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
Standards Content (sample)
Hybrid-electric road vehicles —
Guidelines for charge balance
Véhicules routiers électriques hybrides — Lignes directrices pour le
mesurage de la balance de charge
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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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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 systemfor 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. stabilityin 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 requiredtotal accuracy prescribed in ISO 23274.
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TECHNICAL REPORT ISO/TR 11955:2008(E)
Hybrid-electric road vehicles — Guidelines for charge balance
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 isconducted based on ISO 23274 (see Bibliography).
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
〈of a battery〉 change of charge in a battery during test period
NOTE Normally expressed in ampere-hours.
〈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 thenominal voltage, normally expressed in Wh (ISO 23274)”.
efficiency of the battery, based on energy for a specified charge/discharge procedure, expressed by outputenergy divided by input energy
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 testhave 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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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 thefirst 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 requiredaccuracy.
X charge balance per distance of battery in Watt-hours per kilometre or ampere-hours per kilometreY 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
Investigations into the required accuracy for charge balance measurement systems and procedures forretaining 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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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 foranother test cycle.
X charge balance, ∆Q in ampere-hours
Y fuel consumption in litres per kilometre
2 10-15 mode
Figure 2 — Fuel consumption — ∆Q characteristics in two test modes
X charge balance per distance in ampere-hours per kilometre
Y fuel consumption in litres per kilometre
2 10-15 mode
Figure 3 — Fuel consumption — Charge balance per distance characteristics in two test modes© ISO 2008 – All rights reserved 3
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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 testcycle 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 thecharging/d