Improved Voltage Drop Compensation Method for Hybrid Fuel Cell Battery System

Tae-Ho Eom 1, Jin-Wook Kang 1, Jintae Kim 1, Min-Ho Shin 1, Jung-Hyo Lee 2 and Chung-Yuen Won 1,* 1 Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Korea; taehooo@skku.edu (T.-H.E.); kjw2171@naver.com (J.-W.K.); jintae.kim75@gmail.com (J.-T.K.); smh5480@nate.com (M.-H.S.) 2 Department of Electrical Engineering, Kunsan National University, Kunsan 54150, Korea; jhlee82@kunsan.ac.kr * Correspondence: woncy@skku.edu; Tel.: +82-031-290-7164


Introduction
Throughout recent decades, renewable energies and hydrogen energy for electric vehicles have been studied as alternative power source for the conventional internal combustion engines, and to improve environmental pollution by reducing carbon dioxide emitted to the air as environmental regulation is gradually tightened [1][2][3].
To decrease the carbon dioxide emission, research on fuel cell, lithium battery, and super capacitor have been actively progressing [4][5][6][7].Among them, the hydrogen fuel cell has been a focus of many studies, as it is considered one of the solutions.Basically, electrical energy can be generated in the hydrogen fuel cell by the reaction of hydrogen and oxygen through the membrane, in which hydrogen is supplied externally as compressed hydrogen and oxygen is utilized in the atmosphere after being purified.Among the different types, the proton exchange membrane fuel cell (PEMFC) is widely utilized in independent applications, such as a fuel cell vehicle, due to advantages of a low operating temperature of less than 100 • C, high power density, and fast start-up [8][9][10][11][12][13].
Meanwhile, other battery technologies for electric vehicles have also been researched as alternatives to the conventional internal combustion engine.However, these other batteries for electric vehicles have the disadvantages of long battery charging time and short mileage, unlike the hydrogen fuel cell system, which offers higher energy density, shorter charging time, and better mileage [14][15][16].
In the past, research has been done to improve the performance of the fuel cell, including work on the dead-end method and hydrogen oversupply method [17][18][19][20].The dead-end method is when the fuel cell outputs a large current, and fuel loss occurs due to flooding phenomenon, leading to the deterioration of the fuel cell [21][22][23].For this reason, various solutions using the water management of fuel cell have been studied [24][25][26][27][28][29][30][31][32][33][34].Results show that the addition of a recirculation line and solenoid valve alone can increase fuel utilization rate, at low cost.Furthermore, the operation time increases compared to the dead-end method.The other method is the hydrogen oversupply method, efficiency of which is low due to high hydrogen fuel consumption.For this reason, hydrogen recirculation, with high hydrogen utilization, has been widely used recently.Hydrogen recirculation is highly fuel efficient, because it recycles discharged hydrogen that has not reacted during the reaction process of oxygen and hydrogen [35].
However, the recycled hydrogen could contain impurities that are created during the recirculation process, which results in unstable electrical output characteristics.In addition, the output side of the fuel cell is typically connected to a DC-DC converter.Therefore, the operation of the DC-DC converter, which works to supply constant output voltage could cause the fuel cell voltage to drop.In the worst case scenario, the fuel cell may shut down due to severe voltage drop.For that reason, a voltage drop compensation method is needed to prevent system shutdown.
In this paper, three factors that cause voltage drop during hydrogen recirculation are analyzed: (1) The voltage drop varying depending on the amount of hydrogen supply; (2) the temperature of the fuel cell stack; and (3) the current density of platinum catalyst.
When hydrogen recirculation occurs, the amount of pure hydrogen supplied to the fuel cell can differ due to impurities.This results in fuel cell output voltage drop.This voltage drop during initial operation can vary depending on the temperature of the fuel cell, and inherent resistance can vary during steady state operation, depending on the current density of the platinum catalyst.
In this paper, an electrical compensation method is proposed, in order to overcome voltage drop in hybrid fuel cell battery system, using a hydrogen recirculation system for a forklift.The proposed compensation method has a function detects the three causations for the voltage drop, and generates compensation signals to the DC-DC converter.As a command of the compensation signal, the proposed method even decreases output current of the DC-DC converter, compulsively, to prevent fuel cell voltage drop, and uses an additional battery supply in case of a lack of output current during the operation of DC-DC converter [36][37][38][39][40][41].Accordingly, the proposed compensation method can result in fuel cell battery shutdown caused by low voltage during hydrogen recirculation [41].
To verify the feasibility and validity of the proposed voltage drop compensation method, it is applied to the buck-boost converter for a fuel cell used by a forklift.The problem of fuel cell shutdown due to voltage drop during hydrogen recirculation is solved by applying the proposed method.
This paper is organized as follows: In Section 2, the fuel cell system configuration for forklift, and the advantages of the hydrogen recirculation system, are explained.The voltage characteristics of the fuel cell for the three factors for the voltage drops are also analyzed.In Section 3, the reason why the output voltage of the fuel cell stack drops during hydrogen recirculation is analyzed, and the proposed voltage drop compensation method of the buck-boost converter is introduced.In Section 4, the three factors that cause voltage drops in fuel cells is analyzed by experiment, and the feasibility and validity of the proposed control method are confirmed through the fuel cell forklift experiment.Finally, Section 5 provides the conclusion.

Analysis of Voltage Drop Characteristics of the Fuel Cell System during Hydrogen Recirculation
The fuel cell forklift system consists of a hybrid fuel cell battery system and a forklift, as shown in Figure 1.The rated power of the fuel cell for the forklift used in this paper is 10 kW.Additionally, the fuel cell system includes a recirculation function.The hybrid fuel cell battery system consists of a fuel stack of 50 cells, a 35 bar hydrogen tank that is able to supply 70 liter per minute (LPM) to the fuel cell stack, a lithium polymer battery pack with 13 cells-200 Ah, the buck-boost converter, and the total control unit (TCU).Among secondary batteries, the lithium polymer battery using polymer electrolyte is widely used because of its high power density [42].It also controls the input current of the buck-boost converter by means of the controller area network (CAN) communication.

Analysis of Voltage Drop Characteristics of the Fuel Cell System during Hydrogen Recirculation
The fuel cell forklift system consists of a hybrid fuel cell battery system and a forklift, as shown in Figure 1.The rated power of the fuel cell for the forklift used in this paper is 10 kW.Additionally, the fuel cell system includes a recirculation function.The hybrid fuel cell battery system consists of a fuel stack of 50 cells, a 35 bar hydrogen tank that is able to supply 70 liter per minute (LPM) to the fuel cell stack, a lithium polymer battery pack with 13 cells-200 Ah, the buck-boost converter, and the total control unit (TCU).Among secondary batteries, the lithium polymer battery using polymer electrolyte is widely used because of its high power density [42].It also controls the input current of the buck-boost converter by means of the controller area network (CAN) communication.
The forklift consists of an inverter, drive motor, and hydraulic motor.The inverter is designed with a power of 50 kW, which is used to simultaneously perform a 5 ton lift and drive the forklift.In addition, the fuel cell used PEMFC, including hydrogen recirculation.In the system, compressed hydrogen from a hydrogen tank is supplied to the fuel cell stack.The supplied hydrogen reacts with oxygen in the atmosphere to generate electric energy.During the reactions between the two gases, some conditions can cause voltage drop due to several reasons.In a worst case scenario, the voltage drop at the PEMFC system can cause system shutdown.There are three main causal factors for this lack of hydrogen supply, temperature at the fuel cell stack, and the current density of the platinum catalyst.
Figure 2 shows voltage drop characteristics depending on current density loaded on the fuel cell system.In ideal state, an initial open circuit voltage (OCV) of a fuel cell unit is 1.2 V.The output voltage gradually decreases from the OCV as the load current increases.However, voltage drop can be drastically accelerated at area A because the electrochemical reaction is abruptly reduced as the hydrogen fuel is consumed.In addition, voltage drop of area A can be increased due to impurities during the hydrogen recirculation.
Meanwhile, a detailed explanation of the curve is given in Reference [35].The forklift consists of an inverter, drive motor, and hydraulic motor.The inverter is designed with a power of 50 kW, which is used to simultaneously perform a 5 ton lift and drive the forklift.
In addition, the fuel cell used PEMFC, including hydrogen recirculation.In the system, compressed hydrogen from a hydrogen tank is supplied to the fuel cell stack.The supplied hydrogen reacts with oxygen in the atmosphere to generate electric energy.During the reactions between the two gases, some conditions can cause voltage drop due to several reasons.In a worst case scenario, the voltage drop at the PEMFC system can cause system shutdown.There are three main causal factors for this lack of hydrogen supply, temperature at the fuel cell stack, and the current density of the platinum catalyst.
Figure 2 shows voltage drop characteristics depending on current density loaded on the fuel cell system.In ideal state, an initial open circuit voltage (OCV) of a fuel cell unit is 1.2 V.The output voltage gradually decreases from the OCV as the load current increases.However, voltage drop can be drastically accelerated at area A because the electrochemical reaction is abruptly reduced as the hydrogen fuel is consumed.In addition, voltage drop of area A can be increased due to impurities during the hydrogen recirculation.
Meanwhile, a detailed explanation of the curve is given in Reference [35].
The output voltage of the fuel cell, v out , can be described by subtracting the voltage drop, ∆v drop , from the OCV, V OCV , in Equation (1).In addition, the voltage drop is determined by the three factors mentioned, above which can be expressed as Equation (2).
where, ∆v hydro is voltage drop depending on amount of hydrogen supply, ∆v temp is voltage drop depending on temperature of a fuel cell stack, and ∆v den is voltage drop depending on the current density of platinum catalyst.The output voltage of the fuel cell, out v , can be described by subtracting the voltage drop, , in Equation (1).In addition, the voltage drop is determined by the three factors mentioned, above which can be expressed as Equation (2).
where, is voltage drop depending on the current density of platinum catalyst.

Voltage Drop Characteristics Depending on Hydrogen Supply
When the hydrogen recirculation is applied to the stacks, hydrogen usage can be reduced by recirculating the not-reacted hydrogen.The hydrogen recirculation is very easy to apply to conventional hydrogen fuel cell system by simply installing a hydrogen recirculation line and solenoid valve.
However, if the hydrogen containing impurities at the recirculation is supplied to the fuel cell stacks, their output voltage can be significantly decreased, as can be seen in Figure 3.The y-axis of Figure 3 is the fuel cell stack consisting of 50 cells voltage.
At the fuel cell unit, if the cell voltage is reduced to 0.5 V or less, the fuel cell stacks can be broken because high pressure applies to the separation membrane.

Voltage Drop Characteristics Depending on Hydrogen Supply
When the hydrogen recirculation is applied to the stacks, hydrogen usage can be reduced by recirculating the not-reacted hydrogen.The hydrogen recirculation is very easy to apply to conventional hydrogen fuel cell system by simply installing a hydrogen recirculation line and solenoid valve.
However, if the hydrogen containing impurities at the recirculation is supplied to the fuel cell stacks, their output voltage can be significantly decreased, as can be seen in Figure 3.The y-axis of Figure 3  In order to generate high quality electricity, hydrogen fuel and oxygen have to be continuously supplied in the fuel cell.Equation (3) shows relationship between a voltage drop and hydrogen supply.
( ) At the fuel cell unit, if the cell voltage is reduced to 0.5 V or less, the fuel cell stacks can be broken because high pressure applies to the separation membrane.
In order to generate high quality electricity, hydrogen fuel and oxygen have to be continuously supplied in the fuel cell.Equation (3) shows relationship between a voltage drop and hydrogen supply. where, . Figure 4 shows a graph of voltage drop depending on hydrogen supply, plotted based on Equation (3).For example, when hydrogen supply is 60 LPM continuously, then the fuel cell can support maximally a 200 A load condition.Meanwhile, if hydrogen supply is reduced to 40 LPM due to impurities in the hydrogen recirculation, it can be estimated that the fuel cell can be damaged at a 110 A load current.Namely, voltage drop can be easily anticipated by measuring hydrogen supply and Equation (3).In order to generate high quality electricity, hydrogen fuel and oxygen have to be continuously supplied in the fuel cell.Equation (3) shows relationship between a voltage drop and hydrogen supply.
( ) Figure 4 shows a graph of voltage drop depending on hydrogen supply, plotted based on Equation (3).For example, when hydrogen supply is 60 LPM continuously, then the fuel cell can support maximally a 200 A load condition.Meanwhile, if hydrogen supply is reduced to 40 LPM due to impurities in the hydrogen recirculation, it can be estimated that the fuel cell can be damaged at a 110 A load current.Namely, voltage drop can be easily anticipated by measuring hydrogen supply and Equation (3).

Voltage Drop Characteristics Depending on Temperature of the Fuel Cell Stack
Besides the hydrogen supply, a voltage drop depends on temperature of the fuel cell stack.The area B of Figure 2 is the voltage drop caused by the activation polarization, and it varies depending on temperature of the fuel cell stack.
Figure 5 shows the output voltage ∆v out of the fuel cell decrease as the temperature increases.Since the fuel cell used in vehicle requires a fast starting time, the operating temperature is controlled to 60 The area B of Figure 2 is the voltage drop caused by the activation polarization, and it varies depending on temperature of the fuel cell stack.appears to increase linearly as the temperature increase.However, when the temperature is increased, the exchange current density, o i , is heavily decreased based on Equation (5).Therefore, where, The voltage drop ∆v temp is predicted, as shown in Equation ( 4) from the Butler Volmer equation.∆v temp appears to increase linearly as the temperature increase.However, when the temperature is increased, the exchange current density, i o , is heavily decreased based on Equation (5).Therefore, ∆v temp is heavily reduced. where, where,

Voltage Drop Characteristics Depending on Current Density of Platinum Catalyst
Platinum is mainly used as a catalyst for promoting the activation reaction of the fuel cell.Platinum catalyst is very efficient for the oxidation of hydrogen, but the oxygen reduction reaction (ORR) is slow.Because of this characteristic, the voltage drop is dependent on the current density of platinum catalyst.The current density of platinum catalyst represents the energy activity per unit area of the catalyst layer [43][44][45].Area C of Figure 2 is the voltage drop depending on the current density of the platinum catalyst and the inherent resistance.Figure 6 shows the output voltage ∆v out of the fuel cell decrease as the catalyst current density increases.At high current density, the voltage drop effect large because the diffusion of reactants is slow.
platinum catalyst.The current density of platinum catalyst represents the energy activity per unit area of the catalyst layer [43][44][45].Area C of Figure 2 is the voltage drop depending on the current density of the platinum catalyst and the inherent resistance.Figure 6 shows the output voltage out v Δ of the fuel cell decrease as the catalyst current density increases.At high current density, the voltage drop effect large because the diffusion of reactants is slow.is calculated, as shown in Equation ( 6) from the Ohm's law.
As explained previously, a voltage drop can be predicted by calculating the above three factors.Thus, the proposed compensation method in Section 3 prevents shutdown of the fuel cell system due to voltage drop.

The Proposed Voltage Drop Compensation Method of Buck-Boost Converter Considering Voltage Drop during Hydrogen Recirculation
In Section 2, the three factors that cause voltage drop of the hydrogen fuel cell system were analyzed.The first is the voltage drop depending on the amount of hydrogen supply.The voltage drop varies as the amount of hydrogen supply decreases in the operation of the fuel cell.This situation usually happens due to influx of impurities during hydrogen recirculation.Even the impurities cause that the fuel cell system may shutdown due to excessive voltage drop, as shown in Figure 3.The amount of hydrogen supplied to the fuel cell stack is sensed through the sensing device All conductors have inherent resistance, R ir , that causes the voltage drop, ∆v den , in the flow of charges.When R ir and cross section of the catalyst A cell are constant, the voltage drop varies with the current density of platinum catalyst, i pc .∆v den is calculated, as shown in Equation ( 6) from the Ohm's law. where, , As explained previously, a voltage drop can be predicted by calculating the above three factors.Thus, the proposed compensation method in Section 3 prevents shutdown of the fuel cell system due to voltage drop.

The Proposed Voltage Drop Compensation Method of Buck-Boost Converter Considering Voltage Drop during Hydrogen Recirculation
In Section 2, the three factors that cause voltage drop of the hydrogen fuel cell system were analyzed.The first is the voltage drop depending on the amount of hydrogen supply.The voltage drop varies as the amount of hydrogen supply decreases in the operation of the fuel cell.This situation usually happens due to influx of impurities during hydrogen recirculation.Even the impurities cause that the fuel cell system may shutdown due to excessive voltage drop, as shown in Figure 3.The amount of hydrogen supplied to the fuel cell stack is sensed through the sensing device as in Reference [33].In addition, other factors affecting voltage drops are the temperature of fuel cell stack and current density of the platinum catalyst.
Figure 7 shows the control block diagram of the buck-boost converter that compensates for voltage drop due to these three factors.As shown in the control block diagram, the proposed control algorithm includes three different compensation values, corresponding to each of the voltage drop factors.
Firstly, ∆v hydro represents the compensation value of the voltage drop depending on the amount of hydrogen supply.The compensation value of the voltage drop depending on the measured hydrogen, H stack , supplied to the fuel cell, including recirculated hydrogen can be calculated using Equation (3).In addition, the ∆v hydro value increases as more impurities.∆v hydro is used to calculates the ∆v drop as Equation (2).The voltage compensation is performed by decreasing reference current I * in , which is the reference current of the current controller according to the ratio of the calculated voltage drop.Secondly, ∆v temp is the compensation value of the voltage drop depending on the temperature of the fuel cell stack.∆v temp varies frequently as the fuel cell operates.Therefore, the temperature of the fuel cell stack T stack , is continuously sensed to estimate ∆v temp using Equation (4).
Thirdly, ∆v den is the compensation value of the voltage drop depending on the current density of platinum catalyst D stack .The higher D stack is, the faster the electrochemical reaction rate due to the increased exchange current density.However, when D stack is high, the voltage drop due to the inherent resistance of the fuel cell increases, as shown in Equation (6).Therefore, in this paper, the current density of platinum catalyst is set to 700 mA/cm 2 , which is adequate considering the capacity of the fuel cell, and the compensation value ∆v den is calculated through Equation (6).
Using those three calculated control values, ∆v hydro , ∆v temp , ∆v den , voltage drop, ∆v out , is estimated by Equation (2), and the voltage drop compensation is performed by feed-forwarding the estimated voltage drop into the input of the current controller, as shown in Figure 7.
Figure 8a shows the simulation waveforms that prove the proposed voltage drop compensation method prevents shutdown of the fuel cell during hydrogen recirculation by adjusting the output current of the fuel cell from 180 A to 170 A. As the current supplied from the fuel cell decreases, the output current from the lithium polymer battery parallel-connected to the fuel cell increases to meet the load requirement.
current of the fuel cell from 180 A to 170 A. As the current supplied from the fuel cell decreases, the output current from the lithium polymer battery parallel-connected to the fuel cell increases to meet the load requirement.
Figure 8b shows the simulation waveforms when the output current of the battery increases after compensating for the voltage drop.The battery current supplied to the load has increased from 150 A to 160 A. Figure 9 shows the supplied power variation to the load when the proposed compensation method is operating.As shown in Figure 9, the battery supplies as much extra power as the decreased fuel cell power during the voltage drop compensation due to hydrogen recirculation, maintaining the power supplied to the load.Thus, using the proposed compensation control, constant power can be supplied to the load.Figure 8b shows the simulation waveforms when the output current of the battery increases after compensating for the voltage drop.The battery current supplied to the load has increased from 150 A to 160 A.
Figure 9 shows the supplied power variation to the load when the proposed compensation method is operating.As shown in Figure 9, the battery supplies as much extra power as the decreased fuel cell power during the voltage drop compensation due to hydrogen recirculation, maintaining the power supplied to the load.Thus, using the proposed compensation control, constant power can be supplied to the load.

The Experiment Hardware Configuration
The hardware configuration of the buck-boost converter for applying the proposed voltage drop compensation method is shown in Figure 10.The size reduction of the fuel cell system is required because a hybrid fuel cell battery system is placed in limited area of electric forklift.In order to reduce the size, module type device is used that combined MOSFET and the diode.In addition, the pin-type heat sink is used to reduce the size of the cooling system, and TMS320F28335 is used as a microprocessor for the TCU.The electrical specification of the buck-boost converter is shown in Table 1.The rated power is 10 kW, and it operates at 25 kHz of high switching frequency to minimize output ripple.

Gate Board
Current Sensor Buck Module

The Experiment Hardware Configuration
The hardware configuration of the buck-boost converter for applying the proposed voltage drop compensation method is shown in Figure 10.The size reduction of the fuel cell system is required because a hybrid fuel cell battery system is placed in limited area of electric forklift.In order to reduce the size, module type device is used that combined MOSFET and the diode.In addition, the pin-type heat sink is used to reduce the size of the cooling system, and TMS320F28335 is used as a microprocessor for the TCU.The electrical specification of the buck-boost converter is shown in Table 1.The rated power is 10 kW, and it operates at 25 kHz of high switching frequency to minimize output ripple.
because a hybrid fuel cell battery system is placed in limited area of electric forklift.In order to reduce the size, module type device is used that combined MOSFET and the diode.In addition, the pin-type heat sink is used to reduce the size of the cooling system, and TMS320F28335 is used as a microprocessor for the TCU.The electrical specification of the buck-boost converter is shown in Table 1.The rated power is 10 kW, and it operates at 25 kHz of high switching frequency to minimize output ripple.

The Voltage Drop Characteristics of the Three Factors
The characteristics of an estimated voltage drop, that could take place in three circumstances analyzed in Section 2, are verified through experiments.First, the voltage drop variation depending on the amount of hydrogen supplied to the fuel cell stack is verified.Table 2 shows the experimental conditions of the output characteristics depending on the amount of hydrogen supplied to the fuel cell stack.As per the experiment results shown in Figure 11a-c, it can be confirmed that the operating range of the fuel cell increases as the amount of hydrogen supplied increases.In addition, Figure 11d shows the voltage drop graph depending on the amount of hydrogen supply through the experiment result.If the amount of hydrogen supply is low, shutdown may occur.In this paper, when performing drive experiment of forklift, the amount of hydrogen supply is 70 LPM.As per the experiment results shown in Figure 11a-c, it can be confirmed that the operating range of the fuel cell increases as the amount of hydrogen supplied increases.In addition, Figure 11d shows the voltage drop graph depending on the amount of hydrogen supply through the experiment result.If the amount of hydrogen supply is low, shutdown may occur.In this paper, when performing drive experiment of forklift, the amount of hydrogen supply is 70 LPM.Second, the voltage drop depending on the temperature variation of the fuel cell is verified, which is related to the magnitude of the output voltage of the fuel cell.Table 3 shows the experimental conditions of the output characteristics depending on the temperature of the fuel cell stack.As per the experiment results, shown in Figure 12a-c, it can be confirmed that the voltage drop decrease as the temperature of fuel cell stack increases.In addition, Figure 12d shows the voltage drop graph depending on the temperature of the fuel cell stack.In accordance with experiment results, it is beneficial to operate the system at high temperature, but the temperature should be controlled to 70 • C or less because of the forklift application requirement.
The last factor is the voltage drop depending on the current density of the platinum catalyst.It is mostly caused by the IR drop due to the inherent resistance of the fuel cell.Table 4 shows experimental conditions of the fuel cell stack with the current density of platinum catalyst variation.
As per the experiment results shown in Figure 13a-c, it can be confirmed that the voltage drop increases as the current density of the platinum catalyst increases.In addition, Figure 13d shows the voltage drop graph depending on the current density of the platinum catalyst.In this paper, when performing the forklift drive and lift experiment, the current density of the platinum catalyst is 700 mA/cm 2 .As per the experiment results, shown in Figure 12a-c, it can be confirmed that the voltage drop decrease as the temperature of fuel cell stack increases.In addition, Figure 12d shows the voltage drop graph depending on the temperature of the fuel cell stack.In accordance with experiment results, it is beneficial to operate the system at high temperature, but the temperature should be controlled to 70 °C or less because of the forklift application requirement.The last factor is the voltage drop depending on the current density of the platinum catalyst.It is mostly caused by the IR drop due to the inherent resistance of the fuel cell.Table 4 shows experimental conditions of the fuel cell stack with the current density of platinum catalyst variation.As per the experiment results shown in Figure 13a-c, it can be confirmed that the voltage drop increases as the current density of the platinum catalyst increases.In addition, Figure 13d shows the voltage drop graph depending on the current density of the platinum catalyst.In this paper, when performing the forklift drive and lift experiment, the current density of the platinum catalyst is 700 mA/cm 2 .

The Experiment Results for Hybrid Fuel Cell Battery System and Forklift Drive
Based on the characteristics of each voltage drop, the experiment on the hydrogen recirculation of hybrid fuel cell battery system is performed.Figure 14 shows the hardware configuration of the hybrid fuel cell battery system for a forklift.It includes a battery management system (BMS) to monitor the battery's condition and the protection circuit module (PCM).Based on the characteristics of each voltage drop, the experiment on the hydrogen recirculation of hybrid fuel cell battery system is performed.Figure 14 shows the hardware configuration of the hybrid fuel cell battery system for a forklift.It includes a battery management system (BMS) to monitor the battery's condition and the protection circuit module (PCM).

The Experiment Results for Hybrid Fuel Cell Battery System and Forklift Drive
Based on the characteristics of each voltage drop, the experiment on the hydrogen recirculation of hybrid fuel cell battery system is performed.Figure 14 shows the hardware configuration of the hybrid fuel cell battery system for a forklift.It includes a battery management system (BMS) to monitor the battery's condition and the protection circuit module (PCM).Figure 15a shows fuel cell shutdown due to voltage drop when the voltage drop compensation method is not applied during hydrogen recirculation.When the fuel cell performs hydrogen recirculation, a rapid voltage drop to 28 V occurs.Then, shutdown of the fuel cell is occurred that stops all system operations.

TCU&BMS, PCM
Electronics 2018, 7, x FOR PEER REVIEW 14 of 18 Figure 15a shows fuel cell shutdown due to voltage drop when the voltage drop compensation method is not applied during hydrogen recirculation.When the fuel cell performs hydrogen recirculation, a rapid voltage drop to 28 V occurs.Then, shutdown of the fuel cell is occurred that stops all system operations.
Figure 15b shows the fuel cell output waveform when the proposed voltage drop compensation method is applied.When the voltage drop occurs, it can be confirmed that the voltage is restored by reducing the current reference from 180 A to 170 A, thereby preventing shutdown of the fuel cell during hydrogen recirculation.Figure 15b shows the fuel cell output waveform when the proposed voltage drop compensation method is applied.When the voltage drop occurs, it can be confirmed that the voltage is restored by reducing the current reference from 180 A to 170 A, thereby preventing shutdown of the fuel cell during hydrogen recirculation.
Figure 16 shows the output waveform of the lithium polymer battery.As the current of the fuel cell decreases, the lithium polymer battery supplies additional current, as shown in Figure 16.Therefore, constant power can be supplied to the load.Figure 16 shows the output waveform of the lithium polymer battery.As the current of the fuel cell decreases, the lithium polymer battery supplies additional current, as shown in Figure 16.Therefore, constant power can be supplied to the load.Figure 17a shows the forklift with hybrid fuel cell battery system.The experimental method is lifting a 5 ton weight and driving at 25 km/h.The drive experiment of forklift was repeated six times.
Figure 17b shows the total output current of the hybrid fuel cell battery system flowing through the inverter, while the forklift operates; when lifting 5 tons and driving at 25 km/h, the forklift requires a current of 300 A or more maximally.However, it can be seen that the hybrid fuel cell battery system works normally without the shutdown during hydrogen recirculation, thanks to the proposed voltage drop compensation method.Figure 17a shows the forklift with hybrid fuel cell battery system.The experimental method is lifting a 5 ton weight and driving at 25 km/h.The drive experiment of forklift was repeated six times.

Conclusions
In this paper, a fuel cell voltage drop compensation method is proposed.During hydrogen recirculation, voltage drop occurs due to various factors and causes shutdown of the fuel cell.To solve these problems, three factors to inducing voltage drop of fuel cell (the amount of hydrogen supply, the temperature of fuel cell stack, and the current density of platinum catalyst) were analyzed.Using the analyzed three factors, the voltage drop was calculated and compensated by controlling the output current of the buck-boost converter used in the fuel cell system.At this time, the current of the lithium polymer battery increases as the current of the fuel cell decreases.Thereby, constant power is supplied to the load.
To verify the feasibility and effectiveness of the proposed voltage drop compensation method, the simulation was performed depicting the hybrid fuel cell battery system.Stable operation during hydrogen recirculation was verified by the fuel cell system and the drive experiments of the forklift.Conventional control method occurs voltage drop from 32 V to 28 V during hydrogen recirculation, which can cause fuel cell system breakdown.However, the shutdown problem was solved by using the proposed compensation method, and the operation stability of the fuel cell system depending on the various conditions was improved.Through this, the consumption of hydrogen fuel can be reduced by improving the stability of the fuel cell system, unlike in the past, when excessive Figure 17b shows the total output current of the hybrid fuel cell battery system flowing through the inverter, while the forklift operates; when lifting 5 tons and driving at 25 km/h, the forklift requires a current of 300 A or more maximally.However, it can be seen that the hybrid fuel cell battery system works normally without the shutdown during hydrogen recirculation, thanks to the proposed voltage drop compensation method.

Conclusions
In this paper, a fuel cell voltage drop compensation method is proposed.During hydrogen recirculation, voltage drop occurs due to various factors and causes shutdown of the fuel cell.To solve these problems, three factors to inducing voltage drop of fuel cell (the amount of hydrogen supply, the temperature of fuel cell stack, and the current density of platinum catalyst) were analyzed.Using the analyzed three factors, the voltage drop was calculated and compensated by controlling the output current of the buck-boost converter used in the fuel cell system.At this time, the current of the lithium polymer battery increases as the current of the fuel cell decreases.Thereby, constant power is supplied to the load.
To verify the feasibility and effectiveness of the proposed voltage drop compensation method, the simulation was performed depicting the hybrid fuel cell battery system.Stable operation during hydrogen recirculation was verified by the fuel cell system and the drive experiments of the forklift.Conventional control method occurs voltage drop from 32 V to 28 V during hydrogen recirculation, which can cause fuel cell system breakdown.However, the shutdown problem was solved by using the proposed compensation method, and the operation stability of the fuel cell system depending on the various conditions was improved.Through this, the consumption of hydrogen fuel can be reduced by improving the stability of the fuel cell system, unlike in the past, when excessive hydrogen fuel was used.Given the experimental results, the proposed voltage drop compensation method could be a good candidate for commercial fuel cell systems.

Figure 1 .
Figure 1.Configuration diagram of hybrid fuel cell battery for a forklift, including hydrogen recirculation.TCU, total control unit.

Figure 1 .
Figure 1.Configuration diagram of hybrid fuel cell battery for a forklift, including hydrogen recirculation.TCU, total control unit.

Figure 2 .
Figure 2. Voltage characteristic curve of fuel cell.
is voltage drop depending on amount of hydrogen supply, tem p v Δ is voltage drop depending on temperature of a fuel cell stack, and den v Δ

Figure 2 .
Figure 2. Voltage characteristic curve of fuel cell.

Figure 3 .
Figure 3. Fuel cell stack waveforms for conventional control method during hydrogen recirculation.

Figure 3 .
Figure 3. Fuel cell stack waveforms for conventional control method during hydrogen recirculation.

Figure 3 .
Figure 3. Fuel cell stack waveforms for conventional control method during hydrogen recirculation.

Figure 4 .
Figure 4. Voltage characteristic curve depending on the amount of hydrogen supply.

Figure 4 .
Figure 4. Voltage characteristic curve depending on the amount of hydrogen supply.

Figure 5 Figure 5 .
Figure5shows the output voltage

Figure 5 .
Figure 5. Voltage characteristic curve depending on the temperature of fuel cell stack.

2 FFigure 6 .
Figure 6.Voltage characteristic curve depending on the current density of platinum catalyst.All conductors have inherent resistance, ir R , that causes the voltage drop,

Figure 6 .
Figure 6.Voltage characteristic curve depending on the current density of platinum catalyst.
the fuel cell, including recirculated hydrogen can be calculated using Equation(3).In addition, the hydro v Δ value increases as more impurities.

ForkliftFigure 7 .
Figure 7.Control block diagram of the proposed voltage drop compensation method.

Figure 7 .
Figure 7.Control block diagram of the proposed voltage drop compensation method.

Figure 8 .
Figure 8. Voltage and current waveforms with proposed voltage drop compensation method during hydrogen recirculation for (a) fuel cell stack and (b) lithium polymer battery.

Figure 8 .
Figure 8. Voltage and current waveforms with proposed voltage drop compensation method during hydrogen recirculation for (a) fuel cell stack and (b) lithium polymer battery.

Figure 9 .
Figure 9. Output power waveforms of load, fuel cell and battery.

Figure 9 .
Figure 9. Output power waveforms of load, fuel cell and battery.

Figure 11 .
Figure 11.Fuel cell waveforms for supplied hydrogen of (a) 50 LPM, (b) 60 LPM, (c) 70 LPM, and (d) voltage drop curve depending on the amount of hydrogen supply.

FFigure 12 .
Figure 12.Fuel cell waveforms for stack temperature of the (a) 40 °C, (b) 70 °C, (c) 90 °C, and (d) voltage drop curve depending on the temperature of fuel cell stack.

Table 4 .Figure 12 .
Figure 12.Fuel cell waveforms for stack temperature of the (a) 40 • C, (b) 70 • C, (c) 90 • C, and (d) voltage drop curve depending on the temperature of fuel cell stack.

Figure 13 .
Figure 13.Fuel cell waveforms for current density of the (a) 500 mA/cm 2 , (b) 700 mA/cm 2 , (c) 900 mA/cm 2 , and (d) voltage drop curve depending on current density of platinum catalyst.

Figure 13 .
Figure 13.Fuel cell waveforms for current density of the (a) 500 mA/cm 2 , (b) 700 mA/cm 2 , (c) 900 mA/cm 2 , and (d) voltage drop curve depending on current density of platinum catalyst.

Figure 13 .
Figure 13.Fuel cell waveforms for current density of the (a) 500 mA/cm 2 , (b) 700 mA/cm 2 , (c) 900 mA/cm 2 , and (d) voltage drop curve depending on current density of platinum catalyst.

Figure 14 .
Figure 14.Experiment hardware configuration of hybrid fuel cell battery system.

Figure 14 .
Figure 14.Experiment hardware configuration of hybrid fuel cell battery system.

Figure 15 .
Figure 15.Output waveforms during hydrogen recirculation (a) not applied compensation method and (b) applied proposed compensation method.

Figure 16 Figure 15 .
Figure16shows the output waveform of the lithium polymer battery.As the current of the fuel cell decreases, the lithium polymer battery supplies additional current, as shown in Figure16.Therefore, constant power can be supplied to the load.

Figure 15 .
Figure 15.Output waveforms during hydrogen recirculation (a) not applied compensation method and (b) applied proposed compensation method.

Figure 16 .
Figure 16.Output waveforms of lithium polymer battery during hydrogen recirculation.

Figure 16 .
Figure 16.Output waveforms of lithium polymer battery during hydrogen recirculation.

Figure 17 .
Figure 17.(a) The forklift with hybrid fuel cell battery system and (b) the total output current of the hybrid fuel cell battery system during forklift operation.

Figure 17 .
Figure 17.(a) The forklift with hybrid fuel cell battery system and (b) the output current of the hybrid fuel cell battery system during forklift operation.

Table 1 .
The electrical specification of the buck-boost converter.

Table 2 .
Characteristics experimental conditions of fuel cell.

Table 2 .
Characteristics experimental conditions of fuel cell.

Table 3 .
Characteristic experimental conditions of fuel cell.

Table 4 .
Characteristic experimental conditions of fuel cell.The Experiment Results for Hybrid Fuel Cell Battery System and Forklift Drive