Hybrid-electric road vehicles -- Guidelines for charge balance measurement

ISO/TR 11955 describes procedures of charge balance measurement to ensure necessary and sufficient accuracy of a fuel consumption test on hybrid-electric vehicles (HEV) with batteries, which is conducted based on ISO 23274.

Véhicules routiers électriques hybrides -- Lignes directrices pour le mesurage de la balance de charge

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Status
Withdrawn
Publication Date
12-Oct-2008
Withdrawal Date
12-Oct-2008
Current Stage
9599 - Withdrawal of International Standard
Completion Date
13-Oct-2008
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ISO/TR 11955:2008 - Hybrid-electric road vehicles -- Guidelines for charge balance measurement
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TECHNICAL ISO/TR
REPORT 11955
First edition
2008-10-15
Hybrid-electric road vehicles —
Guidelines for charge balance
measurement
Véhicules routiers électriques hybrides — Lignes directrices pour le
mesurage de la balance de charge
Reference number
ISO/TR 11955:2008(E)
ISO 2008
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ISO/TR 11955:2008(E)
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ii © ISO 2008 – All rights reserved
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ISO/TR 11955:2008(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards

adopted by the technical committees are circulated to the member bodies for voting. Publication as an

International Standard requires approval by at least 75 % of the member bodies casting a vote.

In exceptional circumstances, when a technical committee has collected data of a different kind from that

which is normally published as an International Standard (“state of the art”, for example), it may decide by a

simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely

informative in nature and does not have to be reviewed until the data it provides are considered to be no

longer valid or useful.

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.

ISO/TR 11955 was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 21,

Electrically propelled road vehicles.
© ISO 2008 – All rights reserved iii
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ISO/TR 11955:2008(E)
Introduction

On the fuel consumption test of non-externally chargeable hybrid-electric vehicles (HEV), it is essential to

measure the charge balance in a rechargeable energy storage system (RESS) during a test period in order to

compensate the effect of energy change in a RESS on fuel consumption. ISO 23274, which defines a basic

fuel consumption test method for non-externally chargeable HEVs, does not define required accuracy on a

current measurement system but defines required accuracy on charge balance as required accuracy for the

total current measurement system; so the required accuracy of current sensor or current measuring system

for each test should be individually managed.

To investigate the required accuracy on a current measuring system is a complicated task, due to the fact that

the effect of current measurement error on fuel consumption test accuracy depends on both vehicle

characteristics and test cycle. As the charge balance is normally obtained by integrating battery current

(remainder of “accumulated value of charging current” minus “accumulated value of discharged current”) and

as the battery current is composed of intermittent huge charging current, intermittent huge discharging current

and small current with long duration time, it is necessary to pay special attention to managing the d.c. stability

in the current measurement system to keep the required accuracy.

In consideration of these backgrounds, this Technocal Report describes detailed guidelines for charge

balance measurement methods (including requirements for current measuring systems) to fulfil the required

total accuracy prescribed in ISO 23274.
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TECHNICAL REPORT ISO/TR 11955:2008(E)
Hybrid-electric road vehicles — Guidelines for charge balance
measurement
1 Scope

This Technical Report describes procedures of charge balance measurement to ensure necessary and

sufficient accuracy of a fuel consumption test on hybrid-electric vehicles (HEV) with batteries, which is

conducted based on ISO 23274 (see Bibliography).
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
charge balance
〈of a battery〉 change of charge in a battery during test period
NOTE Normally expressed in ampere-hours.
2.2
energy balance
〈of a battery〉 change of energy in a battery during test period
NOTE 1 Normally expressed in Watt-hours.

NOTE 2 For practical use, following approximate definition is made: “charge balance of battery multiplied by the

nominal voltage, normally expressed in Wh (ISO 23274)”.
2.3
energy efficiency
Wh efficiency

efficiency of the battery, based on energy for a specified charge/discharge procedure, expressed by output

energy divided by input energy
2.4
coulomb efficiency
Ah efficiency

efficiency of the battery, based on electricity (in coulomb) for a specified charge/discharge procedure,

expressed by output electricity divided by input electricity
3 Outline of error in HEV fuel consumption test

As shown in Figure 1, the relationship of fuel consumption and charge balance is estimated by the linear

regression method, using test results in a scheduled driving test, to obtain resultant fuel consumption. The

regression line is scattered by errors caused by various factors. Factors that affect the fuel consumption test

have been classified according to the following three types:
a) errors in the fuel consumption measurement;
b) errors caused by the load simulation on the chassis dynamometer;
c) errors in the charge balance measurement.
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ISO/TR 11955:2008(E)

Whereas the first two types of error scatter the regression line vertically, the third type of error scatters the line

horizontally as shown in Figure 1. Thus, the third error indirectly affects resultant fuel consumption, while the

first two errors directly affect fuel consumption.

As mentioned above, when the fuel consumption of HEVs is expressed as a linear equation in the charge

balance of the battery, ∆Q, the gradient of the regression line will be a function of the distance covered and

the average ratio of the electric power train efficiency to the ICE power train efficiency during the test period.

Consequently, the effect of the third type of error on the resultant fuel consumption will strongly depend on the

test vehicle and the test cycle. Thus, the required accuracy for charge balance measurement will be strongly

dependent on the test cycle and the characteristics of the test vehicle. So, it is important to ascertain the

required accuracy for the charge balance measurement that will ensure that the resultant fuel consumption

test for a specific test cycle and vehicle meets the required accuracy. In addition, it is important to define the

procedures for measuring current and data processing to ensure that the final result meets the required

accuracy.
Key

X charge balance per distance of battery in Watt-hours per kilometre or ampere-hours per kilometre

Y fuel consumption in litres per kilometre
1 electricty measurement error
2 fuel measurement error + load simulation error
Figure 1 — Relationship of the three error factors on tests
4 Guideline for measurement
4.1 General

Investigations into the required accuracy for charge balance measurement systems and procedures for

retaining the required accuracy are described in 4.2 to 4.4.
4.2 Normalization to reduce the effect of the test cycle

Figure 2 shows fuel consumption vs. ∆Q characteristics of an HEV on the market during the Japanese

10-15 mode and the U.S. urban dynamometer driving schedule (U.D.D.S.). The two resultant regression lines

exhibit remarkable differences in their gradients (i.e. the first-order coefficients of the linear regression lines).

2 © ISO 2008 – All rights reserved
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ISO/TR 11955:2008(E)

This fact makes it difficult to compare test results for the same vehicle in different test cycles or to check

whether the regression line of a new result is reasonable by comparing it with a standard regression line for

another test cycle.
Key
X charge balance, ∆Q in ampere-hours
Y fuel consumption in litres per kilometre
1 U.D.D.S.
2 10-15 mode
Figure 2 — Fuel consumption — ∆Q characteristics in two test modes
Key
X charge balance per distance in ampere-hours per kilometre
Y fuel consumption in litres per kilometre
1 U.D.D.S.
2 10-15 mode

Figure 3 — Fuel consumption — Charge balance per distance characteristics in two test modes

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ISO/TR 11955:2008(E)

Figure 3 shows fuel consumption vs. charge balance per distance characteristics of the HEV shown in

Figure 2. The two regression lines show no remarkable differences in their gradients, so that it is possible to

estimate the validity of a newly obtained result by comparing it to the standard regression line of another test

cycle for the HEV.

In order to discuss the accuracy of the charge balance measurement by referring to the accuracy of the fuel

consumption test, the linear regression method should be applied to the fuel consumption as a function of

charge balance per distance (∆Q/L) rather than as a function of the charge balance, ∆Q.

Physically, it indicates that the fuel consumption is not a function of the charge balance per distance [i.e.

charge balance in battery divided by distance travelled (Ah/km)] but rather that it is a function of the energy

balance per distance [energy change in battery divided by distance travelled (Wh/km)]. But the energy

efficiency of the battery (the Wh efficiency) depends on loads, and it varies dynamically corresponding to the

charging/d
...

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