2.3.1 the reverse for charging. Figure 2.B
Perhaps the oldest and most
recognized method of storing electrical energy is the battery. Though other
technologies have emerged, batteries, especially secondary or rechargeable
cells, are still one of the best options available because of their energy
battery types are available with varying chemistries for each major category;
the most common varieties are lead acid (Pb), nickel cadmium (NiCad), nickel
metal hydride (Nimh) and lithium ion (Li+) (Buchmann 2003). The basic mode of
operation is the same in each case, an anode and a cathode are separated by an
electrolyte, which may be a liquid as in Pb or NiCad, or a gel as in Nimh or
Li+. When discharging, positive ions migrate from the anode through the
electrolyte to the cathode, and the reverse for charging. Figure 2.B shows a
basic schematic for a Li+ battery.
Figure 2.B –
Li-ion battery schematic
capacity (C) is measured in amp-hours (Ah), and the total amount of energy
stored in the battery is roughly equal to the capacity multiplied by the
average voltage during discharge:
battery’s state of charge (SoC) is a measure of the energy available from the
battery. Batteries are typically designed to operate within a specific window
of SoC, known as the SoC swing. The SoC with the lowest remaining energy in the
battery is known as the depth of discharge, or DoD.
current output from a battery scales linearly with capacity, and is therefore
measured in terms of capacity, using a parameter called C. A discharge rate
of 1 C indicates the battery will be depleted in one hour, while a rate of 2
C will drain the battery in half of an hour. Current is limited by the rates
for chemical reactions within the cell and by the generation of heat. It is
common for batteries to have a maximum continuous rate of discharge, with a
larger 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 their
continuous rate, and can sustain this output for approximately 10 s (Kokam
Co. Ltd. 2010).
have an internal resistance which accounts for some energy loss from the cell
while charging or discharging. Partly due to internal resistance, batteries
have smaller apparent capacities when discharging at high rates. This is known
as the Peukert effect (Buchmann 2003).
packs consist of multiple cells arranged in series and/or parallel. A set of
batteries connected in series is called a string, the length of which is the
stack height. The product of the stack height and number of strings gives the
total number of cells in a battery pack. For instance, a battery pack with two
sets of three batteries connected in series has 2 strings, a stack height of 3
and six batteries in total.
have limited service life, the length of which depends on cell chemistry, DoD,
SoC swing and temperature, among other factors. With time and use, battery
capacity attenuates and internal resistance grows. For most batteries, this
process is accelerated with higher temperatures during storage and use, deeper
discharge cycles, and high drain rates. Ideal usage conditions for a battery
are moderate temperature, SoC swing and DoD, low and stable current demand with
few current reversals, or microcycles. These conditions will extend the service
life of the battery and yield better energy capacity per charge.
cell chemistry has unique characteristics that make it suitable or not for a
given 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 electrical
systems of combustion engines. Additionally, Pb batteries can be serviced by
careful charging and addition of electrolyte to restore some of their original
capacity. Pb batteries are not ideal for electric vehicle applications because
they are large, heavy, and do not tolerate deep discharge well. While
recyclable, they are not considered environmentally ideal because of their lead
Pb, NiCad batteries are partially serviceable because their electrolyte,
potassium hydroxide, is liquid. NiCad batteries are more tolerant to deep
discharging than Pb and offer greater energy density and power density. When a
current reversal occurs frequently at a similar level of discharge, a ‘memory’
effect occurs that reduces the cell voltage at this level of discharge, and
deep discharging is necessary to reverse the effect. Cadmium is an
environmentally adverse material to extract, process and dispose of, and thus
NiCad batteries are not regarded as environmentally benign.
batteries have increased energy and power density compared to NiCad’s. The
electrolyte is a gel, which removes the possibility of servicing the battery to
restore capacity. There is no memory effect, the cells respond well to deep
discharging and have good cycle life. The contents of Nimh cells are less
adverse than NiCad, and may be recycled into new batteries.
batteries come in many varieties and chemistries. Li+ cells may have a rigid
cylindrical case, or may be contained in a rectangular pouch, known as a
lithium ion polymer battery. Li+ cells are very tolerant of reverse currents,
deep discharge and high drain rate. Compared to other cells, Li+ batteries
maintain their voltage throughout the discharge cycle very well. Owing to a
relatively high cell voltage of 3.7 V, Li+ batteries have the highest energy
and power density of any safe chemistry operating at room-temperature, and are
therefore a foremost consideration for modern EV’s. Li+ batteries do have the
disadvantage of poor performance at low temperatures (< about -20 °C) because their internal resistance increases. Table 2.2 compares several battery types (Masrur and Mi 2006), (Vetter, et al. 2005).