Lithium Modular Battery Bank for Electric Vehicles - UC · Abstract - In electric vehicles, battery...

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Abstract - In electric vehicles, battery management is of utmost importance. Since the batteries are the most expensive and limiting factor that prevents the massive deployment of EV’s, special care must be taken to ensure that the batteries pack supply the maximum possible energy, guaranteeing also reliability and longevity of this critical element. In this article it is presented a modular battery bank intended to be used in electric vehicles, that can be easily connected to the data and control network of the vehicle in order to share all battery relevant information, namely state of charge and critical values, like low or excessive temperature or voltage. Equalization of individual cells is mandatory with Lithium chemistry and in our module this is done by dissipative method, which is of simple implementation, robust and with acceptable energy loss. With this module energy banks of the desired nominal voltage and capacity (connecting modules in series, parallel or series/parallel) can be built. Applications could range from electric bicycles to full-sized electric cars. The batteries in the bank are connected through a CAN bus network, where the communications and equalization algorithms are implemented in a simple 8 bit microcontroller. I. INTRODUCTION Electric vehicles (EV), hybrid or battery only, are considered key elements to sustainable mobility [1]. In fact, today’s mobility is based on vehicles with Internal Combustion Engine (ICE), that consume mainly a scarce source – petrol - with rising costs, supplied from politically and social instable world countries. ICE vehicles contribute heavily to city pollution (CO 2 , particles,…) and the emission of pollutants is a major contributor to the global warming effect and climate changes occurring nowadays. Battery Electric Vehicles (BEV) have, locally, zero emission of pollutants, and if electric energy is supplied by renewable sources – wind, photovoltaic or hydroelectric – they can be truly zero emission globally. Other advantages of electric vehicles are, among others, simplicity of motor construction and drive train, higher reliability and no noise emission. The main limitation of BEV is range. This restriction is directly related to the low value of specific energy of the batteries. On the other hand the ICE engine has a much lower efficiency (somewhere between 20% to 30%) when compared to an electric motor (bigger than 90%) [2]. Since the batteries are the most expensive and limiting factor that prevents the massive deployment of BEV, special care must be taken to ensure that the batteries pack supply the maximum possible energy guaranteeing reliability and longevity of this critical element. The battery bank is constituted by series of cells (grouped in modules) that must be continuously monitored – voltage, temperature and current – when supplying current to the electric motor, recovering energy from regenerative braking, or in the charging process, to ensure that they work within bounds in order to assure their longevity. The electronic system responsible for this task, built around a low cost 8 bit microcontroller, is presented is this article. It monitors a 4 cells module (equivalent to a 12V battery) and shares all the relevant information with a Master node, using CAN network [3]. The microcontroller Single Board Computer, network and protocol used in the battery bank is also described, including the global Batteries Management System (BMS). First prototypes implementation and tests are presented and the article finishes with a conclusion part. II. BATTERY BANK AND 12V MODULES A. Cells and Batteries The cells and batteries used in electric vehicles can be of diverse chemistry, being the most common the ones showed in Fig. 1 and Table 1. Fig. 1. Typical energy densities of different battery types [4] Pb (Lead Acid/Gel) batteries suffer from low energy density, high recharge time (greater than 8 to 10 hours for full charge), but they are tolerant to overcharge voltage. The Peukert effect is noticeable in these kind of batteries, lowering substantially the available energy when high currents are needed (which is true for electric vehicles). NiMH despite the good cycle life stated by manufactures (Table I), that value is usually for low discharge rates. The self discharge of 20-30% per month is also an issue [5]. Lithium batteries have high energy density, long cycle life, can supply large currents without loosing capacity (virtually no Peukert effect). As a matter of fact, the current trend in the automobile industry clearly points to the utilization of Lithium batteries. On the other hand these are less tolerant to extreme Lithium Modular Battery Bank for Electric Vehicles Luís Marques 1 , Verónica Vasconcelos 1 , Paulo G. Pereirinha 1,2,3 , João P. Trovão 1,2 1 IPC/ISEC, Polytechnic Institute of Coimbra, R. Pedro Nunes, P-3030-199 Coimbra, Portugal 2 Institute for Systems and Computers Engineering at Coimbra (INESC-Coimbra), Portugal 3 APVE, Portuguese Electric Vehicle Association. lmarques, veronica, ppereiri, [email protected]

Transcript of Lithium Modular Battery Bank for Electric Vehicles - UC · Abstract - In electric vehicles, battery...

Abstract - In electric vehicles, battery management is of utmost importance. Since the batteries are the most expensive and limiting factor that prevents the massive deployment of EV’s, special care must be taken to ensure that the batteries pack supply the maximum possible energy, guaranteeing also reliability and longevity of this critical element. In this article it is presented a modular battery bank intended to be used in electric vehicles, that can be easily connected to the data and control network of the vehicle in order to share all battery relevant information, namely state of charge and critical values, like low or excessive temperature or voltage. Equalization of individual cells is mandatory with Lithium chemistry and in our module this is done by dissipative method, which is of simple implementation, robust and with acceptable energy loss. With this module energy banks of the desired nominal voltage and capacity (connecting modules in series, parallel or series/parallel) can be built. Applications could range from electric bicycles to full-sized electric cars. The batteries in the bank are connected through a CAN bus network, where the communications and equalization algorithms are implemented in a simple 8 bit microcontroller.

I. INTRODUCTION

Electric vehicles (EV), hybrid or battery only, are

considered key elements to sustainable mobility [1]. In fact,

today’s mobility is based on vehicles with Internal Combustion

Engine (ICE), that consume mainly a scarce source – petrol -

with rising costs, supplied from politically and social instable

world countries. ICE vehicles contribute heavily to city

pollution (CO2, particles,…) and the emission of pollutants is a

major contributor to the global warming effect and climate

changes occurring nowadays.

Battery Electric Vehicles (BEV) have, locally, zero

emission of pollutants, and if electric energy is supplied by

renewable sources – wind, photovoltaic or hydroelectric – they

can be truly zero emission globally. Other advantages of

electric vehicles are, among others, simplicity of motor

construction and drive train, higher reliability and no noise

emission. The main limitation of BEV is range. This restriction

is directly related to the low value of specific energy of the

batteries. On the other hand the ICE engine has a much lower

efficiency (somewhere between 20% to 30%) when compared

to an electric motor (bigger than 90%) [2].

Since the batteries are the most expensive and limiting

factor that prevents the massive deployment of BEV, special

care must be taken to ensure that the batteries pack supply the

maximum possible energy guaranteeing reliability and

longevity of this critical element.

The battery bank is constituted by series of cells (grouped

in modules) that must be continuously monitored – voltage,

temperature and current – when supplying current to the

electric motor, recovering energy from regenerative braking,

or in the charging process, to ensure that they work within

bounds in order to assure their longevity. The electronic

system responsible for this task, built around a low cost 8 bit

microcontroller, is presented is this article. It monitors a 4

cells module (equivalent to a 12V battery) and shares all the

relevant information with a Master node, using CAN network

[3].

The microcontroller Single Board Computer, network and

protocol used in the battery bank is also described, including

the global Batteries Management System (BMS). First

prototypes implementation and tests are presented and the

article finishes with a conclusion part.

II. BATTERY BANK AND 12V MODULES

A. Cells and Batteries

The cells and batteries used in electric vehicles can be of

diverse chemistry, being the most common the ones showed in

Fig. 1 and Table 1.

Fig. 1. Typical energy densities of different battery types [4]

Pb (Lead Acid/Gel) batteries suffer from low energy

density, high recharge time (greater than 8 to 10 hours for full

charge), but they are tolerant to overcharge voltage. The

Peukert effect is noticeable in these kind of batteries, lowering

substantially the available energy when high currents are

needed (which is true for electric vehicles). NiMH despite the

good cycle life stated by manufactures (Table I), that value is

usually for low discharge rates. The self discharge of 20-30%

per month is also an issue [5].

Lithium batteries have high energy density, long cycle life,

can supply large currents without loosing capacity (virtually no

Peukert effect). As a matter of fact, the current trend in the

automobile industry clearly points to the utilization of Lithium

batteries. On the other hand these are less tolerant to extreme

Lithium Modular Battery Bank for Electric Vehicles

Luís Marques1, Verónica Vasconcelos

1, Paulo G. Pereirinha

1,2,3, João P. Trovão

1,2

1IPC/ISEC, Polytechnic Institute of Coimbra, R. Pedro Nunes, P-3030-199 Coimbra, Portugal

2Institute for Systems and Computers Engineering at Coimbra (INESC-Coimbra), Portugal

3APVE, Portuguese Electric Vehicle Association.

lmarques, veronica, ppereiri, [email protected]

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conditions, namely over or undervoltage limits. To cope with

this, electronic system must be used to exercise tight control

on voltage and cell temperature.

TABLE I

Battery Types Available for Electric Vehicles [6]

Property (Unit) Lead Acid NiMH Lithium

Cell Voltage (V) 2 1,2 3,2-3,6

Energy Density (Wh/Kg) 30-40 50-80 100-200

Power Density (W/Kg) 100-200 100-500 500-8000

Maximum Discharge Rate 6-10C 15C 100C

Useful Capacity (DOD %) 50 50-80 >80

Charging Efficiency (%) 60-80 70-90 ~100

Self Discharge (%/Month) 3-4 30 2-3

Cycle Life (Number of

Cycles)

600-900 >1000 >2000

Robust (Over/Under Voltage) Yes Yes Needs BMS

Technology Maturity Old Mature New

Price Low Medium Medium to

High

From Fig. 1, Table 1 and subsequent comments it should

be evident that Lithium battery is nowadays the available

technology that best fits the needs of electric vehicles. Among

them, one of the variants of Lithium cells, the LiFePO4

(Lithium Iron Phosphate), has a good compromise between

electrical characteristics and price [7]. Example of LiFePO4

cylindrical cells (from Headway) and discharge graph are

showed in Fig. 2 and Fig. 3.

Fig. 2. Cylindrical LiFePo4 Cells from Headway

Fig. 3. Discharge Profile of LiFePO4 cell

These Lithium cells have nominal voltage of 3.2V,

maximum charge voltage of 3.65V and cut off discharge

voltage of 2.0V. They can be charged to 5C and the maximum

discharge current is 10C. They present very good cycle life of

2000 cycles with discharge rate of 1C and 80% depth of

discharge (DOD). The energy density is 105Wh/kg and power

density 850 W/kg.

B. 12V Module and Battery bank

The battery bank can be composed of any number of 12V

modules - 4 Lithium cells in series. The 12V Module is

showed in Fig. 4.

Fig. 4. LiFePO4 12V Module Battery

The 12V module is composed of four LiFePO4 cells

connected in series (with 12,8 V nominal voltage), a thermistor

attached to each cell to sense individual temperature and a

board with an 8-bit microcontroller - the Single Board

Computer, SBC – internally named SBC2680, view figure 4 –

plus an equalization board. This SBC was designed and built

by the author’s and is used in other projects related to electric

mobility, also running in our institution [8]. For the first

version of equalization system a dissipative type was used.

The global battery bank schematic is depicted in figure 5.

Fig. 5. Modular Battery Bank (with N 12V modules)

The battery bank network architecture follows the

distributed paradigm, with all 12V modules connected using a

digital shared serial network, namely the Controller Area

Network (CAN), which is the most popular networking

technology used in the automotive domain, today. This

popularity is due to several features, namely:

• high efficiency with short data transfers;

. . .

CAN

SBC2680 Equal.

Board

CAN Bus (Vehicle data and control network)

CAN Bus (battery bank)

CAN

SBC2680 Equal.

Board

12V Module - 1 12V Module - N

CAN

SBC2680

CAN

Master

node

CS

Modular Battery Bank

+

CAN

SBC2680 Equal.

Board

-

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• medium to high information data rate (up to 1Mbit/s);

• robust error detection and automatic retransmission of

corrupted messages;

• distinction between transient and permanent errors;

• asynchronous medium access with priority arbitration;

• low cost.

The CAN protocol is, however, rather simple, which is an

advantage but also a limitation. Therefore, there are several

higher layer protocols developed to work on top of CAN and

provide extra features as needed. One such protocol, which

was selected for use in our bank, is FTT-CAN (which stands

for Flexible Time Triggered communication over CAN [9].

III. SINGLE BOARD COMPUTER AND BATTERIES

MANAGEMENT SYSTEM

The Single Board Computer - SBC2680 - is based in a

Microchip PIC18F2680 microcontroller [10]. This

microcontroller has very appealing characteristics to be used in

the 12V Battery Module, namely:

• clock speed of 40 MHz;

• internal memory of 64 kbytes;

• 24 digital I/O, that can be configured as input or

outputs;

• 10 bit ADC, with 10 multiplexed inputs;

• integrated CAN controller;

• serial UART;

• SPI communication;

• low cost.

The SBC2680 construction is modular, allowing easy

replication and adaptation to all subsystems, resulting in a

homogeneous network with simplified deployment and

management.

Fig. 6. SBC2680 module, based on Microchip 18F2680 microcontroller

In Lithium batteries the voltage of individual cells must be

monitored closely in order to not allow a voltage bigger than

maximum or lower then a minimum. If this is not assured,

irreversible damage will occur in the cell(s), rendering the

bank unusable, leading to full stop of vehicle and to big

maintenance costs.

Battery cells are never identical. Even batteries produced

in the same batch present always small differences in self-

discharge rate, capacity and impedance. Therefore,

individual cells in a battery pack will show different

voltage levels after a full charge.

The management system must guarantee that individual

cells never pass a maximum value and do not go lower than

minimum voltage. In the charging process, if any cell reaches

the maximum value, the process must be stopped and one or

more cells in the pack are not fully charged. The obvious

consequence is that the battery pack full capacity is not

available for discharge (see Fig. 7). In the discharging process,

the available capacity cannot be fully used because the output

of battery must be cut if any of the cells reaches minimum

capacity (first cell in Fig. 7.b). Even worse, with charge-

discharge cycles cell capacities start to drift, leading to a

decrease in the battery available capacity [11].

a) b)

Fig. 7. Example of charge level imbalance in the four cells of the battery

pack: a) during charge; b) during discharge

To cope with the above stated problem the battery pack

needs to be balanced. So, to avoid damaging the cells due

to imbalance in the battery pack, cell equalization is used

to reduce the difference in voltage between the cells. This

gives cells longer lifetime and more available capacity [11]

[12].

The algorithm used in the charge process and to do cell

equalization was the following:

Charge and Equalization algorithm for 12V Module

Do 20 times per second, for each cell (i=0 to 3)

1. Measure cell voltage

2. if voltage(Cell[i]) > THRESH_HIGH then bypass

Cell[i] (turn ON parallel MOSFET[i])

3. if voltage (Cell[i]) > THRESH_OVER then send

ALARM message, in order to cutt-off charger.

Do each second

1. Measure all individual cell temperature

2. if temperature(Cell[i]) > THRESHOLD_TEMP

then send ALARM message (in order to cutt-off

charger).

The action 3 in the algorithm is performed by the Master

battery node and when done it sends a message to all 12V

modules in order to disconnect all the MOSFETS and bypass

resistors.

In the discharge process the SBC2680 is responsible for

monitoring individual cells voltage and to cut-off output of

module (by sending Alarm message to the Master) if any cell

voltage goes lower than a preset minimum value.

The module will also send a CAN alarm message to the

MASTER to inform if over temperature situation occurs.

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IV. IMPLEMENTATION AND TESTS

In order to test the battery module, the algorithms to

determine the SOC and to equalize individual cells, batteries

from A123 were used, depicted in Fig. 3. These cells are a

scale down of the intended cells allowing carrying out the first

tests with the proposed system.

Fig. 8. Lithium cells used in testing of concept

To get the charge profile of cells several charge/discharge

cycles were done, being the values monitored with a PC

connected to the microcontroller trough the serial port. The

charge algorithm used constant current/constant voltage, as

recommended in the cells manual. Discharge was done using

resistive loads. In Fig. 4 the profile of charge and discharge of

cell number 1 of the 12V LiFePO4 module is shown.

Fig. 9. Charge profile of tested cell nº1 (1C)

Fig. 10. Discharge profile of tested cell nº1 (1C rate)

From the experimental results it can be seen that the

capacity of cell is 1000 mAh, as stated in the cell manual.

The system used to test the charge and discharge and

equalization algorithms is shown in Fig. 11.

The next step to test the module is to implement in the

microcontroller the algorithm to control the module charge and

the discharge limits. The implementation was done using C

language.

The Master SBC is also responsible for the State of Charge

(SOC) estimation. In the first prototype of the battery bank, the

SOC estimation was done using one of the various techniques

used in the literature, as for instance in [12]. We used

Coulomb counting plus heuristics with satisfactory results.

Fig. 11. Experimental setup

V. CONCLUSIONS AND FUTURE WORK

In this article it was presented a modular battery bank

intended to be used in electric vehicles, that connects the

various 12V modules through a CAN bus. The 12V modules

are continuously monitored by SBCs, being these also

responsible for the equalization process of individual cells.

Using the Master Node and the CAN network, the battery bank

shares the relevant information with the vehicle data and

control network. Several charge/discharge cycles were done in

order to test the proposed system. Future work involves the

design and implementation of more accurate SOC estimation

algorithms. On the other hand, techniques to minimize losses

in equalization process [13][14][15] will be evaluated in terms

of price, effectiveness, energy conservation and complexity.

VI. REFERENCES

[1] AVERE, (2011, April 20). Electronic Publication: “Battery, Hybrid and

Fuel Cell Electric vehicles key to sustainable mobility”, [Online].

Available: www.avere.org

[2] M. Ehsani, Y. Gao, and E. Amadi, Modern Electric, Hybrid Electric,

and Fuel Cell Vehicles: Fundamentals, Theory, and Design”, 2nd

Edition, Boca Raton, CRC Press, August 2009.

[3] Controller Area Network (CAN) specification – version 2.0., Bosch

GmbH, 1991.

[4] Maxim Integrated Products, Inc, (2011, April 20). Electronic

Publication: Application Note 3958. [Online]. Available:

www.maxim-ic.com

[5] J. Aditya and M. Ferdowsi, “Comparison of NiMh and Li-ion Batteries

in Automotive Applications”, IEEE Conference on Vehicle Power and

Propulsion Conference, 2008.

2

2,5

3

3,5

4

0 600 1200 1800 2400 3000 3600 4200

Cell Voltage

1C

Time (s)

2

2,5

3

3,5

4

0 600 1200 1800 2400 3000 3600 4200

Cell Voltage (V)

1C

Time (s)

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[6] Axeon. Inc, (2011, April 22). Electronic Publication: “Our Guide

Batteries”. [Online]. Available: www.axeon.com

[7] W. Jiayuan, S.Zechang and W. Xuezhe, “Performance and characteristic

research in LiFePO4 battery for electric vehicle applications”, IEEE

Vehicle Power and Propulsion Conference, September 2009.

[8] P. Pereirinha, J. Trovão, L. Marques, J. Silvestre, F. Santos, A. Campos,

M. Silva and P. Tavares, “The Electric Vehicle VEIL Project: A Modular

Platform for Research and Education”, EET-2007 European EleDrive

Conference, May – June, Brussels, Belgium, 2007.

[9] L. Almeida, P. Pedreiras, J.A. Fonseca, “The FTT-CAN protocol: Why

and how”, IEEE Transactions on Industrial Electronics, 49 (6), 2002.

[10] Microchip Technology Inc, Electronic Publication:

“PIC18F2585/2680/4585/4680 Data Sheet” [Online]. Available:

www.microchip.com

[11] J. Xu et al, “Estimation of SOC for Lithium-ion Battery Pack in Electric

Vehicle”, Int. Conf. Artificial Intelligence and Computational

Intelligence, Shanghai, China, November 07.

[12] J. Cao, N. Schofield and A. Emadi, “Battery Balancing Methods: A

Comprehensive Review”, IEEE Vehicle Power and Propulsion

Conference (VPPC), September 2008, China.

[13] K. Cheng, B. Divakar. H. Wu, K.Ding and H. Ho, “Battery

Management System (BMS) and SOC Development for Electrical

Vehicles”, IEEE Transactions on Vehicular Technology, Vol. 60, No. 1,

January 2011.

[14] J. Cao and A. Emadi, “Batteries Need Electronics”, IEEE Industrial

Electronics Magazine, March 2011.

[15] R. Lu, C. Zhu, L. Tian and Q. Wang, “Super-Capacitor Stacks

Management System With Dynamic Equalization Techniques”, IEEE

Transactions on Magnetics, Vol. 43, No. 1, January 2007.

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