Measuring Lithium-ion Polymer Cell Internal Resistance

8/17/2019 Measuring Lithium-ion Polymer Cell Internal Resistance

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Aerospace

Engineering.

Aerospace Engineering

Individual Investigative Project

Designing, Building and Testing a Lithium-

ion Polymer Battery Charger With State of

Health Monitoring

James E Stott

May 2016

Supervisor: Professor David Stone

Dissertation submitted to the University of Sheffield in partial

fulfilment of the requirements for the degree ofBachelor of Engineering


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Abstract

The explosive development of lithium-ion polymer (LiPo) batteries has revolutionised

the way we interact with technology not previously portable. In particular, remote

control vehicle users have unanimously adopted LiPo batteries as their main power

source. Despite their widespread use, the largely hobbyist nature of RC users has not

proved conducive to the adoption of battery health measurement techniques commonly

used in commercial applications that enable informed battery replacement decisions.

Consequently, RC vehicle users are incurring extra cost with unnecessary battery

replacement due to the inability to decide definitively when to replace them. To

minimise waste and cost, a method of quantifying the state of health of RC vehicle

batteries enabling informed battery replacement is of interest to users. Internal

resistance – the largest variable affecting battery performance – is the strongest

indicator of state of health and is therefore the main report consideration.

The report aim was to develop a system for RC vehicle users - governed by user

requirements for low cost and ease of use - which facilitates informed and

unambiguous battery replacement decisions. In order to achieve this, a circuit to

measure the internal resistance of a single cell LiPo battery with multiple cell LiPo

scalability was produced. With future development the circuit has the potential to yield

state of health information and integrate with current hobbyist charging products. The

project hypothesis states that with successive cell cycles, the internal resistance will

increase.

Testing and evaluation of the circuit revealed large variations in cell internal resistance

measurements, predominantly due to unstable ambient temperatures during the testing

procedure, rendering most data unusable. Despite the test data inconsistencies, trend

lines indicate a gradual increase in cell internal resistance with cycle number,

permitting the tentative conclusion that the results are congruent with the hypothesis.

The circuit will therefore fulfil the project aim with further development of the state of

health quantification feature and does meet the user requirements for low cost and ease

of use.

Overall, the project provides some promising results that indicate the aims and

objectives will be fulfilled with further testing and development of the SOH

quantification feature to enable better informed battery replacement decisions.


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Contents

Acknowledgements i

1. INTRODUCTION 1

1.1 Background 1

1.2 Motivation 1

1.3 Aims and Objectives 1

1.4 Hypothesis and Solution 2

1.5 Report Overview 2

2. LITERATURE REVIEW 4

2.1 The Impact of Internal Resistance 4

2.2 Internal Resistance Components 4

2.3 Internal Resistance Measurement Products 6

2.4 Internal Resistance Measurement Techniques 6

2.5 Battery Charging 8

2.6 Circuit Design and Operation 9

2.7 Hypothesis 10

3. LITERATURE REVIEW AND RESEARCH CONCLUSIONS 11

3.1 Literature Review Conclusions 11

3.2 Research Conclusions 12

4. DC ‘DUAL PULSE’ TECHNICAL DESCRIPTION 13

4.1 DC ‘Dual Pulse’ Theory 13

4.2 Method of Internal Resistance Measurement 14

5. CIRCUIT SPECIFICATION 155.1 User Specification 15

5.2 Operation Specification 15

6. CIRCUIT DESIGN 16

6.1 Component Selection 16

6.2 Software Design 17

6.3 Circuit Diagram 19

7. CIRCUIT TESTING 20

7.1 Testing Considerations 20

7.2 Testing Method 21


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8. TEST RESULTS AND EVALUATION 22

8.1 Test Results 22

8.2 Test Results Evaluation 24

9. CONCLUSION 26

10. FUTURE DEVELOPMENT 27

10.1 State of Health Quantification 27

10.2 Charger Integration 27

10.3 Temperature Control 27

11. PROJECT MANAGEMENT 28

11.1 Gantt Charts 28

11.2 Project Progress 28

12. SELF REVIEW 30

12.1 Project Progress 30

12.2 Personal Development 30

13. REFERENCES 31


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i

Acknowledgements

I would like to offer my sincere thanks to my project supervisor, Professor David

Stone, for his continued and boundless support throughout this project, including his

provision of sound careers and further study advice. His reassuring manner has proved

invaluable for re-focussing on objectives and re-directing efforts in times of confusion.

Thanks are also due to my second project supervisor, Dr Daniel Gladwin, for kindly

providing his signature for part orders and advice in David’s absence.


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1 INTRODUCTION

1.1 Background

The invention of the lithium-ion battery in the 1970s (Whittingham, M.S., 1976) never

looked set to transform the portable electronics field. It wasn’t until research into stable

electrode materials at The University of Oxford in 1979 (Mizushima, K. et al., 1980), that

their use in consumer electronics became a key focus. In 1991, Sony and chemical

company Asahi Kasei released the first commercial lithium-ion battery (Sony Energy

Devices Corporation, 2016), laying the foundation for a revolution in portable technology.

1.2

Motivation

A major use of lithium battery technology has emerged among amateur RC (remote

control) vehicle users. LiPo (Lithium-ion Polymer) batteries are the predominant power

source for RC planes, cars and UAVs (Unmanned Aerial Vehicles) due to their high

energy densities, large discharge currents and low weight. The phenomenon of decreasing

battery performance with successive charge cycles is recognised by RC vehicle users, but

is rarely quantified, making battery replacement decisions largely ambiguous resulting in

unnecessary expense and waste. Clearly, this presents a problem to the user. Battery SOH

(State Of Health) is a health metric encompassing parameters that contribute to

performance reductions and represents the condition of an ageing battery compared to a

new one. A key component of battery SOH is internal resistance, which directly affects

performance criteria such as discharge current and power transfer efficiency. A method of

quantifying battery SOH through internal resistance measurement is of interest to RC users

to enable unambiguous replacement decisions, minimising cost and waste.

1.3

Aims and Objectives

The aim of the project was to develop a battery charging system for RC vehicle users that

facilitates informed and unambiguous battery replacement decisions. The objectives of the

project were to design and build a LiPo charging circuit to charge and measure the internal

resistance of single cell LiPo batteries, and to test and evaluate the circuit performance. A

final objective was to demonstrate that the system is scalable to function with multiple cell

batteries and that with further development it is capable of producing SOH readings. The


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project aims and objectives are defined by user requirements for low cost and ease of use

as well as technical and time constraints limiting the breadth of research and development.

1.4 Hypothesis and Solution

The project hypothesis states that with successive cell cycles, the internal resistance will

increase to a point greater than the manufacturer defined limit, rendering the battery unable

to deliver the expected performance. The solution is based upon a widely studied battery

internal resistance measurement method called the DC ‘dual pulse’ method that utilises

Ohm’s law, and was recommended for its mathematical simplicity and ease of

implementation. The circuit comprises a microcontroller and power electronics to measure

cell internal resistance using the DC ‘dual pulse’ method, subjecting a battery to varying

loads, measuring the voltage drop and calculating the result using Ohm’s law.

1.5 Report Overview

The report opens with a literature review that compares and contrasts various literary

sources relevant to the project in order to identify past developments that can be utilised in

achieving the aims and defining the scope of the project.

Following on, chapter 3 evaluates the findings of the literature review and clearly identifieselements of previous works that will be used in the project.

Chapter 4 describes the theory and operation of the DC ‘dual pulse’ method and highlights

some considerations for the circuit design, build and test stages.

Chapter 5 defines a circuit specification to provide an easy means of evaluating the circuit

test results through comparison with the circuit specification.

Chapter 6 guides the reader through the circuit design process, introduces the circuit

diagram and shows the program code from which the reader can gain a technical

understanding of the circuit operation.

In chapter 7, considerations during the test procedure are outlined and followed by a

description of the circuit testing method.

Chapter 8 lays out the circuit test results in graphical form and analyses them for

reliability, accuracy and congruency with the circuit specification, aims and hypothesis.


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Spurious test data results are identified and their respective causes highlighted with

analysis into the sources of uncertainty and how they manifest themselves in the test data.

Chapter 9 provides a project conclusion before chapter 10 suggests areas for future

development to fully achieve the project aims and objectives, as well as suggesting

methods of reducing the uncertainty in internal resistance measurements.

Chapter 11 delivers an analysis of the project management and contrasts expected with

actual progression, allowing conclusions to be drawn as to the effectiveness of time

management and planning techniques.

Chapter 12 gives a self review of the performance and personal developments made during

the project.

Finally, chapter 13 sets out the references used throughout the report.


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2 Literature Review

The following chapter analyses existing internal resistance research and technical articles,

and discusses their relationship to the investigation in order to justify the report aim and

define the report scope. Throughout this chapter, the review is grouped into themes with

appropriate headings to effectively identify both similar and conflicting research

connections.

2.1 The Impact of Internal Resistance

Typically, designers look to maximise the power transfer efficiency between source and

load to minimise the required battery capacity and hence weight. Internal resistance

reduces power transfer efficiency and is therefore an important consideration in RC vehicle

design. Linden (2010) describes that, “The power capability of a battery is determined by

its cell voltage and its impedance” (p. 24.68). At this point, it is important to make clear

the difference between internal impedance and resistance. Internal impedance is a measure

of the opposition to current flow when a battery is subjected to a complex AC load,

whereas internal resistance is the equivalent when subjected to a simple DC load. As RC

vehicles present simple DC loads, internal resistance constitutes the main opposition to

current flow in the RC application of LiPo batteries and will remain the measurement of

interest throughout the report. Physically, internal resistance limits the flow of energy from

the battery to the load due to Ohm’s Law, which in turn curtails the deliverable power and

reduces the power transfer efficiency. Buchmann (2001) highlights the impact of internal

resistance on high current delivery, reinforcing the importance of minimising internal

resistance; “A battery with low internal resistance can deliver high current on demand; a

battery with high internal resistance cannot deliver high current” (p. 109). An article

discussing the prediction of battery performance using internal resistance states that, “Theresistance of the internal circuit path is what influences the performance of a cell and is

therefore the important parameter that needs to be measured” (Albér, n.d., p. 2).

2.2 Internal Resistance Components

Resistance or impedance – a measure of the opposition to current flow in a circuit –seems a

simple concept. However, in the context of a battery, it becomes a topic requiring greater

consideration due to multiple resistance and impedance components. Using the assumption

that battery total resistance obeys Ohm’s law and is independent of frequency, internal


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resistance can be used in battery SOH measurement. Although this is an appropriate

assumption for the DC applications in question, later chapters discuss measurements made

using complex AC loads, yielding impedances as opposed to resistances. According to

Buchmann (2001), “A battery as a power source combines ohmic, inductive and capacitive

resistances” (p. 150), which when subjected to a complex load will exhibit an internal

impedance. The various resistive components of a battery can be visualised using an

equivalent electrical circuit called the Randles model shown in figure 1. For complex AC

loads, each resistive component must be considered, as opposed to only the ohmic

resistance R O for DC loads.

Internal impedance aside, the

internal resistance of a battery is

comprised of both ohmic and

electrochemical resistances,

combining to yield the total TER

(Total Effective Resistance).

According to Albér (n.d.), the total

conductance path through a battery

includes, “the metallic or ohmic path, as well as the path that is

involved electrochemically” (p. 2), where each has an associated resistance. A technical

bulletin produced by Energizer (2005) and Linden (2010) define the resistances as the

electronic and ionic resistances respectively, which combine to yield TER. In a technical

article on internal resistance measurement by Schweiger et al. (2010), ohmic resistance is

defined as the resistivity of battery components including anode and cathode materials,

current collectors and electrolyte. Energizer (2005) attributes ionic (electrochemical)

resistance to electrochemical factors including electrolyte conductivity, ion mobility and

electrode surface area. This is in line with the definitions found in Linden (2010), Albér

(n.d.) and Buchmann (2001). The Energizer (2005) technical article makes reference to the

different development times of electronic (ohmic) and ionic (electrochemical) resistances

during discharge, which will become an important consideration when analysing internal

resistance measurement methods later in the report.

Figure 1 The Randles model showing the various

resistances, impedances and capacitances of a typical battery. Buchmann (2001). (P. 150).

R O = ohmic resistance ZW = Warburg Impedance

QC = constant phase loop R t = transfer resistanceL = inductor


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2.3 Internal Resistance Measurement Products

Despite the requirement for low internal resistance to maximise power delivery in RC

applications, a poor range of consumer measurement products is available. A hobby

website by Battery University (2011) presents several methods of internal resistance

measurement with accompanying theory, but fails to address the accuracy and reliability of

the methods so important to making good estimates of battery SOH. This is in keeping

with many other hobbyist sources located on the Internet. Turnigy – a major manufacturer

of hobbyist batteries and chargers – produces an ‘all in one’ LiPo battery checker that

includes an internal resistance measurement capability. This represents the type of unit a

typical RC hobby user might purchase to measure battery SOH. The accompanying

Turnigy (n.d.) data sheet stresses, “there is no perfect test for cell internal resistance” (p.9). It states “internal resistance can vary considerably even within the same cell, this makes

the task of determining the internal resistance a lot harder” (p. 9). The Turnigy (n.d.) data

sheet identifies that internal resistance is a function of variables including cell temperature,

charge state and age, highlighting the difficulty of accurately and reliably measuring

battery internal resistance. As the cell temperature decreases, the internal resistance

increases, becoming particularly poor around zero degrees Celsius. The lower the cell

charge, the higher the internal resistance, which also increases with cell age. Thesevariables and their relevance to the project will be considered in later sections.

2.4 Internal Resistance Measurement Techniques

According to Buchmann (2001), “several methods of measurement are available of which

the most common are applying DC loads and AC signals. Depending on the level of

capacity loss, each technique provides slightly different readings” (p. 111). Albér (n.d.)

also highlights the range of internal resistance measurement techniques available –

“Instruments presently available use either an AC current injection method or a momentary

load test” (p. 3). According to Buchmann (2001) and Schweiger et al. (2010), applying an

AC current to a battery reveals the voltage and current phase shift, allowing internal

impedance to be calculated and battery SOH to be quantified. EIS (Electrochemical

Impedance Spectroscopy) is an application of the AC current injection method. EIS “is an

effective method of analysing the mechanisms of interfacial structure and to observe the

change in formation when cycling the battery as part of everyday use” (p. 202) according

to Buchmann (2001). Schweiger et al. (2010) highlights the wide range of battery


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parameters measurable with EIS - “EIS can provide detailed information of the cell under

examination; parameters such as corrosion rate, electrochemical mechanisms, battery life

and of course internal resistance” (p. 5609). The Energizer (2005) technical bulletin

describes EIS as “an impedance test across a range of frequencies to portray internal

resistance accurately” (p. 1). Typical RC vehicle users rarely cycle batteries daily,

diminishing the usefulness of EIS to measuring the SOH of amateur RC vehicle batteries.

Internal impedance measurements obtained through EIS are highly dependent upon battery

characteristics and are not universal according to Buchmann (2001) – “Each battery type

generates its own set of signatures, and without a library of well defined reference readings

with which to compare the measurements, EIS has little meaning” (p. 203). For EIS to be

an effective method of RC vehicle battery SOH measurement, a large library of battery

reference readings must exist. Despite the in depth battery analysis EIS offers, it requires

extensive measurement equipment and is generally time consuming according to

Schweiger et al. (2010). Finally Albér (n.d.) highlights several problems with AC

measurement methods – “The problem with AC measurements is that they are susceptible

to charger ripple currents and other noise sources. Some instruments cannot be used while

the battery is on-line” (p. 3). In order to mitigate the effects of noise on internal resistance

measurement, Albér (n.d.) offers the DC load method as an alternative to AC methods due

to its use of A/D convertors capable of ignoring AC signals flowing through the battery.

“A common method of measuring internal resistance is the DC load test which applies a

discharge current to the battery whilst measuring the voltage drop” (p. 145) according to

Buchmann (2001). The voltage drop method “is a fast and convenient method for the

measurement of internal resistance” (p. 5623) according to Schweiger et al. (2010). One

DC method effectively shorts the battery to deliver maximum current for a very short

period of time, allowing an approximation of internal resistance to be made according to

Energizer (2005) and Linden (2010). However, Linden (2010) states that the ammeter

resistance must be extremely low, no more than 10% of the battery internal resistance.

Alternatively, a DC load test called the ‘dual pulse’ or ‘voltage drop’ method consecutively

applies two smaller loads to a battery and calculates the internal resistance from the known

load resistance and voltage drop. The resulting smaller discharge currents permit the use of

larger resistors less susceptible to thermal effects, improving the internal resistance

measurement accuracy. Linden (2010) writes, “a more accurate method of calculation (as

opposed to flash amps) is the voltage drop method. In this method, a small initial load is


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applied to stabilise the battery. A load approximating the application load is then applied”

(p. 9.27).

A variety of other internal resistance measurement methods are available including energy

loss and calorimetric methods according to Schweiger et al. (2010). However, these

methods do not present reliable, easily obtainable internal resistance measurements

according to the article findings.

2.5 Battery Charging

Typically, a CC/CV (constant current/constant voltage) method is employed to charge

LiPo batteries according to Buchmann (2001). Buchmann (2001) defines a complete LiPo

charge as when the upper battery voltage threshold has been reached – typically 4.1V – and

the charge current has reached 3% of the nominal charge current. Various charging circuit

assemblies can be implemented using MOSFETs, digital electronics or ‘off the shelf’ ICs

(Integrated Circuits) to achieve the CC/CV charging scheme shown in figure 2.

Figure 2 A graph showing the CC/CV LiPo charging scheme implemented in battery

chargers. It is clear from the graph that at the upper battery threshold voltage 4.1V, the

charging scheme switches from constant current to constant voltage. Buchmann (2001).


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2.6 Circuit Design and Operation

According to Linden (2010), Energizer (2005) and Buchmann (2001), various battery

conditions including temperature, state of charge and age affect internal resistance. The

hobby website produced by Battery University (2011) states that “resistance levels are

highest at low state of charge and immediately after charging.” As previously mentioned,

the internal resistance of LiPo batteries varies inversely proportional to temperature,

exhibiting high internal resistance around electrolyte freezing level. Figure 3 shows the

effect of temperature on the internal resistance of a fresh AA battery; the rapid internal

resistance increase is clearly visible

around the water-based electrolyte

freezing temperature negative twentydegrees Celsius. Therefore, an

important consideration in the circuit

design and operation is to ensure a

constant temperature during the

measurement process to yield

consistent readings. Buchmann

(2001) suggests that internal

resistance measurements taken from

a fully charged battery immediately after charging are higher than those taken several

hours after removal from the charger. Buchmann (2001) also defines a full LiPo cell cycle

as a discharge to 3V and warns that many modern batteries contain protection circuits that

can distort internal resistance measurements.

Finally, Buchmann (2001) gives insight into the method by which some battery quick

testers calculate SOH readings - “Some quick testers simulate the equipment load and

observe the voltage signature of the battery under these conditions. The readings are

compared to reference settings in the tester. The resulting discrepancies are calculated

against the anticipated or ideal settings and displayed as SOH readings.” (p. 192). In order

to quantify battery SOH it is clear that a form of trend monitoring must be implemented,

consisting of historical internal resistance measurements and manufacturer defined limits

to compare against readings.

Figure 3 A graph showing the change ininternal resistance of an AA battery with

temperature. Energizer (2005).


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2.7 Hypothesis

According to Buchmann (2001), the usage of LiPo batteries does not contribute as much to

the increase in internal resistance as ageing. However, “the wear down effects caused by

usage and aging are more pronounced in LiPo batteries” (p. 164), confirming the

hypothesis that with cycling, the internal resistance of LiPo batteries will increase. It

describes that cell oxidisation increases internal resistance and is “the ultimate cause of

failure” (p. 110). Albér (n.d.) attributes increasing battery internal resistance to cycling and

aging as corrosion, sulfation and grid growth occur within the cell, which is congruent with

the hypothesis.


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3 Literature Review and Research Conclusions

From the literature review and an analysis of the state of the art, the following chapter

draws conclusions regarding the market requirement, circuit design and hypothesis. This

chapter also highlights some issues encountered during research and makes clear the

reasons for narrowing the project aims and objectives scope.

3.1 Literature Review Conclusions

The requirement from RC vehicle users for LiPo batteries with low internal resistance is

clear, confirming that there is an opportunity to market an internal resistance measurement

circuit capable of quantifying battery SOH. A distinct lack of currently available internal

resistance measurement products offers a unique chance to introduce a useful new

technology into the RC vehicle market.

On analysis of the sources, it’s clear there is unanimous agreement that internal resistance

has a pronounced effect on the ability of a battery to deliver the required power. The

definition of SOH - a metric encompassing parameters that contribute to performance

reductions - can now be expanded to identify ‘performance reductions’ as a reduction in

the ability to deliver the required power, due to increased battery internal resistance.

The DC ‘dual pulse’ or ‘voltage drop’ method is best suited to satisfying the report aim.

Due to the non-complex nature of RC vehicle loads, internal resistance as opposed to

impedance is deemed to be an appropriate measure to quantify battery SOH. Although

used extensively in commercial applications where high accuracy is required, AC current

injection methods do not fulfil the ease of use and low cost requirements of the project.

The simplicity of the DC ‘dual pulse’ method allows readily available power electronics to

be assembled into a basic circuit, minimising cost and simplifying the measurement

process. Despite the improved accuracy of AC over other measurement methods, the

extensive equipment required and large measurement times of EIS do not lend themselves

to fulfilling the project aim and objectives. The usability of readings obtained via AC

methods, EIS in particular, are dependent upon a well-defined library of internal resistance

readings for various battery types. Suppliers of LiPo batteries used by RC vehicle users do

not currently provide the required data libraries for validating internal measurement

readings, rendering AC methods unsuitable for this project.


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As internal resistance is a function of variables including cell temperature and state of

charge, the requirement to control these variables during internal resistance measurement

has been clearly demonstrated. Therefore a major consideration during circuit operation

will be to ensure constant cell temperature and charge level to produce consistent internal

resistance measurements.

Finally, the hypothesis is backed up by several literary sources, validating the motivation

of this report to carry out the design, build and test stages.

3.2 Research Conclusions

Research into charging methods revealed that purchasing an ‘off the shelf’ LiPo battery

charging IC offered the best solution to fulfil the project aim. Although the construction of

a CC/CV charger from scratch was technically possible, the required theory and

development time proved too big a hurdle to overcome, leaving the ‘off the shelf’ solution

as the preferred option for implementing charging into the final circuit.

Despite multiple attempts to assemble an ‘off the shelf’ battery charging solution, each

attempt ended with the destruction of the charging IC. Various factors contributed to the

difficulty in producing a working LiPo battery charging circuit including limited personal

technical ability and time constraints. Therefore, it was decided to separate the battery

charging and internal resistance measurement circuits, assigning the charging circuit to

potential future system development. Consequently the scope of the project narrowed,

leaving extra time for refined and persistent work on an internal resistance measurement

circuit where the best potential for development lies. As a result of the narrowed scope, the

aim of the project became to develop a system for RC vehicle users that facilitates

informed and unambiguous battery replacement decisions, whilst objectives related to

battery charging were withdrawn.


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4 DC ‘Dual Pulse’ Technical Description

The following chapter describes the theory of the DC ‘dual pulse’ internal resistance

measurement method and the steps required to measure battery internal resistance.

4.1 DC ‘Dual Pulse’ Theory

The ‘dual pulse’ method allows the simple calculation of battery internal resistance using

Ohm’s law. When placed under a load and discharged, the battery terminal voltage will

drop due to the voltage drop across the internal ohmic resistance as governed by Ohm’s

law. Using equation 1, the battery internal resistance can be calculated from the terminal

voltage change and the known load resistance.

Both Energizer (2005) and Albér (n.d.) make a recommendation for short discharge times

to minimise the effect of polarization on internal resistance measurements. As previously

mentioned, the TER is combined from internal ohmic and ionic resistances. The effect of

ohmic resistance is seen immediately after load application, whilst the effect of ionic

resistance occurs later during the discharge period. Studying figure 4, these phenomena can

be visualised; the voltage drop dv1 occurs instantaneously and is due to the ohmic

resistance of the battery. The non-linear voltage drop dv2 occurs over a period of

milliseconds and is due to ionic resistance resulting from polarisation effects. In order to

minimise the amount of ionic resistance measured and accurately measure the ohmic

(internal) resistance, Energizer (2005) and Albér (n.d.) recommend keeping the discharge

duration between 50 and 100ms.

Figure 4 A LiPo discharge curve showing terminal voltage against time, highlighting

the effects of ohmic and ionic resistances on the terminal voltage. Linden (2010).


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Energizer (2005) and Linden (2010) recommend applying a light background load prior to

the full load application to stabilise the battery discharge and provide equilibration for

consistent measurements.

Equation 1 can be used to calculate the internal resistance of a LiPo battery using the DC

‘dual pulse’ method.

Where R in = internal resistance, ohms

V1 = stabilised closed circuit voltage, V

V2 = closed circuit voltage at load application, V

R L = application load, ohms

4.2 Method of Internal Resistance Measurement

1. Connect the cell to a light stabilisation load for a short period of time to stabilise the

discharge and provide equilibration for consistent internal resistance measurements.

2. Measure the cell terminal voltage under light load application.

3. Disconnect the light stabilisation load and connect the cell to a heavy discharge load

for a short period of time.

4. Measure the cell terminal voltage under heavy load application.

5. Disconnect the heavy discharge load and use equation 1 to calculate the cell internal

resistance from the voltage measurements and the known load resistance.

Rin = (V 1 − V 2)RL

V 2

(1) Linden (2010)


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5 Circuit Specification

The following chapter defines what the internal resistance measurement circuit should do

and how it should operate. In later chapters, it will be used as a benchmark for evaluation

of the actual test results against expected results.

5.1 User Specification

• The circuit must accurately calculate the internal resistance of a single LiPo cell and

present the internal resistance value to the user.

• The circuit must be scalable to measure the internal resistance of multiple cell LiPo

batteries.

•

The circuit must integrate easily with common LiPo battery charging technologies.

• The circuit must be easy to operate and inexpensive.

• The circuit must be able to facilitate internal resistance trend monitoring.

5.2 Operation Specification

• The circuit must apply stabilisation and application loads to the cell consecutively.

• The circuit must not discharge the cell lower than 3V.

•

The circuit must apply the stabilisation load for 20ms and the application load for

100ms.


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6 Circuit Design

The following chapter describes the circuit design process and makes the justifications for

technical and operational decisions clear. After defining the component selection, the

software design is outlined before the final circuit diagram is presented.

6.1 Component Selection

In order to fulfil the user requirements for low cost and ease of use, as well as providing

good scope for future development, the implementation of the DC ‘dual pulse’ method in

software with a microcontroller and accompanying power electronics has been selected.

The highly popular Arduino Uno microcontroller offers a wide array of input and output

ports to facilitate the scalability requirement whilst the strong program library and

supporting community offers quick and easy troubleshooting solutions. The large on board

memory also offers space to implement internal resistance trend monitoring as detailed in

the specification. The Arduino Uno receives power via the on-board USB power

connection from a PC host and is programmed in the C programming language using the

standard Arduino IDE software.

The DC ‘dual pulse’ method requires the consecutive application – switching - of two

loads in order to accurately measure cell internal resistance. Using power MOSFETs in a

switch configuration best fulfils the low cost requirement due to their widespread use and

economical purchase prices, whilst also offering good power handling capabilities able to

withstand the large discharge currents. The stabilisation and discharge currents have been

set at 0.5A and 2A respectively, requiring MOSFETs capable of handling at least 8.4W,

assuming a maximum cell voltage of 4.2V. To further reduce cost, the MOSFETs will be

logic compatible to negate the need for gate driver chips, enabling switch control directly

from the Arduino Uno.

Due to the large currents encountered during internal resistance measurement, the

discharge resistors must also possess appropriate power handling capabilities and low

temperature coefficients to maintain accurate internal resistance readings. Assuming the

stabilisation and discharge currents previously mentioned, power resistors capable of

handling at least 8.4W have been selected. The large current through the power resistors

will lead to ohmic heating, reducing the resistances. As equation 1 requires a constant load

resistance to produce accurate and consistent results, load resistors with low temperature


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coefficients have been selected, minimising the variation in load resistance over the

discharge period.

6.2 Software Design

The internal resistance measurement method described in chapter 4 has been implemented

in software in the C programming language and is shown below. Using the I/O ports and

A/D convertors of the Arduino Uno, the required load switching scheme and voltage

measurements for cell internal resistance calculations as shown in chapter 4 have been

realised in software.

int dischargeOnPin = 10; // Assign discharge switch variable to I/O port 10

int stabiliseOnPin = 11; // Assign stabilise switch variable to I/O port 11int depleteOnPin = 12; // Assign deplete switch variable to I/O port 12

void setup() {

pinMode(dischargeOnPin, OUTPUT); // Set I/O port 10 to output

pinMode(stabiliseOnPin, OUTPUT); // Set I/O port 11 to ouput

pinMode(depleteOnPin, OUTPUT); // Set I/O port 12 to output

Serial.begin(9600); // Initialise serial comms with PC

}

void loop() {

int openV = analogRead(A0); // Read the open cell voltagefloat openVoltage = openV * (5.0 / 1023.0); // Convert to floating point

if (openVoltage > 3.0) { // If the cell has not fully discharged

Serial.print(openVoltage,3); // Print the open cell voltage to the

// serial monitor

Serial.print("\t"); // Tabulate the serial monitor

digitalWrite(stabiliseOnPin, HIGH); // Start the stabilisation discharge

delay(20); // Discharge at 0.5A for 20msint stabiliseV = analogRead(A0); // Read the stabilisation discharge cell

// voltagefloat stabiliseVoltage = stabiliseV * (5.0 / 1023.0); // Convert to floating point

digitalWrite(stabiliseOnPin, LOW); // Stop the stabilisation dischargedigitalWrite(dischargeOnPin, HIGH); // Start the application load

// discharge

delay(100); // Discharge at 2A for 100ms

int dischargeV = analogRead(A0); // Read the application load

// discharge cell voltage

float dischargeVoltage = dischargeV * (5.0 / 1023.0); // Convert to floating pointdigitalWrite(dischargeOnPin, LOW); // Stop the application load

// dischargeSerial.print(((stabiliseVoltage*2)/dischargeVoltage)-2,3); // Calculate the cell internal

// resistance and print to the

// serial monitorSerial.println(); // Start a new line on the serial monitor


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digitalWrite(depleteOnPin, HIGH); // Start the depletion discharge

delay(10000); // Discharge at 2A for 10s

digitalWrite(depleteOnPin, LOW); // Stop the depletion discharge

}

else { // If the cell has fully dischargedSerial.println("Discharge Complete"); // Print discharge complete to the serial

// monitorwhile (true) { // Enter an infinite loop waiting for reset

}

}

}


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6.3 Circuit Diagram

The following circuit diagram shows the complete circuit utilised in the battery internal

resistance measurement. The host PC undertaking the data recording and the connection to

the Arduino ATmega328 microcontroller chip have been omitted for diagram

simplification. Pins with no connection are not required for the operation of the circuit.

Pins with the prefix PD are digital I/O ports used in the MOSFET switching process whilst

pin AC7 is an analogue I/O port connected to an ADC for cell terminal voltage

measurement.


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7 Circuit Testing

The following chapter highlights some testing procedure considerations and describes the

testing method.

7.1 Testing Considerations

As previously mentioned, the internal resistance of LiPo cells as well as the discharge

resistances are dependent on temperature. In order to accrue consistent and accurate

internal resistance measurements, the ambient temperature during testing must remain

constant at twenty-one degrees Celsius. However, during the component selection process,

discharge resistors with low temperature coefficients were selected and are therefore less

affected by varying temperatures. Despite the load resistors having tolerances of 1%, to

increase the reliability of internal resistance measurements the load resistances must be

measured with an accurate ohmmeter to obtain their true resistances.

As the cell internal resistance is also dependent upon the state of charge, the testing

procedure will use a fully charged LiPo cell taken straight from the charger to equilibrate

internal resistance measurements.

Finally, in order to evaluate whether the hypothesis is correct, a method of showingincreasing cell internal resistance with successive charge cycles is required. Therefore, the

bulk of the testing procedure will consist of cycling the cell, measuring the internal

resistance and discharging the cell whilst looking for an upward trend in the internal

resistance against cell cycle graph. To facilitate this, an extra MOSFET switch and

discharge resistor has been installed on the circuit as shown in the circuit diagram. This

additional circuitry will discharge the LiPo cell at 1A until empty before it is recharged and

the testing process repeats.


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7.2 Testing Method

1. Ensure the LiPo cell is fully charged.

2.

Ensure the ambient temperature is stable at twenty-one degrees Celsius.

3.

Connect the circuit to a computer and initialise serial communications and data capture.

4. Remove the cell from the charger and connect it to the circuit.

5.

Reset the circuit to start the internal resistance measurement process.

6. Monitor the discharge process until complete ensuring a consistent ambient

temperature.

7. Remove the cell from the circuit and recharge for the next cycle.

8. Record the cell cycle number and transfer the internal resistance measurements to data

processing software.


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8 Test Results and Evaluation

The following chapter presents the test data and results in graphical form before analysing

them for accuracy and reliability. Sources of outlying data points are highlighted before

conclusions are drawn as to the congruency of the results with the hypothesis, project aims

and circuit specification.

8.1 Test Results

After cycling the cell using the testing method described in chapter 7, the data collected

has been aggregated and processed into a graph at figure 5 showing the variance of average

LiPo cell internal resistance with cycle number. Due to variations in the fully charged cell

voltage prior to the test procedure commencing, it was not possible to create a graph of

fully charged cell internal resistance against cycle number. Therefore, figure 5 displays the

average LiPo cell internal resistance calculated from internal resistance measurements

taken every ten seconds during discharge.

Figure 5 A graph showing the variance in the average LiPo cell internal resistance with

cell cycle number.

Studying figure 5, the linear trend line clearly indicates an upward trend in the internal

resistance with cycle number, which is in agreement with the hypothesis and expectations.

However, it is clear from figure 5 that the internal resistance measurements become

significantly more spurious and unreliable with a large deviation beyond 35 cycles. As a

result of this spread, the weighting of the trend line beyond 35 cycles gives a false

0

0.02

0.04

0.06

0.08

0.1

0.12

0 5 10 15 20 25 30 35 40 45

I n t e r n a l R e s i s t a n c e / O h m s

Cell Cycle Number

A graph showing the variance in average LiPo cell internal resistance

with cycle nymber


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indication of increasing internal resistance with cell cycle. In order to allow reliable

conclusions to be drawn, figure 6 presents the same graph as figure 5 with the spurious

results beyond 35 cell cycles omitted. Prior to the 36th cell cycle, the LiPo cell was left

fully charged, off the charger for several days which appears to have introduced large

variations in the cell internal resistance measurements for later tests.

Figure 6 A graph showing the variance in the average LiPo cell internal resistance with

cell cycle number with data beyond 35 cell cycles omitted.

As previously mentioned, the intended plot of internal resistance measurements taken at

the fully charged cell voltage 4.1V against cell cycle number has not been possible to

draw. For comparison with figure 6, figure 7 shows the variation in internal resistance

measurements taken at the fully discharged cell voltage 3.0V against cell cycle number.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 5 10 15 20 25 30 35 I n t e r n a l R e s i s

t a n c e / O h m s

Cell Cycle Number

A graph showing the variance in average LiPo cell internal resistance

with cycle number


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Figure 7 A graph showing the variance in the fully discharged LiPo cell internal

resistance with cell cycle number with data beyond 35 cell cycles omitted.

As expected, figure 7 shows the fully discharged cell internal resistance is higher than the

average cell internal resistance shown in figure 6. This confirms that internal resistance is a

function of cell charge state and agrees with common theory that says battery internal

resistance increases with decreasing charge level.

The internal resistance measurements are presented with an uncertainty of ±0.02!,

calculated from the discharge resistor tolerance ±1%. It should be noted that this is a large

uncertainty with respect to the maximum working cell internal resistance 0.16! as detailed

in the cell data sheet. Other sources of uncertainty are difficult to quantify as they have not

been monitored throughout the project, but they are highlighted in the test results

evaluation.

8.2

Test Results Evaluation

It is difficult to draw conclusions regarding the agreement of the test results with the

hypothesis due to insufficient testing data. The lack of data is due in part to the time

consuming process of charging and discharging the LiPo cell, but the elimination of

unreliable data due to various sources of uncertainty is the largest contributor. As

previously highlighted, careful control of cell temperature and charge level during internal

resistance measurement was required to ensure consistently accurate results. In practice

however, these are difficult variables to control without great levels of attention and large

0

0.02

0.04

0.06

0.08

0.1

0 5 10 15 20 25 30 35 I n t e r n a l R e s i s t a n c e / O

h m s

Cell Cycle Number

A graph showing the variance in fully discharged LiPo cell internal

resistance with cycle nymber


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amounts of scientific equipment. The testing procedure was carried out in a converted

church, which possessed poor thermal control capability due to the nature of the building.

Therefore, large uncontrollable temperature variations occurred during the internal

resistance measurement procedure and due to the aforementioned dependency of internal

resistance on cell temperature, affected the consistency and reliability of the test results.

Despite the aforementioned sources of error and inconsistencies with the test data, the

circuit appears to be measuring cell internal resistance correctly. According to the cell data

sheet, the normal working cell internal resistance should be under 160m! for 300 cycles.

Forecasting the trend lines of figures 6 and 7 forward, according to the data the cell

internal resistance is approximately 120m! at 300 cycles, which is a surprisingly accurate

forecast considering the internal resistance measurement uncertainty of ±0.02! and lack of

data.

The gentle upward slope of the trend lines displayed in figures 6 and 7 shows increasing

internal resistance with successive cell cycles and is therefore in agreement with the

hypothesis. A significantly larger amount of test data would display the upward trend more

clearly and allow the conclusion that the test results are in line with the hypothesis to be

stated with much greater confidence.

The project aim - to develop a system for RC vehicle users that facilitates informed and

unambiguous battery replacement decisions – has been partially fulfilled by creating an

internal resistance measurement circuit, but will be completed with further development

into SOH quantification as detailed in later chapters.

The circuit does present scalability to work with multiple cell LiPo batteries thanks to the

multitude of analogue input and output ports on the Arduino microcontroller. The use of

balance leads found on many commercially available LiPo batteries facilitates multiple cell

internal resistance measurements. A simple circuit expansion and a small amount of

program code change is required to adapt the circuit to measure multiple cell LiPo battery

internal resistance.


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9 Conclusion

The ability of RC vehicle users to decide confidently when to replace their batteries is

hampered by a lack of battery health measurement products available to them, creating

unnecessary wastage and expenditure. Internal resistance is a key indicator of battery

performance and can be measured to provide RC vehicle users with an overview of the

state of health of their batteries, permitting better informed replacement decisions and

minimising cost. This project set out to find a solution to this real problem through a

design, build and test exercise. By performing thorough research of the issue at hand, a

clear set of aims and objectives was defined, permitting the design of a solution which best

serves the needs of RC vehicle users. Despite numerous project scope changes during the

design and build stages caused by technical ability and time limits, the overarching testresults suggest the main aims and objectives have been fulfilled. With further refinement

and development, the project promises to provide a legitimate marketable solution to save

RC vehicle users money.


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10 Future Development

This chapter highlights areas for future project development and briefly discusses the

reasons for developments and their practical implementation.

10.1 State of Health Quantification

As previously mentioned, battery SOH is a battery health metric encompassing parameters

which contribute to performance reductions. Although internal resistance measurement has

been identified as a key battery performance indicator, various other battery measurements

including charge acceptance and charge rate are required to yield accurate and consistent

battery SOH measurements. Furthermore, raw internal resistance readings are difficult for

the average hobbyist battery user to interpret, strengthening the case for development of asystem that produces SOH health measurements as a percentage – significantly easier for

users to understand. Through further circuit development, the production of a complex

algorithm and the implementation of battery heath trend monitoring the circuit will deliver

SOH measurements to the user, fulfilling the aim of the project.

10.2 Charger Integration

Typically, consumers prefer to purchase ‘all in one’ units as opposed to separate items.

Despite the inability of the project to deliver a combined battery charger and SOH

measurement system, the user requirements for ease of use and low cost are best fulfilled

by combining the two systems to enable the purchase and use of a single product. Further

development will reveal the best solution for integration with a charging system. Options

include utilising an ‘off the shelf’ charging IC solution, or striking a deal with

manufacturers to include the SOH measurement system with current charging solutions

available on the market.

10.3 Temperature Control

The importance and difficulty of maintaining careful temperature control during internal

resistance measurement has been continually stressed throughout this report. A significant

amount of further development is required in order to mitigate the difficulties of

temperature control for remote control vehicle users to facilitate the ease of use

requirement. Further development options include implementing an active thermal control

system or using modelling to provide some form of temperature compensation.


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11 Project Management

This chapter compares the actual project progress with the expected progress as detailed by

figures 8 and 9 and provides a summary of the overall project progress.

11.1 Gantt Charts

Figure 8 The first project Gantt chart produced 21/11/15

Figure 9 The revised Gantt chart produced 22/04/16

11.2 Project Progress

Overall, the project progress has been consistent and in line with the expectations as

planned and displayed in figure 7. With the exception of the circuit testing objective, all

parts of the project have remained on track and have been completed before or on the


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planned deadline. The extensive amount of time required to charge and discharge the LiPo

cell during testing was not foreseen and as such, twice the amount of planned time was

required to complete the testing objective.


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12 Self Review

This chapter provides some insight into my progression and performance as a final year

student and highlights areas that I have developed throughout the project.

12.1 Project Progress

Overall, I am very pleased with how the project has progressed and developed. After some

initial teething problems defining the project scope and aims, the project rapidly took shape

and allowed me to formulate some definitive objectives to work towards. The failure to

build a functioning battery charging circuit was disheartening and frustrating to begin with,

but in hindsight, was a useful mistake to make as it helped me to review my technical

capability and reduced the workload down to a more manageable size.

12.2 Personal Development

Personally, I feel this project has been thoroughly useful in developing my confidence

when posed with a challenge I know little about. It has helped me to not feel intimidated

when faced with a task requiring significant time dedication and technical ability, and to

break it down into smaller objectives with a meticulously planned approach to complete it.

My professional time and crisis management skills have benefitted significantly by facing

all of the challenges during this project and will translate well into a commercial

environment as a graduate. My technical writing and formatting skills have improved

dramatically and enabled me to approach future technical projects and papers with

increased confidence. The importance of correct and consistent referencing to maintain

project integrity has been clearly demonstrated and stands me in good stead to produce

similar high quality, reputable pieces of work in the future. Crucially, my research and

analysis skills have improved as I have learned to locate alternative sources of information

such as library textbooks and technical papers. I now understand how to perform a

literature analysis, extracting key information and assessing the source reliability and

objectivity. Overall, this final year project has been thoroughly enjoyable and invaluable in

highlighting the level of responsibility and professionalism required to undertake large

research and development tasks. I would not hesitate to use the experience I have gained as

guidance for future projects in further education and a foundation upon which to build my

professional career.


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13 References

Albér, G. (n.d.) Predicting Battery Performance Using Internal Cell Resistance. Florida,

Albércorp.

Battery University. (2011) BU-902: How to Measure Internal Resistance [online]. Canada,

Cadex. Available from: http://batteryuniversity.com/learn/article/how_to_measure_internal

_resistance [Accessed 10th April 2016].

Buchmann, I. (2001) Batteries In A Portable World. Canada, Cadex Electronics.

Energizer. (2005) Battery Internal Resistance. Technical Bulletin, [online], 1(1), 1 – 2.

Available from: http://data.energizer.com/PDFs/BatteryIR.pdf [Accessed 10th April 2016].

Linden, D and Reddy, Thomas B. (2011) Linden's Handbook Of Batteries. New York,

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Schweiger, G. H. et al. (2010) Comparison of Several Methods for Determining the

Internal Resistance of Lithium Ion Cells [online], Sensors. 10(1), 5604 – 5625. Available

from: www.mdpi.com/1424-8220/10/6/5604/pdf [Accessed 10th April 2016].

Sony Energy Devices Corporation. (2016) “Keywords to Understanding Sony Energy

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onyenergy-devices.co.jp/en/keyword/ [Accessed 10th April 2016].

Turnigy. (n.d.) Instruction Manual [online]. Hong Kong, Turnigy. Available from:

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Measuring Lithium-ion Polymer Cell Internal Resistance

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