EVS 26 Los Angeles , California , May 6-9 , 2012 Analysis of Adverse Effects on Vehicle Performance Due to Hybrid Vehicle Battery Deterioration

This paper presents the results of analysis into the adverse effects on vehicle performance caused by the deterioration in capacity and increase in resistance of lithium-ion batteries installed in hybrid electric vehicles (HEVs). Detailed evaluations were conducted based on simulations to determine the adverse effects on HEV performance. It was concluded that the battery resistance at which the CO2 emissions rate and the Fuel Consumption reaches 120% of the level of a new vehicle is approximately 350% of the initial battery resistance value.


Introduction
In recent years, the electrification of the automobile has been vigorously promoted from the standpoint of addressing environmental and energy problems.This paper summarizes the results of research into the permissible deterioration of the battery, which is considered the heart of an electrically powered automobile.Battery power deterioration naturally has an adverse effect on the power and environmental performance of the vehicle.However, the specifics and extent of this effect on vehicle performance differ greatly depending on the type of vehicle system.For example, the effects on vehicle performance are completely different if batteries with the same power deterioration characteristics are installed in a battery electric vehicle (BEV) or a hybrid electric vehicle (HEV).Consequently, it is important to determine the permissible battery deterioration for each type of vehicle system separately, based on the characteristics of the effects.Consequently, this study examines the effects of battery deterioration on vehicle performance using an HEV equipped with a lithium-ion battery as the test vehicle.First, the study analyzed in detail the effect on vehicle performance as the battery deteriorated.
Here, the research studied deterioration in battery capacity as well as deterioration in internal resistance.Subsequently, this battery deterioration data was utilized to determine the permissible deterioration of the HEV lithium-ion battery, with a particular focus on the effect on CO 2 emissions and fuel consumption.

Outline of Test HEV and Simulation
Table 1 shows the specifications of the vehicle, which is a passenger vehicle installed with a seriesparallel hybrid system.The simulation model reproduces the power split device, which links the motor, engine, and generator, as well as phenomena such as engine cranking (Fig. 1) [1], [2], [3].

Engine ON Conditions
Table 2 shows the conditions under which the engine is switched on.The following three elements are used as the triggers: 1) required drive power, 2) vehicle speed, and 3) the state of charge (SOC) of the battery.These three elements are further divided into controls for high and low SOCs, and values were set independently for these two categories.When the vehicle is started, the engine is operated following the operating curve.

Test Battery
Figure 2 shows the SOC dependency of the opencircuit voltage and the internal resistance of the test lithium-ion battery.In this study, the upper limit of the terminal voltage is 4.3 V and the lower limit is 2.5 V for both the new and the deteriorated batteries.In addition, a maximum discharge rate of 20C and a maximum charge rate of 10C were also set as limitations.This paper defines capacity retention (Cs [%]) as the capacity of a new battery compared to the capacity of the battery after deterioration.Internal resistance increase (Rs [%]) is defined in the same way.The criteria for battery SOC are the capacity when the battery is new and the capacity of the battery after deterioration.Furthermore, battery capacity deterioration is assumed to have no effect on the SOC dependency of the open-circuit voltage.

SOC Correction and Hot/Cold Start Correction for HEV
An evaluation of the fuel economy of an HEV must be performed under conditions where there is no difference in SOC before and after the vehicle is driven.Therefore, the fuel economy when ΔSOC=0 is estimated based on the results of a fuel consumption simulation with various initial SOC levels.The evaluation then uses this data as the corrected fuel economy.Furthermore, this research performed a test cycle under both cold and warm start conditions.The fuel economy value was then calculated based on the following proportions: cold start = 25% and hot start = 75%.This made it possible to obtain results that considered the effect of warming up on fuel economy.

Effects of Deterioration in Capacity and Internal Resistance on Vehicle Performance
This section analyzes and evaluates the effects of lithium-ion battery capacity and internal resistance deterioration on vehicle performance, and describes the results of the tests.To make it easier to identify the effects on vehicle performance, the study first assumed that each of the following types of battery deterioration advanced independently.

Effects of Battery Capacity Deterioration
Table 3 and Fig. 3 show the simulation results for the effect of decreases in battery capacity on vehicle performance.These results confirmed that fuel economy (i.e., km driven per liter of fuel) worsened slightly when the capacity retention was 40%.This is because the SOC is more susceptible to fluctuation when the battery capacity decreases (Fig. 4), which increases the frequency at which a SOC of 35% is reached.Since the signal to switch the engine off is transmitted when the SOC reaches 35%, the number of times that the engine is turned ON and OFF also increases.This also leads to an increase in engine cranking, which involves large current flows, and causes an increase in internal resistance loss.However, the results also show that battery capacity deterioration has a very minor effect on vehicle performance.

Effects of Internal Resistance Increase
Table 4 and Fig. 5 show the simulation results for the effect of increases in internal resistance on vehicle performance.As shown in these results, an increase in internal resistance has a significantly greater effect on fuel economy than a reduction in battery capacity.This section evaluates the following performance values to consider the reason for this result.Charge and discharge efficiency The discharge efficiency (η dis ) and charge efficiency (η chg ) are derived via the following equations. (1) (2) Permissible recovery rate The permissible recovery rate is an index that expresses how much the battery can be charged from the available power (through regenerative braking etc.) from the standpoint of input performance (determined by the upper limit voltage and maximum charge rate).See Equation 3.
Fuel economy declines due to an increase in internal resistance for the following reasons.The first reason is that both the charge and discharge efficiency of the battery decline.Figure 6 shows the efficiency values derived using Equations 1 and 2 above.In particular, the decrease in the discharge efficiency is quite pronounced.To clarify this effect, Fig. 7 shows the total battery energy and internal resistance loss per increase in internal resistance.The increase in internal resistance during discharge makes it more likely that the lower limit voltage will be reached, thereby limiting and reducing the current.The engine then must compensate for the insufficient battery power.As a result, the total battery discharge greatly reduces as the internal resistance rises.In contrast, during charging, the internal resistance can increase to 200% without reaching the upper limit voltage limit so the total energy input will rise in this range.However, when the internal resistance increase exceeds 300%, the upper limit voltage limit is reached and the energy input from charging then decreases.Nonetheless, this decrease is smaller than the total output.The loss increases for both energy input and output.Consequently, in the case of Equations 1 and 2 above, the increase in internal resistance has a larger adverse effect on discharge efficiency compared to charge efficiency.The second reason is that when the internal resistance increase reaches 300% or higher, the power for charging the battery when the vehicle decelerates is limited.Figure 8 shows the charge efficiency (derived from Equation 2) and the permissible recovery rate of regenerated power (derived from Equation 3) per increase in internal resistance.It is possible to recover all regenerated power when the battery is new, but the permissible recovery rate is greatly reduced when the internal resistance increases.This results in a reduction in fuel economy.Finally, the third reason for the reduction in fuel economy is that the engine ON time increases.Figure 9 shows the engine running time and increase as the internal resistance rises.This graph indicates that the engine running time increases in accordance with the internal resistance, which has an adverse effect on fuel economy.Furthermore, increased engine cranking also reduces fuel economy.However, compared to the three reasons described above, the adverse effect of additional engine cranking is only minor.This section describes the results of an evaluation of simultaneous deterioration in battery capacity and internal resistance deterioration to reproduce actual usage conditions more closely.Table 5 compares these two types of battery deterioration.This table was created by referring to the results of a lithium-ion battery storage capacity loss test and then adopted in this analysis [4]. Figure 10 shows the relationship between the internal resistance increase Rs (with simultaneous deterioration in battery capacity) and fuel economy, fuel consumption.These results confirmed that simultaneous deterioration has a larger adverse effect on vehicle performance than the independent deterioration described in the previous sections.This can be explained by a combination of the previous results and observations of the adverse effects of these two battery deterioration phenomena.1.It was confirmed that deterioration in the battery capacity has only a minor adverse effect on the fuel economy and CO2 emissions of an HEV. 2. In contrast, deterioration in the battery internal resistance has a large adverse effect on the fuel economy and CO2 emissions of an HEV.Test results and analysis showed that this is directly caused by reductions in the charge and discharge efficiency, an increase in the engine running time due to greater limitations on battery discharge, and a decrease in the permissible recovery rate (an index that expresses how much the battery can be charged from the available power (through regeneration and the like) from the standpoint of input performance).

CO2 emissions and fuel consumption
were e valuated as indices of the effect on vehicle performance to examine the permissible battery deterioration.This research determined the permissible vehicle performance reduction to be 20%.It was confirmed that a 350% increase in internal resistance (with capacity retention of approximately 60% compared to a new battery) results in a 20% reduction in vehicle performance.Therefore, it is proposed that 350% internal resistance increase can be used to determine the permissible deterioration of lithium-ion batteries in an HEV.

Figure 2 :
Figure 2: Characteristics of Internal Resistance and Open-circuit Voltage of the Lithium-ion Battery (Single Cell)

Figure 3 :Figure 4 :
Figure 3: Relationship between Capacity Retention (Cs) and Fuel Economy

Figure 5 :
Figure 5: Increase in Battery Resistance and Fuel Economy Performance

Figure 6 :
Figure 6: Charge and Discharge Efficiency when Internal Resistance Increases

Figure 8 :
Figure 8: Charge Efficiency and Permissible Recovery Rate of Regenerated Power per Internal Resistance Increase

Figure 11 :
Figure 11: Examination of Permissible Battery Deterioration under Actual Usage Conditions

Table 1 :
Vehicle Specifications

Table 3 :
Relationship between Capacity Retention (Cs) and Fuel Economy

Table 4 :
Relationship between Internal Resistance Increase (Rs) and Fuel Economy

Table 5 :
Relationship between Cs and Rs of Deteriorated