Development of Microprocessor Based Capacity Estimating System for LiFePO 4 Battery

International Journal of Energy and Power (IJEP) Volume 4, 2015 www.ijep.org doi: 10.14355/ijep.2015.04.006

Development of Microprocessor Based Capacity Estimating System for LiFePO4 Battery Ming‐YuanCho*1, Chien‐HsingLee2 Department of Electrical Engineering National Kaohsiung University of Applied Sciences, Taiwan

1

Chinese Petroleum Corporation, Taiwan

2

*1,

mycho@mail.ee.kuas.edu.tw; 2242322@cpc.com.tw

Abstract This paper applies the technologies of battery capacity measurement, microprocessor/hardware, and web‐based programming to develop a prototype of Battery Capacity Estimating System (BCES). It aims to combine battery charging/discharging curves and closed loop load voltage associated with look up table method to estimate the battery residual capacity. Meanwhile, the column method is adopted to measure and calculate the incremental battery capacity during charging/discharging period.

Voltage/current extracting circuit and microprocessor‐based module are developed for its integration and testing. Finally, this web based BCES is designed to communicate with microprocessor via Modbus protocol and display battery information on its Graphical User Interface (GUI). The practicality of the proposed system has been justified by the proposed system.

Keywords LiFePO4 Battery; Closed Loop Load Voltage Method; Coulomb Method; Microprocessor; Battery Management

Introduction Amid ever‐growing technology development and increasing uses of fossil fuel, the global environment is polluted and the climate changed. More and more countries started tackling with these two problems by means of energy saving and carbon reduction and by exploring the use of renewable energy. This results in more focus on the R&D of green energy and electric vehicles which, in turn, leads to the use of batteries as the storage for powers generated by renewable energy. Thanks to its low costs and matured technology, the acid‐lead battery remains the first choice for large capacity power storage in site of the huge benefits possessed by LiFePO4 battery’s large current charging and discharging capacity over its acid‐lead counterpart [1‐2] and its fast adoption in electric vehicle and motorcycle applications. This paper is aimed at creating a good battery capacity inspection system and battery management interface for LiFePO4 battery measurement. This would enable users to get immediate battery status data including remaining battery level and capacity increment during battery charging for more efficient battery operation. This will help the fast adoption of electric vehicle (EV)and electric motorcycle(EM) with a good battery measurement information platform and accompanying peripheral devices.

Battery’s charging and discharging characteristics vary with its type which is determined by its material and internal structure. The most appropriate battery capacity measurement method is selected for individual type of battery based on its charging and discharging characteristics including specific gravity method [3], open circuit voltage method [4], closed circuit voltage method [5], table method [6], Coulomb method [7], internal resistance method [8] and the method proposed herein.

The Estimate System for Battery Residual and Incremental Capacity Structure of Battery Capacity Estimate System The battery capacity estimate system, Figure 1, employs a three‐stage switch to change the battery for charging and discharging. The front‐end battery parameter retrieval circuit captures voltage and current data and sends them to the DSPIC microprocessor based computing center. The LiFePO4 battery residual and incremental capacity estimate system (BRICES) firmware running on the latter then calculates individual parameter and estimated capacity to a PC through RS‐485 communication line for immediate web page display.

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PC Database RS485 communication Electronic load

Three stage switch Microprocessor computing

Charger

center (DSPIC)

Data retrieval circuit Voltage

current

LiFePO4 battery

Electric motorcycle

FIGURE 1: BATTERY CAPACITY ESTIMATE SYSTEM STRUCTURE

LiFePO4 Battery Specification Scores of LiFePO4 battery brands are now competing in the markets. Each supplier employs exclusive anode, electrode, and electrolyte of different material, quality, and production process, which, in turn, leads to varying performance. Actually, batteries of the same model may vary in their capacities as well. A charging and discharging test is used to learn about battery characteristics for improving battery capacity measurement accuracy based on the charging and discharging curve. Here we test one LiFePO4 battery of EV (brand A, simulate) and one LiFePO4 battery of EM (brand B), as shown in Table 1.

TABLE 1: SPECIFICATION OF LIFEPO4 BATTERY FOR EV (BRAND A, SIMULATE) AND EM (BRAND B )

Test battery

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Name

EV LiFePO4 battery (brand A)

EM LiFePO4 battery (brand B)

Nominal voltage

DC 13.2V (3.3V cell x 4)

DC 48V (3V cell x 16)

Nominal capacity

20Ah

10Ah

Charging voltage

14.6V

58.4V

Cut‐off voltage

DC 10~12V

DC 38~42V

Recommended charging current

1C

1C

Max. continuous discharging current

300A (15C)

30A (3C)

Max. instant charging current

400A<1second

33A<1second

Operation temperature

‐20~75

‐20~55

Protection device

BMS

BMS (with 40A fuse)

Weight

4.5Kg

8.5Kg

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BRICES Algorithm The BRICES estimates residual capacity of battery by means of closed loop voltage method and look up table method and incremental capacity by Coulomb method. These methods are reviewed in detail here.

1)

Battery Residual Capacity Estimate Algorithm

This algorithm discharges the battery with closed loop load voltage and gets its average voltage in a short period of discharging time. The average value is then used to get the residual battery capacity by interpolating values derived from the look up table. The voltage and capacity cross reference table is based on relation between voltage at the closed loop end and residual capacity of a battery. That is, a battery‐discharging test is needed to identify characteristics of battery discharging.

Table 2 and Table 3 show the relation of residual capacity and average end voltage of battery by measuring voltage for every discharge of capacity from 0% to 99%. The discharging amount is the average voltage output of 10~20 discharging at current of ʺ0.5Cʺ. The residual capacity estimate program is then coded in the estimated module saved in the microprocessor as shown in Figure 2.

Start

N

Charging button ON

Y Y

A motorcycle battery connected?

N

Vehicle battery

N

“No Battery” message display on LCD

Y

Motorcycle battery

Vehicle battery

Retrieve voltage and current value

A

Get actual value by calculating A/D values

Calculate average voltage

Display instant “V” and “I” on LCD

Electronic load activated for discharging

Y

N

Is current “I” > 15A?

N

0.5C current discharging

IC current discharging

Is percentage of residual capacity available in the 0.5C table

Is percentage of residual capacity available in the IC table

Y Wait for 10 sec. till voltage get steady

Wait of 1 sec.

N

Get voltage readings

N

Got 5 voltage readings?

Y

Interpolation

N

Y

Display percentage of residual capacity on LCD

Interpolation

Y A

FIGURE 2: FLOW CHART OF LIFEPO4 BATTERY RESIDUAL CAPACITY ESTIMATE PROGRAM

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TABLE 2: CROSS‐REFERENCE TABLE OF RESIDUAL CAPACITY AND END VOLTAGE FOR VEHICLE BATTERY DISCHARGING AT CURRENT OF 0.5C

殘電量(%) 99% 98% 97% 96% 95% 94% 93% 92% 91% 90% 89% 88% 87% 86% 85% 84% 83% 82% 81% 80% 79% 78% 77% 76% 75%

Residual capacity (%)

VOL(V) 12.93 12.912 12.906 12.903 12.9 12.896 12.893 12.888 12.886 12.884 12.88 12.878 12.876 12.874 12.868 12.866 12.864 12.858 12.854 12.849 12.844 12.838 12.834 12.83 12.824

殘電量(%) 74% 73% 72% 71% 70% 69% 68% 67% 66% 65% 64% 63% 62% 61% 60% 59% 58% 57% 56% 55% 54% 53% 52% 51% 50%

Residual capacity (%)

VOL(V) 12.816 12.812 12.806 12.798 12.796 12.79 12.787 12.783 12.781 12.777 12.774 12.77 12.766 12.763 12.76 12.757 12.754 12.751 12.748 12.745 12.742 12.739 12.735 12.73 12.725

殘電量(%) 49% 48% 47% 46% 45% 44% 43% 42% 41% 40% 39% 38% 37% 36% 35% 34% 33% 32% 31% 30% 29% 28% 27% 26% 25%

Residual capacity (%)

VOL(V) 12.722 12.717 12.714 12.71 12.706 12.701 12.696 12.692 12.688 12.684 12.68 12.672 12.67 12.66 12.654 12.648 12.64 12.632 12.62 12.61 12.598 12.584 12.574 12.562 12.55

殘電量(%) 24% 23% 22% 21% 20% 19% 18% 17% 16% 15% 14% 13% 12% 11% 10% 9% 8% 7% 6% 5% 4% 3% 2% 1% 0%

Residual capacity (%)

VOL(V) 12.532 12.522 12.502 12.49 12.472 12.456 12.436 12.416 12.394 12.374 12.352 12.324 12.3 12.272 12.244 12.212 12.18 12.14 12.102 12.058 12.006 11.95 11.882 11.845 11.787

TABLE 3: CROSS‐REFERENCE TABLE OF RESIDUAL CAPACITY AND END VOLTAGE FOR MOTORCYCLE BATTERY DISCHARGING AT CURRENT OF 0.5C

殘電量(%) 99% 98% 97% 96% 95% 94% 93% 92% 91% 90% 89% 88% 87% 86% 85% 84% 83% 82% 81% 80% 79% 78% 77% 76% 75%

Residual capacity (%)

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VOL(V) 52.305 52.08 52.02 51.99 51.97 51.955 51.943 51.932 51.921 51.911 51.901 51.892 51.883 51.874 51.855 51.845 51.83 51.815 51.8 51.782 51.762 51.74 51.715 51.695 51.67

殘電量(%) 74% 73% 72% 71% 70% 69% 68% 67% 66% 65% 64% 63% 62% 61% 60% 59% 58% 57% 56% 55% 54% 53% 52% 51% 50%

Residual capacity (%)

VOL(V) 51.647 51.622 51.597 51.577 51.555 51.535 51.515 51.495 51.482 51.465 51.447 51.435 51.422 51.412 51.4 51.387 51.377 51.367 51.357 51.347 51.337 51.327 51.317 51.307 51.297

殘電量(%) 49% 48% 47% 46% 45% 44% 43% 42% 41% 40% 39% 38% 37% 36% 35% 34% 33% 32% 31% 30% 29% 28% 27% 26% 25%

Residual capacity (%)

VOL(V) 51.287 51.275 51.26 51.247 51.237 51.22 51.2 51.19 51.17 51.155 51.135 51.115 51.087 51.067 51.037 51.012 50.977 50.94 50.9 50.87 50.827 50.785 50.735 50.685 50.632

殘電量(%) 24% 23% 22% 21% 20% 19% 18% 17% 16% 15% 14% 13% 12% 11% 10% 9% 8% 7% 6% 5% 4% 3% 2% 1% 0%

Residual capacity (%)

VOL(V) 50.572 50.507 50.442 50.375 50.3 50.222 50.14 50.052 49.97 49.887 49.81 49.735 49.655 49.57 49.47 49.345 49.172 48.94 48.635 48.227 47.705 47.05 46.202 45.067 43.435

International Journal of Energy and Power (IJEP) Volume 4, 2015 www.ijep.org

2)

Battery Capacity Increment Estimate Algorithm

The more accurate Coulomb method (Amp‐hour method) is employed to measure battery capacity. Identify residual capacity of battery with closed loop load voltage method and the lookup table before it gets charged.

The initial capacity (Ah) is added continuously with the Amp‐hour values to get the battery capacity increments and convert the capacity value (Ah) to actual capacity ratio (%).

In the formula below, tn represents charging time, tn+1 current time, C(tn) initial battery capacity, C(tn) current capacity, and i(tn+1) the charging current. Battery capacity is fractal accumulated and coded in the microprocessor‐computing module in the estimate flow chart shown in Figure 3 for the calculation of incremental capacity calculation during charging.

N

i n0

(1)

FIGURE 3: FLOW CHART OF LIFEPO4 BATTERY CAPACITY INCREMENT ESTIMATE PROGRAM

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Battery Capacity Measurement Hardw Are Design 1)

The Voltage‐current Getting Circuit Design

Dual getting circuit for voltage and current of vehicles and motorcycles is employed for improving measurement accuracy, as shown in Figure 4. The first retrieval circuit is designed for motorcycle LiFePO4 battery (10Ah, 48V) with voltage in range of 40V~59V and maximum current at 10A (1C). The second is designed for vehicle LiFePO4 battery (20Ah, 13.2V) with voltage in range of 11V~15V and maximum current at 40A (2C). The latter features cable diameter of 8mm2 and maximum withhold current of 60A as the fast charging current of car may reach up to 40A(2C).

CT Group 1 Group 2 GND CT

FIGURE 4: VOLTAGE‐CURRENT GETTING CIRCUIT

2)

Computing Core of Microprocessor

The module employs a microprocessor (MCU) as its computing core as shown in Figure 5. The module composes of a DSPIC33FJ256MC710 chip from Microchip as the control center. The front end gets analog signal of voltage and current with the data retrieval circuit and converts them into digital ones with the MCU embedded 12‐bit A/D converter. These digital values are then computed by the DSPIC chip for displaying on LCD module and serial communicated to PC through RS485 protocol.

FIGURE 5: COMPUTING CORE OF MICROPROCESSOR

Web based Battery Management GUI The analog voltage and current signal provided by the front‐end retrieval circuit are converted into digital ones by the MCU’s A/D converted before computing into actual voltage and current values by the DSPIC chip. These values are then sent to and saved in a database by the ModBus communication protocol through the RS485 serial communication line. Fields of the database are shown in Table 4.

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TABLE 4: FIELDS CONTAINED IN MODBUS REGISTER

Date/month Hour/minutes Year/seconds

Communicati on board code

During charging and discharging of a LiFePO4 battery the MCU identifies its type before switching the web page to vehicle or motorcycle GUI display along with instant data of voltage, current, and current residual capacity measurements. It also detects current use status and refreshes the page for battery information and charging curve every 30 seconds, shown in Figure 6 and Figure 7.

Battery measurement and management system System

Instant information Automatically update after 24 seconds

Update time:

Battery information Battery type

Current status

Vehicle LiFePO4 battery Charging

Battery voltage Battery curve Battery current Residual capacity now

Volta ge (V)

Current (A)

@ 2014. CPC Corporation Taiwan. All rights reserved.

FIGURE 6: GUI OF EV LIFEPO4 BATTERY Battery measurement and management system System

Instant information Automatically update after 24 seconds

Update time:

Battery information Battery type

Current status

Vehicle LiFePO4 battery Charging

Battery voltage Battery curve Battery current Residual capacity now

Voltage (V)

A Current (A)

@ 2014. CPC Corporation Taiwan. All rights reserved.

FIGURE 7: GUI OF EM LIFEPO4 BATTERY

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Conclusion Out of scores of battery types and characteristics this paper employs vehicle and motorcycle LiFePO4 battery for testing as both are widely adopted for their large current charging and discharging at good conversion efficiency.

This paper measures battery residual capacity and incremental capacity with closed loop load voltage accompanied by lookup table and Coulomb methods. The MCU based residual capacity and increment estimate firmware and retrieval circuit then captures the voltage‐current data and sends these battery measurements to a web based battery management GUI through Modbus communication protocol. This helps users in learning about instant charging information of the battery, that is, a good battery capacity estimate system enhances accuracy of battery capacity measurement.

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[2]

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[3]

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[4]

Aylor, J. H., Thieme, A., Johnso, and B. W., “A Battery State‐of‐charge Indicator for Electric Wheelchairs’’, IEEE Transactions on Industrial Electronics, vol.39, No.5, pp.398‐409, 1992.

[5]

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Fugang Zheng, “Management on and diagnosis for charging and residual capacity of battery of smart electric motorcycle”, Master thesis, National Sun Yat‐sen University, 2001

[8]

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Ming‐Yuan Cho*1: Professor of Department of Electrical Engineering NKUAS, Taiwan.

Chien‐Hsing Lee2: Engineer of CPC Corporation, Taiwan.

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