Chapter 1
Electric vehicles Batteries
1 .1 Ov erv iew
A basic requirement for electric vehicles (EVs) is a portable source of
e l e c t r i c a l e n e r g y, w h i c h i s c o n v e r t e d t o m e c h a n i c a l e n e r g y i n t h e e l e c t r i c
m o t o r f o r v e h i c l e p r o p u l s i o n . E l e c t r i c a l e n e r g y i s t yp i c a l l y o b t a i n e d
through conversion of chemical energy stored in devices such as
b a t t e r i e s a n d f u e l c e l l s . A f l yw h e e l i s a n a l t e r n a t i v e p o r t a b l e s o u r c e i n
which energy is stored in mechanical form to be converted into electrical
energy on demand for vehicle propulsion. The portable electrical energy
source presents the biggest obstacle in commercializa tion of EVs. A
near-term solution for minimizing the environmental pollution problem
due to the absence of a suitable, high -energy-density energy source for
E V s i s p e r c e i v e d i n t h e h yb r i d e l e c t r i c v e h i c l e s ( H E V s ) t h a t c o m b i n e
propulsion efforts from gasoli ne engines and electric motors.
Among the available choices of portable energy sources, batteries
have been the most popular choice of energy source for EVs since the
beginning of research and development programs in these vehicles. The
EVs and HEVs commercially available today use batteries as the
electrical energy source. The various batteries are usually compared in
t e r m s o f d e s c r i p t o r s , s u c h a s s p e c i f i c e n e r g y, s p e c i f i c p o w e r , o p e r a t i n g
life, etc. Similar to specific energy, specific power is the p ower
available per unit mass from the source. The operating life of a battery
i s t h e n u m b e r o f d e e p d i s c h a r g e c yc l e s o b t a i n a b l e i n i t s l i f e t i m e o r t h e
number of service years expected in a certain application. The desirable
features of batteries for EV and HEV applications are high specific
p o w e r , h i g h s p e c i f i c e n e r g y, h i g h c h a r g e a c c e p t a n c e r a t e f o r r e c h a r g i n g
a n d r e g e n e r a t i v e b r a k i n g , a n d l o n g c a l e n d a r a n d c yc l e l i f e . A d d i t i o n a l
technical issues include methods and designs to balance the battery
s e g m e n t s o r p a c k s e l e c t r i c a l l y a n d t h e r m a l l y, a c c u r a t e t e c h n i q u e s t o
d e t e r m i n e a b a t t e r y ’ s s t a t e o f c h a r g e , a n d r e c yc l i n g f a c i l i t i e s o f b a t t e r y
components. And above all, the cost of batteries must be reasonable for
EVs and HEVs to be commercially viable.
Battery technology has been undergoing extensive research and
d e v e l o p m e n t e f f o r t s o v e r t h e p a s t 3 0 ye a r s , ye t t h e r e i s c u r r e n t l y n o
b a t t e r y t h a t c a n d e l i v e r a n a c c e p t a b l e c o m b i n a t i o n o f p o w e r , e n e r g y, a n d
l i f e c yc l e f o r h i g h - v o l u m e p r o d u c t i o n v e h i c l e s . T h e s m a l l n u m b e r o f E V s
and HEVs that were introduced in the market used batteries that were too
expensive and have short calendar life, making the batteries the biggest
impediment in commercializing EVs and HEVs.
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Chapter 1
Electric vehicles Batteries
1 .2 Ba ttery Basics
The batteries are made of unit cells containing the chemical energy that
i s c o n v e r t i b l e t o e l e c t r i c a l e n e r g y. O n e o r m o r e o f t h e s e e l e c t r o l yt i c c e l l s
a r e c o n n e c t e d i n s e r i e s t o f o r m o n e b a t t e r y. T h e g r o u p e d c e l l s a r e
enclosed in a casing to form a battery module. A battery pack is a
collection of these individual Battery modules connected in a series and
parallel combination to deliver the desired volta ge and energy to the
p o w e r e l e c t r o n i c d r i v e s ys t e m .
The energy stored in a battery is the difference in free energy between
chemical components in the charged and discharged states. This
available chemical energy in a cell is converted into electrica l energy
only on demand, using the basic components of a unit cell, which are the
p o s i t i v e a n d n e g a t i v e e l e c t r o d e s , t h e s e p a r a t o r s , a n d t h e e l e c t r o l yt e s .
The electrochemically active ingredient of the positive or negative
electrode is called the active mat erial. Chemical oxidation and reduction
processes take place at the two electrodes, thereby bonding and releasing
e l e c t r o n s , r e s p e c t i v e l y. T h e e l e c t r o d e s m u s t b e e l e c t r o n i c a l l y c o n d u c t i n g
and are located at different sites, separated by a separator, as sho wn in
Figure3.1. During battery operation, chemical reactions at each of the
electrodes cause electrons to flow from one electrode to another;
however, the flow of electrons in the cell is sustainable only if electrons
generated in the chemical reaction ar e able to flow through an external
electrical circuit that connects the two electrodes. The connection points
between the electrodes and the external circuit are called the battery
terminals. The external circuit ensures that most of the stored chemical
e n e r g y i s r e l e a s e d o n l y o n d e m a n d a n d i s u t i l i z e d a s e l e c t r i c a l e n e r g y. I t
must be mentioned that only in an ideal battery does current flow only
w h e n t h e c i r c u i t b e t w e e n t h e e l e c t r o d e s i s c o m p l e t e d e x t e r n a l l y.
U n f o r t u n a t e l y, m a n y b a t t e r i e s d o a l l o w a s l o w d i s c h a r g e , d u e t o
diffusion effects, which is why they are not particularly good for long term energy storage. This slow discharge with open -circuit terminals is
known as self-discharge, which is also used as a descriptor of battery
q u a l i t y.
FIGURE 3.1 Components of a battery cell. (a) Cell circuit symbol; (b) cell
cross-section.
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Chapter 1
Electric vehicles Batteries
The components of the battery cell are described as follows:
1. Positive electrode: The positive electrode is an oxide or sulfide
or some other compound that is capable of being reduced during
cell discharge. This electrode consumes electrons from the external
circuit during cell discharge. E xamples of positive electrodes are
l e a d o x i d e ( P b O 2 ) a n d n i c k e l o x y h yd r o x i d e ( N i O O H ) . T h e
electrode materials are in the solid state.
2. Negative electrode: The negative electrode is a metal or an alloy
that is capable of being oxidized during cell discharge . This
electrode generates electrons in the external circuit during cell
discharge. Examples of negative electrodes are lead (Pb) and
cadmium (Cd). Negative electrode materials are also in the solid
state within the battery cell.
3 . E l e c t r o l y t e : T h e e l e c t r o l yt e i s t h e m e d i u m t h a t p e r m i t s i o n i c
conduction between positive and negative electrodes of a cell. The
e l e c t r o l yt e m u s t h a v e h i g h a n d s e l e c t i v e c o n d u c t i v i t y f o r t h e i o n s
that take part in electrode reactions, but it must be a nonconductor
for electrons in order to avoid self -discharge of batteries. The
e l e c t r o l yt e m a y b e l i q u i d , g e l , o r s o l i d m a t e r i a l . A l s o , t h e
e l e c t r o l yt e c a n b e a c i d i c o r a l k a l i n e , d e p e n d i n g o n t h e t yp e o f
b a t t e r y. T r a d i t i o n a l b a t t e r i e s s u c h a s l e a d - a c i d a n d n i c k e l - c a d m i u m
use liquid electrolytes. In lead -acid batteries, the electrolyte is the
aqueous solution of sulfuric acid [H 2SO4(aq)]. Advanced batteries
currently under development for EVs, such as sealed lead -acid,
n i c k e l - m e t a l - h yd r i d e ( N i M H ) , a n d l i t h i u m - i o n b a t t e r i e s u s e a n
e l e c t r o l yt e t h a t i s g e l , p a s t e , o r r e s i n . L i t h i u m - p o l ym e r b a t t e r i e s
use a solid electrolyte.
4 . S e p a r a t o r : T h e s e p a r a t o r i s t h e e l e c t r i c a l l y i n s u l a t i n g l a ye r o f
m a t e r i a l t h a t p h ys i c a l l y s e p a r a t e s e l e c t r o d e s o f o p p o s i t e p o l a r i t y.
S e p a r a t o r s m u s t b e p e r m e a b l e t o t h e i o n s o f t h e e l e c t r o l yt e a n d
may also have the function of storing or immobilizing the
e l e c t r o l yt e . P r e s e n t d a y s e p a r a t o r s a r e m a d e f r o m s yn t h e t i c
p o l ym e r s .
T h e r e a r e t w o b a s i c t yp e s o f b a t t e r i e s : p r i m a r y b a t t e r i e s a n d s e c o n d a r y
batteries. Batteries that cannot be recharged and are designed for a single
discharge are known as primary batteries. Examples of these are the
lithium batteries used in watches, calculators, cameras, etc., and the
m a n g a n e s e d i o x i d e b a t t e r i e s u s e d t o p o w e r t o ys , r a d i o s , t o r c h e s , e t c .
Batteries that can be recharged by flowing current in the direction
opposite to that during discharge are known as secondary batteries. The
chemical reaction process during cell charge operation when electrical
energy is converted into chemi cal energy is the reverse of that during
discharge.
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Chapter 1
Electric vehicles Batteries
The batteries needed and used for EVs and HEVs are all secondary
b a t t e r i e s , b e c a u s e t h e y a r e r e c h a r g e d d u r i n g r e g e n e r a t i o n c yc l e s o f
v e h i c l e o p e r a t i o n o r d u r i n g t h e b a t t e r y r e c h a r g i n g c yc l e i n t h e s t o p p e d
condition using a charger. All the batteries that will be discussed in the
following are examples of secondary batteries.
T h e m a j o r t yp e s
applications are:
•
•
•
•
•
•
•
of
rechargeable
batteries
considered
for
EV s
Lead-acid (Pb-acid)
Nickel-cadmium (NiCd)
N i c k e l - m e t a l - h yd r i d e ( N i M H )
Lithium-ion (Li -ion)
L i t h i u m - p o l ym e r ( L i - p o l y)
Sodium-sulfur (NaS)
Zinc-air (Zn-Air)
1 .3 Lea d -Acid Ba ttery
Lead-acid batteries have been the most popular choice of batteries for
EVs. Lead-acid batteries can be designed to be high powered and are
inexpensive, safe, and reliable. A recycling infrastructure is in place
f o r t h e m . H o w e v e r , l o w s p e c i f i c e n e r g y, p o o r c o l d t e m p e r a t u r e
performance, and short calendar and cycle life are among the obstacles
to their use in EVs
The lead-acid battery has a history that dates to the middle of the 19th
c e n t u r y, a n d i t i s c u r r e n t l y a m a t u r e t e c h n o l o g y. T h e f i r s t l e a d - a c i d
battery was produced as early as in 1859. In the early 1980s, over
100,000,000 lead -acid batteries were produced per year. The long
existence of the lead -acid battery is due to the following:
• Relatively low cost
• Easy availability of raw materials (lead, sulfur)
• Ease of manufacture
• Favorable electromechanical characteristics
The battery cell operation consists of a cell discharge operation, when
the energy is supplied from the battery to the electric motor to develop
propulsion power, and a cell charge operation, when energy is supplied
f r o m a n e x t e r n a l s o u r c e t o s t o r e e n e r g y i n t h e b a t t e r y.
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