Space systems -- Space batteries -- Guidelines for in-flight health assessment of lithium-ion batteries

This document provides detailed information on the various methods of assessing the health status of lithium-ion space batteries in flight and makes recommendations to battery suppliers, spacecraft manufacturers and operators to ease this assessment.

Systèmes spatiaux - Batteries spatiales - lignes directrices pour l'évaluation en vol de la santé des batteries lithium-ion

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Publication Date
21-Oct-2020
Current Stage
5060 - Close of voting Proof returned by Secretariat
Start Date
17-Sep-2020
Completion Date
17-Sep-2020
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TECHNICAL ISO/TR
REPORT 20891
First edition
2020-10
Space systems — Space batteries
— Guidelines for in-flight health
assessment of lithium-ion batteries
Systèmes spatiaux - Batteries spatiales - lignes directrices pour
l'évaluation en vol de la santé des batteries lithium-ion
Reference number
ISO/TR 20891:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TR 20891:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 20891:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions and abbreviated terms ................................................................................................................................ 1

3.1 Term and definitions .......................................................................................................................................................................... 1

3.2 Abbreviated terms ............................................................................................................................................................................... 1

4 Overview ....................................................................................................................................................................................................................... 2

4.1 General ........................................................................................................................................................................................................... 2

4.2 Battery capacity ..................................................................................................................................................................................... 4

4.3 Battery impedance ......... ...................................................................................................................................................................... 4

4.3.1 General...................................................................................................................................................................................... 4

4.3.2 Electrochemical impedance spectroscopy (EIS) .................................................................................. 5

4.4 Battery internal resistance ........................................................................................................................................................... 8

5 Specificities of spacecraft telemetry and resulting limitations ........................................................................11

5.1 General ........................................................................................................................................................................................................11

5.2 Signal digitization ..............................................................................................................................................................................11

5.3 Temperature ...........................................................................................................................................................................................12

5.4 Voltage .........................................................................................................................................................................................................12

5.5 Current ........................................................................................................................................................................................................12

5.6 Sampling frequency .........................................................................................................................................................................13

5.7 Synchronisation ..................................................................................................................................................................................13

5.8 On-board memory .............................................................................................................................................................................13

6 Main methods for the evaluation of battery ageing parameters .....................................................................14

6.1 Global method: fitting of a numerical model to in-flight data .....................................................................14

6.1.1 General...................................................................................................................................................................................14

6.1.2 Model structure .............................................................................................................................................................14

6.1.3 Data fitting ..........................................................................................................................................................................16

6.2 Evaluation of battery capacity ................................................................................................................................................25

6.2.1 Direct method ..................................................................................................................................................................25

6.2.2 Indirect method .............................................................................................................................................................30

6.3 Measurement of battery internal resistance ..............................................................................................................32

6.3.1 Direct internal resistance measurement ..................................................................................................32

6.3.2 Indirect measurement of battery resistance ........................................................................................34

6.3.3 Correlation of internal resistance to capacity .....................................................................................46

6.4 Measurement of battery spectral impedance ............................................................................................................47

6.4.1 General...................................................................................................................................................................................47

6.4.2 Time domain identification of a dynamic model ..............................................................................48

6.4.3 Derivation of impedance from frequency domain processing of transients .............49

6.4.4 Derivation of impedance from frequency domain processing of disturbances ......50

7 Recommendations for easing battery in-flight health assessment ..............................................................51

7.1 General recommendations.........................................................................................................................................................51

7.2 Recommendations related to battery characterization prior to flight.................................................52

7.3 Recommendations related to spacecraft telemetry performance ...........................................................52

7.4 Recommendations related to spacecraft operations...........................................................................................52

7.5 Recommendations related to data formatting ..........................................................................................................52

7.6 Recommendations related to data processing .........................................................................................................53

Bibliography .............................................................................................................................................................................................................................54

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/TR 20891:2020(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,

Subcommittee SC 14, Space systems and operations.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 20891:2020(E)
Introduction

The charge and discharge cycle of a battery is not 100 % efficient, with each cycle side reactions can

occur that eventually accumulate and cause degradation of the battery's performance. Understanding

how the battery’s performance changes throughout the mission is a subject of importance; and accurate

determination of the battery’s current SoH is essential in a large number of situations, for example:

— the routine assessment of battery performance to allow early detection of anomalies (by comparing

its actual versus predicted performance);
— the setting of alarm thresholds to ensure adequate energy;

— detection of battery anomalies that can put at risk the spacecraft passivation and/or de-orbiting

strategy;
— decisions regarding mission extension beyond initial target life;

— evaluating the remaining capability of a spacecraft upon occurrence of an anomaly;

— feedback to the battery manufacturer to improve the performance predictions.

However, it is often difficult to properly assess the in-flight status, due to various factors:

— Flight electrical load profiles differ significantly to load profiles used to characterize battery

performance models and the battery’s SoH; for example, the total available battery capacity, which is

the most important parameter, is not directly accessible during flight since its simple measurement

by full discharge of the battery goes against the spacecraft operational safety.

— The quality of the accessible data from telemetry is sometimes poor: insufficient telemetry

resolution and/or accuracy, lack of synchronization between related parameters like current and

voltage, possibly large load consumption fluctuations introducing a high level of noise, delivery of

data under a form not easy to process, etc.

— The battery is operating in flight in a way that is generally very different from the test conditions

at qualification or acceptance. As a consequence, if no in-flight assessment has been made at the

beginning of life, the direct comparison between current in-flight status and available ground

testing data can be difficult and in any case more difficult than a comparison with the initial in-

flight behaviour.

— The battery is operated under time variant conditions in a large bandwidth of different time

scales, e.g. switching heater circuits vs. variations of the charge profile and eclipse length for a LEO

satellite with drifting orbit. Low frequency variations introduced by drifting orbits or seasons are

considered for the computation of trends and averaging over several orbits.

— The processing of data to derive the health status is not straightforward and is usually performed

by identifying the ageing parameters of a model. Therefore, the representativeness of this model is

a key issue. In addition, even with a good model, the results are not always satisfactory.

Therefore, it has been found of interest to provide detailed information about the means currently

used or envisioned to perform in-flight battery health assessment and to make recommendations to

spacecraft builders, operation managers and batteries manufacturers that would make it easier. This is

the subject of this document.

It is important to highlight that, according to the definition given in 3.1.1, assessing the health status

allows to verify that the battery behaves as well as or possibly better than anticipated. It is not aimed at

providing an evaluation of any sort of “absolute ageing” nor to predict further evolution, even if this can

be the case with some methods and their on-board implementation.
© ISO 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 20891:2020(E)
Space systems — Space batteries — Guidelines for in-flight
health assessment of lithium-ion batteries

IMPORTANT — The electronic file of this document contains colours which are considered to be

useful for the correct understanding of the document. Users should therefore consider printing

this document using a colour printer.
1 Scope

This document provides detailed information on the various methods of assessing the health status

of lithium-ion space batteries in flight and makes recommendations to battery suppliers, spacecraft

manufacturers and operators to ease this assessment.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 17546, Space systems — Lithium ion battery for space vehicles — Design and verification requirements

3 Terms, definitions and abbreviated terms
3.1 Term and definitions

For the purposes of this document, the terms and definitions given in ISO 17546 and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1.1
battery health

state of the battery, which is healthy if both the amount and the rate of degradation of its performance

are lower than or equal to the predicted ones at the same time into the mission
3.2 Abbreviated terms
ADC analogue to digital converter
BoL beginning of life
CC constant current
CV constant voltage
DoD depth of discharge
EIS electrochemical impedance spectroscopy
EMF electro-motive force (a.k.a. open circuit voltage)
© ISO 2020 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/TR 20891:2020(E)
EoC end of charge
EoCV end of charge voltage
EoD end of discharge
EoDV end of discharge voltage
EoL end of life
ESA European Space Agency
GEO geostationary earth orbit
LEO low earth orbit
NCA nickel cobalt aluminium (lithium-ion cathode composition)
NIBHM non-Intrusive battery health monitoring
SoC state of charge
SoH state of health
RTL round trip loss
4 Overview
4.1 General

The SoH of a battery reflects its capability to fulfil the needs of a mission, i.e. whether the performance

is at or above the expected level. Typically, the performance itself covers requirements such as;

a) the capability to deliver and absorb a certain amount of energy, under a certain load profile and

with a defined voltage range;

b) the capability to deliver a certain power for a given duration, while maintaining a certain voltage.

Theoretically, assessing this SoH can be conceived as the simply monitoring the battery behaviour (e.g.

the battery voltage) in the orbit and comparing it against previously set baselines but, in reality, it is far

less straightforward.
Figure 1 — Schematic of energy usage in satellite battery
2 © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/TR 20891:2020(E)

As Figure 1 shows, the deliverable energy a) of a battery is dependent on the mission phase and

operational strategy. In many cases this not only shows characterizing the mission energy profile but

also includes understanding the proportion of contingency energy, i.e. the energy needed to reach a safe

mode, as part of the recovery of a major on-board failure. There is also some usable energy that will

not be used at the extremes of SoC (to mitigate the accelerated lifetime degradation that occurs with

repeated use of energy in these segment of the SoC window). As energy is not expected to be drawn

from the contingency or protective margins of the battery in nominal operating conditions, observing

energy in these segments is simply not possible during mission.

Pulsed power profiles in b) may occur for a duration too short to be captured by the telemetry. It is

thus necessary to infer the health status from only the observable data, by estimating the value of the

parameters driving the battery performance.

At a simplistic level, the capabilities of the battery can be expressed using the following fundamental

formulae:
EQ= V (1)
where
E is the battery energy (Wh);
Q is the battery capacity (Ah);
V is the nominal voltage at which the charge is delivered (V).
VV=±IR (2)
TOCV
where
V is the terminal voltage (V);
V is the open circuit voltage of the battery (V);
OCV
I is the current in (or out) of the battery (A);
R is the resistance (or internal impedance) of the battery (Ω).
PI=±VI R (3)
OCV
Where P is the power developed by the battery (W).
RR=+RR+ (4)
Ω CT dif
where
R is the ohmic (electronic) resistance (Ω);
R is the charge transfer resistance (Ω);
R is the diffusion resistance (Ω).
dif

From the formulae, it can be seen that the battery capacity and voltage (driving factor of a) above) and

internal impedance (driving factor of b)) are the main contributors to the battery performance and

© ISO 2020 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO/TR 20891:2020(E)

that the evolution of these factors through ageing leads to a reduction of both the operating voltage at a

[7]
given discharged energy and the discharge rate .

Other parameters, like self-discharge or diffusion time constant, are also quite sensitive to ageing but

have, at most, a second order influence on the performance. This does not mean, though, that they

cannot be useful indicators of the battery SoH.

It is worth noting that many parameters (such as resistance and SoC) have a temperature dependency,

which should be considered when choosing a test temperature or interpreting telemetry data.

4.2 Battery capacity

The simplest and most direct way of measuring a battery capacity is to perform a full discharge at a

known rate (the lower the current the less resistive effects that will be observed). Unfortunately, this is

usually not compatible with safe operation of the spacecraft. Therefore, the difficulty of estimating the

capacity depends on the way the battery is used on board.

On a GEO, the total number of eclipses over the lifetime is small and consequently the battery can be

used at a relatively large DoD, in the range of 70 % to 80 %. In such operating conditions, most of the

discharge curve is accessible directly via the telemetry and, given the large fraction of the orbit period

that is devoted to charging, the battery has time to reach a stable state, usually taper under almost zero

current.

Conversely, in a LEO, the very large number of eclipses forces to limit the operating DoD to values around

20 % and the quick succession of eclipse and sunlight regimes does not allow the battery to reach any

steady state (here the diffusion plays a significant role). Furthermore, due to seasonal variations of

the sun illumination and even more pronounced variations in the case of a drifting orbit, the repetitive

profile of the battery state from orbit to orbit is not even converging towards a stationary profile. It is

therefore much more difficult to observe directly a stable battery behaviour.
4.3 Battery impedance
4.3.1 General

A rechargeable intercalation battery functions by internal ion flow motivating external electron flow

(discharge) or external electron flow motivating internal ionic movement (charge). When subjecting a

cell to a flow of current, a chemical change occurs within it. This chemical change causes the build-up or

dissipation of obstacles to the current. These obstacles are known as polarizations:

a) Ohmic polarization is caused by ohmic internal resistance of the cell against the flow of the current.

This ohmic resistance (R ) consists of an electronic resistance and an ionic one.

— The electronic resistance can be seen in positive current collectors (foil and electrode terminal),

positive active materials, positive conductive materials, negative current collectors (foil and

electrode terminal) and negative active materials. Contact resistance between positive current

collector and positive active materials is also an electronic resistance because the oxide thin

layer is formed on the surface of the aluminium current collector foil. Since the positive active

material shows a characteristic in electronic conductivity similar to a semiconductor, mixed

conductive material like carbon keep electronic network in positive electrode layer. On the

other hand, negative current collectors made of copper and negative active materials made of

carbon have lower electronic resistance than positive ones.

— The ionic resistance is another component of ohmic polarization. The resistance makes obstacle

against the transfer of Li-ion and counter anion in liquid electrolyte impregnated in consecutive

micro-pores inside the positive electrode layer, negative electrode layer and porous separator.

This polarization usually has a very fast response time, i.e. in the order of milliseconds.

b) Activation polarization is the potential difference needed to generate currents depending on the

activation energy of electrode reaction. The activation energy has electrons transferred from

4 © ISO 2020 – All rights reserved
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ISO/TR 20891:2020(E)

electrodes into electrolyte or from electrolyte into electrodes. In the case of charge reaction of

positive electrode, for example, Li de-intercalated from solid active material inside is activated on

the surface of particle, and thereafter is oxidized to Li-ion. The response can be in the order of 10

milliseconds to seconds.

c) Concentration polarization denotes the voltage loss resulting from changes in the electrolyte

concentration due to a flow of electrode reaction current through the electrode/electrolyte

interface. The concentration polarization is equivalent to a difference of the thermodynamic

potential, which is a function of concentration of electrode reaction species.

d) Diffusion polarization is a kind of concentration polarization. The diffusion polarization occurs

when electrode reaction species become insufficient at the electrode surface because of slow

supply rate driven by concentration gradient. This polarization occurs at the positive and negative

electrode surface, for example, when Li transfer from active material to electrode surface through

solid phase by diffusion process, and the intercalation to active material is also accompanied by

diffusion polarization. This response can be in the order of minutes to hours.

Therefore, it is preferable to speak about internal impedance than resistance and to consider the

impedance spectrum (i.e. impedance module and phase versus frequency) as an appropriate way to get

an insight onto these various polarizations. Its evolution with ageing can be a very effective qualitative

[5][6]

health indicator . It may even, under certain conditions, allow prediction of the battery behaviour,

[9]

at least in the short to medium term . The drawback is that the classical way of measuring it, by the

sweeping in frequency of a sine excitation current, is not straightforward to implement on-board. The

internal resistance, which can be seen as a reduced image of the impedance, is easier to access and is

therefore also a parameter of interest.
4.3.2 Electrochemical impedance spectroscopy (EIS)

In a Li-ion battery, the charge and discharge mechanism relies on several electronic and ionic processes

for successful operation. These processes occur across a range of timescales from picoseconds to

minutes and hours. By analysing the response of a battery to voltage or current with respect to time

(or frequency of excitation), the behaviour of some of these processes can be separated and understood

individually. In EIS, this is done by studying the output impedance signal from an applied sinusoidal

current or voltage. The phase shift and magnitude of the output signal can then be used to determine

the impedance.

Different internal mechanisms inside the battery can be linked to specific time domains and hence

respond to specific excitation frequencies. By altering the frequency of the input current or voltage

and investigating how the resulting phase shift (and impedance) changes with input frequency, the

relationship between individual mechanisms and their individual impedances can be isolated and

understood.

As a battery ages the performance characteristics alter. The process governing these performance

characteristics can be traced back to changes in the internal electrochemical mechanisms. These

changes (and the mechanisms responsible for them) can be observed via the changing impedance vs.

frequency relationship with lifetime.

Typically, these results are displayed in a Nyquist plot where the impedance is separated into the real

and imaginary components and the two components plotted on the X and Y axis respectively, as shown

in Figure 2.
© ISO 2020 – All rights reserved 5
---------------------- Page: 10 ----------------------
ISO/TR 20891:2020(E)
Key
X real (Z), mΩ
Y imaginary (Z), mΩ
1 equivalent circuit
2 ohmic resistance (R ) 0 phase shift (i.e. DC)
3 charge transfer (R ) and double layer (C ) region, Hz
ct dl
4 Warburg impedance (Z ) solid state diffusion region, mHz
Figure 2 — Schematic of a Nyquist impedance plot

The key attributes of the battery can then be observed, and an equivalent circuit can be used to map the

attributes to the physical behaviour inside the battery. Figure 3 illustrates schematically.

Over time these attributes may change. This may be due to several factors such as;

— the introduction of surface layers, slowing down to electrolyte /electrode transfer, increasing

resistance;

— poorer electrical connection between the electrode particles, increasing the electrical re

...

TECHNICAL ISO/TR
REPORT 20891
First edition
Space systems — Space batteries
— Guidelines for in-flight health
assessment of lithium-ion batteries
PROOF/ÉPREUVE
Reference number
ISO/TR 20891:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TR 20891:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 20891:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions and abbreviated terms ................................................................................................................................ 1

3.1 Term and definitions .......................................................................................................................................................................... 1

3.2 Abbreviated terms ............................................................................................................................................................................... 1

4 Overview ....................................................................................................................................................................................................................... 2

4.1 General ........................................................................................................................................................................................................... 2

4.2 Battery capacity ..................................................................................................................................................................................... 4

4.3 Battery impedance ......... ...................................................................................................................................................................... 4

4.3.1 General...................................................................................................................................................................................... 4

4.3.2 Electrochemical impedance spectroscopy (EIS) .................................................................................. 5

4.4 Battery internal resistance ........................................................................................................................................................... 8

5 Specificities of spacecraft telemetry and resulting limitations ........................................................................11

5.1 General ........................................................................................................................................................................................................11

5.2 Signal digitization ..............................................................................................................................................................................11

5.3 Temperature ...........................................................................................................................................................................................12

5.4 Voltage .........................................................................................................................................................................................................12

5.5 Current ........................................................................................................................................................................................................12

5.6 Sampling frequency .........................................................................................................................................................................13

5.7 Synchronisation ..................................................................................................................................................................................13

5.8 On-board memory .............................................................................................................................................................................13

6 Main methods for the evaluation of battery ageing parameters .....................................................................14

6.1 Global method: fitting of a numerical model to in-flight data .....................................................................14

6.1.1 General...................................................................................................................................................................................14

6.1.2 Model structure .............................................................................................................................................................14

6.1.3 Data fitting ..........................................................................................................................................................................16

6.2 Evaluation of battery capacity ................................................................................................................................................25

6.2.1 Direct method ..................................................................................................................................................................25

6.2.2 Indirect method .............................................................................................................................................................30

6.3 Measurement of battery internal resistance ..............................................................................................................32

6.3.1 Direct internal resistance measurement ..................................................................................................32

6.3.2 Indirect measurement of battery resistance ........................................................................................34

6.3.3 Correlation of internal resistance to capacity .....................................................................................46

6.4 Measurement of battery spectral impedance ............................................................................................................47

6.4.1 General...................................................................................................................................................................................47

6.4.2 Time domain identification of a dynamic model ..............................................................................48

6.4.3 Derivation of impedance from frequency domain processing of transients .............49

6.4.4 Derivation of impedance from frequency domain processing of disturbances ......50

7 Recommendations for easing battery in-flight health assessment ..............................................................51

7.1 General recommendations.........................................................................................................................................................51

7.2 Recommendations related to battery characterization prior to flight.................................................52

7.3 Recommendations related to spacecraft telemetry performance ...........................................................52

7.4 Recommendations related to spacecraft operations...........................................................................................52

7.5 Recommendations related to data formatting ..........................................................................................................52

7.6 Recommendations related to data processing .........................................................................................................53

Bibliography .............................................................................................................................................................................................................................54

© ISO 2020 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO/TR 20891:2020(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,

Subcommittee SC 14, Space systems and operations.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 20891:2020(E)
Introduction

The charge and discharge cycle of a battery is not 100 % efficient, with each cycle side reactions can

occur that eventually accumulate and cause degradation of the battery's performance. Understanding

how the battery’s performance changes throughout the mission is a subject of importance; and accurate

determination of the battery’s current SoH is essential in a large number of situations, for example:

— the routine assessment of battery performance to allow early detection of anomalies (by comparing

its actual versus predicted performance);
— the setting of alarm thresholds to ensure adequate energy;

— detection of battery anomalies that can put at risk the spacecraft passivation and/or de-orbiting

strategy;
— decisions regarding mission extension beyond initial target life;

— evaluating the remaining capability of a spacecraft upon occurrence of an anomaly;

— feedback to the battery manufacturer to improve the performance predictions.

However, it is often difficult to properly assess the in-flight status, due to various factors:

— Flight electrical load profiles differ significantly to load profiles used to characterize battery

performance models and the battery’s SoH; for example, the total available battery capacity, which is

the most important parameter, is not directly accessible during flight since its simple measurement

by full discharge of the battery goes against the spacecraft operational safety.

— The quality of the accessible data from telemetry is sometimes poor: insufficient telemetry

resolution and/or accuracy, lack of synchronization between related parameters like current and

voltage, possibly large load consumption fluctuations introducing a high level of noise, delivery of

data under a form not easy to process, etc.

— The battery is operating in flight in a way that is generally very different from the test conditions

at qualification or acceptance. As a consequence, if no in-flight assessment has been made at the

beginning of life, the direct comparison between current in-flight status and available ground

testing data can be difficult and in any case more difficult than a comparison with the initial in-

flight behaviour.

— The battery is operated under time variant conditions in a large bandwidth of different time

scales, e.g. switching heater circuits vs. variations of the charge profile and eclipse length for a LEO

satellite with drifting orbit. Low frequency variations introduced by drifting orbits or seasons are

considered for the computation of trends and averaging over several orbits.

— The processing of data to derive the health status is not straightforward and is usually performed

by identifying the ageing parameters of a model. Therefore, the representativeness of this model is

a key issue. In addition, even with a good model, the results are not always satisfactory.

Therefore, it has been found of interest to provide detailed information about the means currently

used or envisioned to perform in-flight battery health assessment and to make recommendations to

spacecraft builders, operation managers and batteries manufacturers that would make it easier. This is

the subject of this document.

It is important to highlight that, according to the definition given in 3.1.1, assessing the health status

allows to verify that the battery behaves as well as or possibly better than anticipated. It is not aimed at

providing an evaluation of any sort of “absolute ageing” nor to predict further evolution, even if this can

be the case with some methods and their on-board implementation.
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TECHNICAL REPORT ISO/TR 20891:2020(E)
Space systems — Space batteries — Guidelines for in-flight
health assessment of lithium-ion batteries

IMPORTANT — The electronic file of this document contains colours which are considered to be

useful for the correct understanding of the document. Users should therefore consider printing

this document using a colour printer.
1 Scope

This document provides detailed information on the various methods of assessing the health status

of lithium-ion space batteries in flight and makes recommendations to battery suppliers, spacecraft

manufacturers and operators to ease this assessment.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 17546, Space systems — Lithium ion battery for space vehicles — Design and verification requirements

3 Terms, definitions and abbreviated terms
3.1 Term and definitions

For the purposes of this document, the terms and definitions given in ISO 17546 and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1.1
battery health

state of the battery, which is healthy if both the amount and the rate of degradation of its performance

are lower than or equal to the predicted ones at the same time into the mission
3.2 Abbreviated terms
ADC analogue to digital converter
BoL beginning of life
CC constant current
CV constant voltage
DoD depth of discharge
EIS electrochemical impedance spectroscopy
EMF electro-motive force (a.k.a. open circuit voltage)
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ISO/TR 20891:2020(E)
EoC end of charge
EoCV end of charge voltage
EoD end of discharge
EoDV end of discharge voltage
EoL end of life
ESA European Space Agency
GEO geostationary earth orbit
LEO low earth orbit
NCA nickel cobalt aluminium (lithium-ion cathode composition)
NIBHM non-Intrusive battery health monitoring
SoC state of charge
SoH state of health
RTL round trip loss
4 Overview
4.1 General

The SoH of a battery reflects its capability to fulfil the needs of a mission, i.e. whether the performance

is at or above the expected level. Typically, the performance itself covers requirements such as;

a) the capability to deliver and absorb a certain amount of energy, under a certain load profile and

with a defined voltage range;

b) the capability to deliver a certain power for a given duration, while maintaining a certain voltage.

Theoretically, assessing this SoH can be conceived as the simply monitoring the battery behaviour (e.g.

the battery voltage) in the orbit and comparing it against previously set baselines but, in reality, it is far

less straightforward.
Figure 1 — Schematic of energy usage in satellite battery
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ISO/TR 20891:2020(E)

As Figure 1 shows, the deliverable energy a) of a battery is dependent on the mission phase and

operational strategy. In many cases this not only shows characterizing the mission energy profile but

also includes understanding the proportion of contingency energy, i.e. the energy needed to reach a safe

mode, as part of the recovery of a major on-board failure. There is also some usable energy that will

not be used at the extremes of SoC (to mitigate the accelerated lifetime degradation that occurs with

repeated use of energy in these segment of the SoC window). As energy is not expected to be drawn

from the contingency or protective margins of the battery in nominal operating conditions, observing

energy in these segments is simply not possible during mission.

Pulsed power profiles in b) may occur for a duration too short to be captured by the telemetry. It is

thus necessary to infer the health status from only the observable data, by estimating the value of the

parameters driving the battery performance.

At a simplistic level, the capabilities of the battery can be expressed using the following fundamental

formulae:
EQ= V (1)
where
E is the battery energy (Wh);
Q is the battery capacity (Ah);
V is the nominal voltage at which the charge is delivered (V).
VV=±IR (2)
TOCV
where
V is the terminal voltage (V);
V is the open circuit voltage of the battery (V);
OCV
I is the current in (or out) of the battery (A);
R is the resistance (or internal impedance) of the battery (Ω).
PI=±VI R (3)
OCV
Where P is the power developed by the battery (W).
RR=+RR+ (4)
Ω CT dif
where
R is the ohmic (electronic) resistance (Ω);
R is the charge transfer resistance (Ω);
R is the diffusion resistance (Ω).
dif

From the formulae, it can be seen that the battery capacity and voltage (driving factor of a) above) and

internal impedance (driving factor of b)) are the main contributors to the battery performance and

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ISO/TR 20891:2020(E)

that the evolution of these factors through ageing leads to a reduction of both the operating voltage at a

[7]
given discharged energy and the discharge rate .

Other parameters, like self-discharge or diffusion time constant, are also quite sensitive to ageing but

have, at most, a second order influence on the performance. This does not mean, though, that they

cannot be useful indicators of the battery SoH.

It is worth noting that many parameters (such as resistance and SoC) have a temperature dependency,

which should be considered when choosing a test temperature or interpreting telemetry data.

4.2 Battery capacity

The simplest and most direct way of measuring a battery capacity is to perform a full discharge at a

known rate (the lower the current the less resistive effects that will be observed). Unfortunately, this is

usually not compatible with safe operation of the spacecraft. Therefore, the difficulty of estimating the

capacity depends on the way the battery is used on board.

On a GEO, the total number of eclipses over the lifetime is small and consequently the battery can be

used at a relatively large DoD, in the range of 70 % to 80 %. In such operating conditions, most of the

discharge curve is accessible directly via the telemetry and, given the large fraction of the orbit period

that is devoted to charging, the battery has time to reach a stable state, usually taper under almost zero

current.

Conversely, in a LEO, the very large number of eclipses forces to limit the operating DoD to values around

20 % and the quick succession of eclipse and sunlight regimes does not allow the battery to reach any

steady state (here the diffusion plays a significant role). Furthermore, due to seasonal variations of

the sun illumination and even more pronounced variations in the case of a drifting orbit, the repetitive

profile of the battery state from orbit to orbit is not even converging towards a stationary profile. It is

therefore much more difficult to observe directly a stable battery behaviour.
4.3 Battery impedance
4.3.1 General

A rechargeable intercalation battery functions by internal ion flow motivating external electron flow

(discharge) or external electron flow motivating internal ionic movement (charge). When subjecting a

cell to a flow of current, a chemical change occurs within it. This chemical change causes the build-up or

dissipation of obstacles to the current. These obstacles are known as polarizations:

a) Ohmic polarization is caused by ohmic internal resistance of the cell against the flow of the current.

This ohmic resistance (R ) consists of an electronic resistance and an ionic one.

— The electronic resistance can be seen in positive current collectors (foil and electrode terminal),

positive active materials, positive conductive materials, negative current collectors (foil and

electrode terminal) and negative active materials. Contact resistance between positive current

collector and positive active materials is also an electronic resistance because the oxide thin

layer is formed on the surface of the aluminium current collector foil. Since the positive active

material shows a characteristic in electronic conductivity similar to a semiconductor, mixed

conductive material like carbon keep electronic network in positive electrode layer. On the

other hand, negative current collectors made of copper and negative active materials made of

carbon have lower electronic resistance than positive ones.

— The ionic resistance is another component of ohmic polarization. The resistance makes obstacle

against the transfer of Li-ion and counter anion in liquid electrolyte impregnated in consecutive

micro-pores inside the positive electrode layer, negative electrode layer and porous separator.

This polarization usually has a very fast response time, i.e. in the order of milliseconds.

b) Activation polarization is the potential difference needed to generate currents depending on the

activation energy of electrode reaction. The activation energy has electrons transferred from

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electrodes into electrolyte or from electrolyte into electrodes. In the case of charge reaction of

positive electrode, for example, Li de-intercalated from solid active material inside is activated on

the surface of particle, and thereafter is oxidized to Li-ion. The response can be in the order of 10

milliseconds to seconds.

c) Concentration polarization denotes the voltage loss resulting from changes in the electrolyte

concentration due to a flow of electrode reaction current through the electrode/electrolyte

interface. The concentration polarization is equivalent to a difference of the thermodynamic

potential, which is a function of concentration of electrode reaction species.

d) Diffusion polarization is a kind of concentration polarization. The diffusion polarization occurs

when electrode reaction species become insufficient at the electrode surface because of slow

supply rate driven by concentration gradient. This polarization occurs at the positive and negative

electrode surface, for example, when Li transfer from active material to electrode surface through

solid phase by diffusion process, and the intercalation to active material is also accompanied by

diffusion polarization. This response can be in the order of minutes to hours.

Therefore, it is preferable to speak about internal impedance than resistance and to consider the

impedance spectrum (i.e. impedance module and phase versus frequency) as an appropriate way to get

an insight onto these various polarizations. Its evolution with ageing can be a very effective qualitative

[5][6]

health indicator . It may even, under certain conditions, allow prediction of the battery behaviour,

[9]

at least in the short to medium term . The drawback is that the classical way of measuring it, by the

sweeping in frequency of a sine excitation current, is not straightforward to implement on-board. The

internal resistance, which can be seen as a reduced image of the impedance, is easier to access and is

therefore also a parameter of interest.
4.3.2 Electrochemical impedance spectroscopy (EIS)

In a Li-ion battery, the charge and discharge mechanism relies on several electronic and ionic processes

for successful operation. These processes occur across a range of timescales from picoseconds to

minutes and hours. By analysing the response of a battery to voltage or current with respect to time

(or frequency of excitation), the behaviour of some of these processes can be separated and understood

individually. In EIS, this is done by studying the output impedance signal from an applied sinusoidal

current or voltage. The phase shift and magnitude of the output signal can then be used to determine

the impedance.

Different internal mechanisms inside the battery can be linked to specific time domains and hence

respond to specific excitation frequencies. By altering the frequency of the input current or voltage

and investigating how the resulting phase shift (and impedance) changes with input frequency, the

relationship between individual mechanisms and their individual impedances can be isolated and

understood.

As a battery ages the performance characteristics alter. The process governing these performance

characteristics can be traced back to changes in the internal electrochemical mechanisms. These

changes (and the mechanisms responsible for them) can be observed via the changing impedance vs.

frequency relationship with lifetime.

Typically, these results are displayed in a Nyquist plot where the impedance is separated into the real

and imaginary components and the two components plotted on the X and Y axis respectively, as shown

in Figure 2.
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ISO/TR 20891:2020(E)
Key
X real (Z), mΩ
Y imaginary (Z), mΩ
1 equivalent circuit
2 ohmic resistance (R ) 0 phase shift (i.e. DC)
3 charge transfer (R ) and double layer (C ) region, Hz
ct dl
4 Warburg impedance (Z ) solid state diffusion region, mHz
Figure 2 — Schematic of a Nyquist impedance plot

The key attributes of the battery can then be observed, and an equivalent circuit can be used to map the

attributes to the physical behaviour inside the battery. Figure 3 illustrates schematically.

Over time these attributes may change. This may be due to several factors such as;

— the introduction of surface layers, slowing down to electrolyte /electrode transfer, increasing

resistance;

— poorer electrical connection between the electrode particles, increasing the electrical res

...

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