An automatic system to test Li-ion batteries

and ultracapacitors for vehicular applications

MIRKO MARRACCI, BERNARDO TELLINI

Department of Energy and Systems Engineering

University of Pisa, Fac. Of Engineering

L.go L. Lazzarino n. 1, 56122 Pisa

ITALY

mirko.marracci@ing.unipi.it, bernardo.tellini@ing.unipi.it

Abstract: - In this paper we present an automatic system to test Li-ion batteries and ultracapacitors. The main

goal of the system is to compare performances of different storage systems during real operating conditions in

order to make possible a fair comparison.

Key-Words: - Li-ion batteries, ultracapacitors, measurement system, hybrid vehicles.

1 Introduction Li-ion batteries and ultracapacitors are commonly

used storage systems that can find application in

different fields like, for example, electric, hybrid,

and fuel cell vehicles [1]-[6]. The common

judgment on these two devices is that

ultracapacitors are more “power oriented”, while

batteries are more “energy oriented”; these terms are

used to indicate that the device operates more

efficiently with relative short (power oriented) or

long (energy oriented) charge or discharge times

respectively.

In some important applications, like hybrid vehicles,

the energy storage system is required to deliver or

absorb electric power in short intervals of time,

typically a few seconds or tens of seconds; therefore they need to have different characteristics from

batteries for pure-electric vehicles, that are

discharged in tens of minutes or hours; in this case

very-high power Li-ion batteries and ultracapacitors

can be both suitable for this kind of application.

Ragone plots [7] are commonly used for

performance comparison of various energy storing

devices; these plots report the values of energy

density (Wh/kg) versus power density (W/kg)

defining for each device family the specific

application fields. However these plots are not

rigorous, but give only trends: when reporting

individual device characteristics into family plots

often a single point per device is used.

Instead, it can be very useful to evaluate the

characteristic curve for these devices through

experimental tests, reporting energy density vs

power density, in relation to the discharge time.

Furthermore, to make a fair, useful comparison

between different storage systems, they must be

subjected to the same stress; in particular, their

specific power must be assessed using the same

discharge duration.

In this paper an automatic system to make a

comparison between a commercial ultracapacitor

and a commercial very-high power battery is

presented. The system can reproduce actual work

operating conditions that devices could find during

the normal use in hybrid vehicles.

2 Devices under test The photographs of the ultracapacitor and very-high

power battery under test are shown in Fig. 1 and

Fig. 2 respectively.

Fig.1 Photograph of the ultracapacitor under test

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ISBN: 978-1-61804-128-9 101

Fig.2 Photograph of the buttery under test

The devices under test are both commercially

available storage systems. The ultracapacitor is a 20

F 15 V system, model Maxwell BPAK0020-P015-

B01 whose manufacturer declared data are available

in [8]. The ultracapacitor is composed of six 2.5 V

modules in series with a nominal capacitance of 120

F.

The battery is one of the highest power Li-ion

batteries commercially findable; it is composed of

eight cells in series with a nominal (two-hour)

capacity of 7.2 Ah. Performance data for this battery

are available for full discharges to up to 15*Cn,

while pulse discharges may be performed to up to

20 *Cn. Manufacturer’s documentation is available

in [9].

According to this manufacturer’s data, therefore, a

single cell, whose mass is 226g, should be able to

deliver (at 15Cn regime, for about 230 s) around

1673 W/kg, (or 1460 W/kg if a overhead of 13% for

case and BMS is taken into account) that is difficult

to compare with the value of 13587 W/kg of the

considered ultracapacitor, much higher, but

available for only a split second.

These data confirm the need of a fair, useful

comparison between the two different storage

systems, defining rigorous laboratory tests analyzing

realistic, comparable operating conditions

3 Experimental setup The schematic architecture of the realized hardware

experimental setup is reported in Fig. 3.

The charging system is driven by a 1500 W Toellner

Power Supply (Model TOE 8872) interfaced to the

PC through a GPIB port. The DC power supply allows to charge the device under test keeping both

voltage or current constant. In order to enable the

execution of charging phases with linearly variable

current, a specific software iterative procedure has

been implemented.

Multimeters

Acquisition

System

Electronic

Load

Battery DC Power

Supply

Switches

Control

IEEE 488 IEEE 488

www

A

B

Fig. 3 schematic architecture of the realized hardware experimental setup

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ISBN: 978-1-61804-128-9 102

The discharging system is driven by a 6000 W

Zentro-Elektrik Electronic Load (Mod-el EL 6000).

The device allows to discharge the equipment under

test in constant current mode (I-Mode), constant

voltage mode (U-Mode), constant power mode (P-

Mode) or constant conductance mode (G-Mode) and

is fully remote controlled via GPIB standard

interface. Also in this case, in order to enable the

execution of discharging phases with linearly

variable current, a specific software iterative

procedure has been implemented.

The connection of the DC power supply or the

electronic load is driven by two switches (A and B

in Fig. 3) controlled via the implemented software

by means of digital signals. When the switch A is

closed the system is in a discharge phase and the

electronic load is controlled while, when the switch

B is closed the system is in a charge phase and the

DC power supply is controlled. Otherwise, when

both A and B switches are open the system is in a

pause phase.

The device under test is inserted in a precision

refrigerating/warming test chamber with program

control (BinderTM MK 53) ranging from T = -40

°C up to T = +180 °C. The temperature uniformity

range between ±0.8 °C (@ -40 °C) to ±2.0 °C (@ +150 °C) while the declared temperature fluctuation

is limited to ±0.3 °C for each selected temperature.

In Fig. 4 a photograph of the climatic chamber is

shown.

Fig.4 Photograph of the climatic chamber

The measuring system acquire voltage at the

terminals of the device under test directly while for the acquisition of the current a transducer is

generally required [10]-[14]. In our case a shunt (for

high currents) and a LEM current transducers (for

small currents) are used respectively.

For security reasons the acquisition system must be

able to acquire and control each cell voltage as well

as the total device voltage in order to assure that the

voltage of each cell remains into a safety range (2.7

– 4.2 V).

In Fig. 5 a photograph of the whole laboratory test

station is reported.

Fig.5 Photograph of the whole test system

Taking into account the quite high common-mode

voltage range required, signals are converted into

digital by means of three NI 9219 modules installed

on a NI CompactDAQ USB data acquisition system.

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ISBN: 978-1-61804-128-9 103

Each module features 4 simultaneous sampling

universal channels that can measure several signals

from sensors; measurement ranges differ for each

type of measurement and include up to ± 60 V for

voltage and ± 25 mA for current, with 250 Vrms

channel-to-channel isolation and 24-bit resolution;

in this way the system can acquire simultaneously

each cell voltage as well as the total voltage across

and the current through the device under test.

The acquisition system measures the ambient and

device temperature by means of two RTD (PT100)

transducers. Acquired signals can be used both to

control the ambient temperature (when the device is

inserted into the climatic chambers) or to generate

an alarm signal and stop the test when the device

temperature reach a prearranged value.

The experimental setup is managed by a dedicated

Virtual Instrument (VI), developed in a LabVIEW

environment, running on a PC. The front panel of

the realized VI during a linear current charge phase

is represented in Fig. 6

4 Results and discussion One of the main results obtained from the test to

compare Li-ion battery and ultracapacitor has been

the specific discharge powers obtained at imposed

fixed discharging currents for times typical of

vehicular applications (5-240 s).

The specific rated power for the ultracapacitor

ranges from 25 W/kg (discharge of 240 s) to 1417

W/kg (discharge of 5 s) while the specific rated

power for the Li-ion battery ranges from 1450 W/kg

(discharge of 240 s) to 2100 W/kg (discharge of 5

s).

The ultracapacitor is on the contrary advantaged

from having nearly symmetrical charge and

discharge powers while the limitation introduced to

charging to I=3*Cn is a major Li-ion battery

disadvantage. In this case the charge specific power

is constant to 330 W/kg for the battery while ranges

from 30 W/kg (charge of 240 s) to 1550 W/kg

(charge of 5 s) for the ultracapacitor.

Fig. 6 Front panel of the VI developed for the test system

Advances in Power and Energy Systems

ISBN: 978-1-61804-128-9 104

In order to make a deep comparison between

commercial ultracapacitor and very-high power

battery, reproducing actual work operating

conditions that devices could find during, for

example, the normal use in hybrid vehicles, a test of

the two systems using the New European Driving

Cycle (NEDC) [15] is currently in progress.

The New European Driving Cycle is a driving cycle

consisting of four repeated urban driving cycles

(ECE-15) and an Extra-Urban driving cycle, or

EUDC. The NEDC is supposed to represent the

typical usage of a car in Europe.

Analyzing the main drive train power fluxes for a

series hybrid vehicle performing a NEDC cycle, we

found the typical current profile required from

Rechargeable Energy Storage System (RESS) that is

represented in the following Fig. 7.

0 200 400 600 800 1000 1200-50

0

50

25

-25

Time (s)

Cu

rren

t (A

)

Fig.7 Current profile of the storage system for a

series hybrid vehicle performing a NEDC cycle.

This complex current profile reproduced in Fig. 7

can be used to analyze the storage system behavior

during a realistic stress condition. For safety

reasons, during the battery tests, the current profile

has been obtained reducing peaks of discharge and charge current of nearly 6*Cn; This limitation does

not constraint markedly vehicle operation and keep

temperatures in a perfectly acceptable range.

In Fig. 8 preliminary results are reported for the

battery system. The figure shows the total voltage

across the battery during the imposed cycle.

0 200 400 600 800 1000 120028

29

30

31

32

33

Time (s)

Volt

age

(V)

Fig.8 Voltage profile of the storage system for a

series hybrid vehicle performing a NEDC cycle.

5 Conclusions An automatic system to test and compare Li-ion

batteries and ultracapacitors has been presented. The

main goal of this system is to compare performances

of different storage systems during real operating

conditions in order to make possible a fair

comparison.

The system has been used to compare performances

of a real commercially available ultracapacitor and

high-power Li-ion battery.

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Advances in Power and Energy Systems

ISBN: 978-1-61804-128-9 105

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Advances in Power and Energy Systems

ISBN: 978-1-61804-128-9 106