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. 1 | Page 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. 2 | Page 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. 3 | Page 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. 4 | Page 5 | Page