð
5
:
3
Þ
However, as both battery voltage and capacity depend on the battery charge and
discharge rate, the evaluation of the energy stored in a battery could reduce sen-
sibly if the electric energy is released or charged quickly, with high discharge or
charge current.
Specific energy is the electric energy stored for one kilogram of battery mass
and its unit is Wh/kg. Since the energy stored in a battery depends on many factors
such as temperature and discharge rate, the specific energy can be only approxi-
mately evaluated for the battery mass.
Specific power is the electric power obtainable per kilogram of battery and its
unit is W/Kg. The evaluation of this variable has to take in consideration that the
real power given out by a battery is highly dependent on the load connected to its
terminal. Moreover, it is convenient to operate the battery at its maximum power
only for a short time of few seconds, otherwise the battery life might be seriously
compromised and the battery would operate in a very inefficiently way.
Generally batteries with high specific energy have a low specific power. In
other words many types of batteries can store a large amount of energy, but they
can only give it out at low rate. That means to drive the vehicle over a long
distance at a low speed.
To compare different types of batteries, in terms of specific power and specific
energy, it is helpful using the Ragone plot, which is a graph of specific power
versus specific energy. Figure
5.14
shown a Ragone plot for the main typologies of
battery currently under development. Ragone plots are also used to make a
comparison between batteries and other technologies of energy storage, such as the
super capacitors and flywheels [
1
,
3
,
5
].
5.3.1.4 Amperhour and Energy Efficiency
The amperhour efficiency is defined as the ratio of the amperhours supplied by a
battery to the amperhours required to give it back to the state before discharge.
The amperhour efficiency is generally less than 100% and the real value depends
on the type of battery, temperature, rate of charge and state of charge. In particular,
