2.3.1 the reverse for charging. Figure 2.B
1 Batteries Perhaps the oldest and mostrecognized method of storing electrical energy is the battery. Though othertechnologies have emerged, batteries, especially secondary or rechargeablecells, are still one of the best options available because of their energydensity. Manybattery types are available with varying chemistries for each major category;the most common varieties are lead acid (Pb), nickel cadmium (NiCad), nickelmetal hydride (Nimh) and lithium ion (Li+) (Buchmann 2003). The basic mode ofoperation is the same in each case, an anode and a cathode are separated by anelectrolyte, which may be a liquid as in Pb or NiCad, or a gel as in Nimh orLi+. When discharging, positive ions migrate from the anode through theelectrolyte to the cathode, and the reverse for charging. Figure 2.
B shows abasic schematic for a Li+ battery. Figure 2.B -Li-ion battery schematic Batterycapacity (C) is measured in amp-hours (Ah), and the total amount of energystored in the battery is roughly equal to the capacity multiplied by theaverage voltage during discharge: (2-vii) Abattery’s state of charge (SoC) is a measure of the energy available from thebattery. Batteries are typically designed to operate within a specific windowof SoC, known as the SoC swing. The SoC with the lowest remaining energy in thebattery is known as the depth of discharge, or DoD.
Maximumcurrent output from a battery scales linearly with capacity, and is thereforemeasured in terms of capacity, using a parameter called C. A discharge rateof 1 C indicates the battery will be depleted in one hour, while a rate of 2C will drain the battery in half of an hour. Current is limited by the ratesfor chemical reactions within the cell and by the generation of heat. It iscommon for batteries to have a maximum continuous rate of discharge, with alarger peak output that may be sustained for a brief period. Kokam Co. Ltd.supplies several types of Li+ batteries with a peak output of twice theircontinuous rate, and can sustain this output for approximately 10 s (KokamCo.
Ltd. 2010). Batterieshave an internal resistance which accounts for some energy loss from the cellwhile charging or discharging.
Partly due to internal resistance, batterieshave smaller apparent capacities when discharging at high rates. This is knownas the Peukert effect (Buchmann 2003). Batterypacks consist of multiple cells arranged in series and/or parallel.
A set ofbatteries connected in series is called a string, the length of which is thestack height. The product of the stack height and number of strings gives thetotal number of cells in a battery pack. For instance, a battery pack with twosets of three batteries connected in series has 2 strings, a stack height of 3and six batteries in total. Batterieshave limited service life, the length of which depends on cell chemistry, DoD,SoC swing and temperature, among other factors. With time and use, batterycapacity attenuates and internal resistance grows.
For most batteries, thisprocess is accelerated with higher temperatures during storage and use, deeperdischarge cycles, and high drain rates. Ideal usage conditions for a batteryare moderate temperature, SoC swing and DoD, low and stable current demand withfew current reversals, or microcycles. These conditions will extend the servicelife of the battery and yield better energy capacity per charge. Eachcell chemistry has unique characteristics that make it suitable or not for agiven application. Pb batteries are simple, cheap and robust. The electrolyte,water, is readily available and so the battery can be conveniently ‘topped up’if necessary.
This makes them a favourite choice for use in the electricalsystems of combustion engines. Additionally, Pb batteries can be serviced bycareful charging and addition of electrolyte to restore some of their originalcapacity. Pb batteries are not ideal for electric vehicle applications becausethey are large, heavy, and do not tolerate deep discharge well. Whilerecyclable, they are not considered environmentally ideal because of their leadcontent. LikePb, NiCad batteries are partially serviceable because their electrolyte,potassium hydroxide, is liquid. NiCad batteries are more tolerant to deepdischarging than Pb and offer greater energy density and power density. When acurrent reversal occurs frequently at a similar level of discharge, a ‘memory’effect occurs that reduces the cell voltage at this level of discharge, anddeep discharging is necessary to reverse the effect. Cadmium is anenvironmentally adverse material to extract, process and dispose of, and thusNiCad batteries are not regarded as environmentally benign.
Nimhbatteries have increased energy and power density compared to NiCad’s. Theelectrolyte is a gel, which removes the possibility of servicing the battery torestore capacity. There is no memory effect, the cells respond well to deepdischarging and have good cycle life. The contents of Nimh cells are lessadverse than NiCad, and may be recycled into new batteries. Li+batteries come in many varieties and chemistries. Li+ cells may have a rigidcylindrical case, or may be contained in a rectangular pouch, known as alithium ion polymer battery. Li+ cells are very tolerant of reverse currents,deep discharge and high drain rate.
Compared to other cells, Li+ batteriesmaintain their voltage throughout the discharge cycle very well. Owing to arelatively high cell voltage of 3.7 V, Li+ batteries have the highest energyand power density of any safe chemistry operating at room-temperature, and aretherefore a foremost consideration for modern EV’s. Li+ batteries do have thedisadvantage of poor performance at low temperatures (< about -20 °C)because their internal resistance increases. Table 2.
2 compares several batterytypes (Masrur and Mi 2006), (Vetter, et al. 2005).