Performance and durability fields: capacity, efficiency, resistance and cycle life in a passport

Performance and durability fields record how a battery model performs under stated test conditions. Annex IV sets out electrochemical performance and durability parameters for rechargeable industrial batteries, LMT batteries and EV batteries, and Annex XIII point 1 requires performance and durability information in the public model summary. The fields include rated capacity, voltage, energy, round trip energy efficiency at beginning of life and at a stated point in cycle life, internal resistance, and the C rate and conditions of the cycle life test.

Each value is a test result. Each test result is meaningful only with its conditions. A capacity figure without temperature, discharge rate and end of discharge conditions is a number without a scale.

What does rated capacity mean in a record?

Rated capacity is the capacity stated by the manufacturer for the battery model under reference conditions, expressed in ampere hours or watt hours as the record provides. Rated capacity is a model value, verified against technical documentation and test results. It is not a measurement of the single battery in front of the reader, and it is not a guarantee of capacity under different conditions. Capacity varies with temperature, discharge rate, age and use. The record should therefore show the rated value together with the reference conditions, so that a later measurement can be compared on a like for like basis. A unit level capacity measurement, where recorded in point 4, is a different field with a date and a measurement basis, and the two must not be merged. Merging them hides the difference between design rating and observed condition.

How are efficiency and resistance recorded?

Round trip energy efficiency is recorded at beginning of life and at 50 percent of cycle life where Annex XIII so requires, with the test conditions stated. Internal resistance is recorded in ohms at beginning of life, with the measurement method and conditions stated. Both values are sensitive to temperature and to the state of charge at measurement. A resistance value without a temperature and a state of charge cannot be compared with a later measurement, and an efficiency value without a cycle definition cannot be compared across models. The record should state the standard or test method relied on, the reference conditions, and the date of the test where the system provides for it. A reader who compares efficiency or resistance across batteries must first check that the conditions match. Comparison without matched conditions is not comparison. It is juxtaposition of numbers.

What does cycle life mean in a record?

Cycle life is the number of cycles a battery model sustains under a defined test before capacity falls to a defined end point, with the C rate, depth of discharge, temperature and end point all stated. Annex XIII point 1 requires the C rate of the relevant cycle life test to be recorded. Without the C rate and the end point, a cycle life figure is uninterpretable: a battery tested to 80 percent retained capacity at a low rate and a battery tested to 70 percent at a high rate cannot be ranked by cycle count alone. The record should also distinguish cycle life as a model test result from the life a unit achieves in service, which depends on use. A model cycle life figure is not a warranty of cycles for a particular unit, and the record must not present it as one.

What does the performance table illustrate?

How are performance fields updated?

Model performance fields are updated on model revision, with a version note and a date. Unit performance fields in point 4 are updated through dated measurements as the battery is used. A new measurement does not overwrite an old measurement. It appends to the history, so that change over time can be read. Where a measurement method changes, the record should note the change, because a step change in values may reflect the method rather than the battery. A performance field that is corrected because the original entry was wrong is corrected as a version change, with the prior value retained in history. The discipline is the same across layers: current value visible, history retained, change explained.

What must performance fields never claim?

Performance fields must never claim future performance, remaining life, or suitability for a new application. A test result describes the past, under stated conditions. A projection describes the future, under assumed conditions. The record can contain an expected lifetime figure where Annex XIII requires it, with the basis stated, but the figure must be labelled as expected and must not be quoted as a measured outcome. This guide gives general educational information only and does not give legal advice for a particular battery or test.