Battery Package and Models

The battery pack is one of the main components of anEV. Many EVs have been devel- oped and are commercially available with different battery types. The ultimate goal for battery performance would be to offer similar energy and power densities to petroleum fuels used in conventional vehicles, with a comparable cost of an ICE. However, this is not feasible with the current technology and a compromise has to be made (Emadi,

2014). Lithium-ion battery technology as an energy storage system is heavily researched. A high energy density, small memory effect, and low self-discharge are the key features of Lithium-ion cells. For more information, follow Emadi (2014).

Equivalent electric circuit and electrochemical modelling are the two main approaches to mimic and model the experimental responses observed from cell characterization data. The electrochemical modelling is based on electrochemical equations of the battery chemistry. In contrast, equivalent circuit modelling is based on the electrical behaviour of the battery. The simplest circuit model is the resistance model with battery voltage Vbat, and current Ibat that can be described as:

Vbat(t) = Vbat.oc− RbatIbat(t). (2.17)

This is based on an open circuit voltage Vbat.oc, and a resistor Rbatto model the equivalent

series internal resistance of the battery. The simplest approach to track the battery’s State Of Charge (SOC) that ranges from empty (SOC = 0) to full (SOC = 1) can be described as follows: SOC(t) = SOC(t0) + 1 Cbat.N Z t t0 Ibat(t)dt (2.18)

where Cbat.N is the battery’s charge storage capacity. The energy delivered from the

battery (Ebat) can be expressed as:

Ebat=

Z t

t0

Capacity Discharge, (Ah) 0 10 20 30 40 50 60 V o lt a g e, (V ) 280 300 320 340 360 380

measurement by INL,17.3 (A) Dynamic Model,17.3 (A) measurement by SnT,10 (A)

Figure 2.4: Voltage-capacity discharged of Smart-ED during the static capacity test.

which is a function of the battery voltage and electric current. For more details about mathematics based equivalent circuit and electrochemical battery models, follow Emadi (2014) and Seaman et al. (2014).

A dynamic battery model for theEVapplications is proposed in (Tremblay et al.,2009). In this model, the battery voltage for lithium-ion discharge model is obtained by:

Vbat= Vbat.oc− Kpol

Cbat.N

Cbat.N − it

(it− i∗) − RbatIbat− Azon.ampexp(−Bzon.invit). (2.20)

The battery voltage for lithium-ion charge model is obtained by: Vbat=Vbat.oc− Kpol

Cbat.N

it− 0.1Cbat.N

i∗ − K_pol Cbat.N

Cbat.N − it

it− RbatIbat+ Azon.ampexp(−Bzon.invit) (2.21)

where it is the battery charge consumption (Ah), equivalent to:

it=

Z

idt. (2.22)

The common time-domain tests to characterise the battery pack are used and compared to measurements. These tests generally are performed in a controlled environment such as constant conditions for temperature and pressure in order to produce unbiased in- formation (for more information, follow Emadi (2014)). Figure 2.4 shows a common test result known as a C-rates static discharge capacity test for the discharge rate of the Smart-ED battery package with a constant discharge to a minimum cut-off volt- age. This test was performed for theUSADepartment of Energy, which was conducted by the Idaho National Laboratory (INL) and Intertek Testing Services, North America

Symbol Description Value, Unit Vbat.oc Open circuit battery voltage 384.1 V

Kpol Polarisation constant 0.017874 V /Ah

Cbat.N Battery charge capacity 52.174 Ah

Rbat Battery internal resistance 0.17507 Ω

Azon.amp Exponential zone amplitude 64.9 V

Bzon.inv Exponential zone time constant inverse 0.052043 Ah−1

Table 2.2: Parameters of the Smart-ED battery dynamic model

Figure 2.5: Charge power capability of Smart-ED versus energy discharged (INL,

2014).

(INL, 2014). In addition, this test is compared with the Smart-ED battery dynamic model proposed by (Tremblay et al., 2009), and the experimental tests was performed by the Automation Research Group Laboratory at Interdisciplinary Centre for Security, Reliability and Trust (SnT), at the University of Luxembourg (Tim Schwickart,2015). The discharge measurement carried out by theINL, and the simulation with the battery dynamic model is based on a three-hour discharged with a constant current rate at 17.3 (A). The parameters of the dynamic model are given in Table 2.2. The battery discharge measurement performed by SnT is based on approximately constant current at 10 (A) within around five hours. Figure 2.5, and Figure2.6 show theSmart-ED 10- second charge and 30-second discharge pulse power capabilities of battery as a function of capacity discharge (INL,2014). These show that the requested power by the powertrain can be supplied by the battery pack over the major SOCrange.

The power consumption values for the low-voltage consumer units is shown in Figure

2.7 (Geringer et al., 2012). In addition, the battery pack can be discharged over a large temperature range. Temperature variations can significantly affect the battery performance. The influence of the ambient temperature on battery efficiency is shown in Figure2.8 (Geringer et al.,2012). Furthermore, Figure2.9shows the ranges achievable

Figure 2.6: Discharge power capability of Smart-ED versus energy discharged (INL,

2014).

Figure 2.7: Smart-ED power consumption of various low-voltage consumer units (Geringer et al.,2012).

Figure 2.8: Efficiency of the Smart-ED battery at different ambient temperature (Geringer et al.,2012).

Figure 2.9: Range of the Smart-ED at different ambient temperature (Geringer et al.,

2012).

in a test with single battery charge on a road gradient of ±2% as a function of the ambient temperature (Geringer et al., 2012). For more detailed information about the

Smart-EDbattery tests results, see INL (2014) and Geringer et al. (2012).

In concluding of this section, it is clear that a detailed exact analytical model for the electric propulsion system of the Smart-ED including all models and relations of the components with unknown parameters can be complex for theADASand ITS applica- tions. Hence, a dynamometer test has been conducted in order to avoid complex models and achieve proper system identifications. The next section will relate to simple models for the propulsion system and energy consumption of theSmart-ED.