Guidance

Guidelines for preparing battery degradation evidence

Updated 8 April 2024

The applicant must demonstrate that the battery or fuel cell used to drive the vehicle’s propulsion system has an acceptably low rate of degradation.

As a guide to applicants, pending the evidence presented in each case, an acceptable level of degradation is defined as:

• for battery electric vehicles, the battery must maintain at least 80% of its initial or rated charge capacity for the initial 3 years or 70% of initial or rated charge capacity for the initial 5 years
• for fuel cell vehicles, the fuel stack must maintain at least 90% of its rated voltage output for the initial 5 years

Given the newness of this technology, we are prepared to work with manufacturers who supply alternative evidence which provides what the Vehicle Certification Agency (VCA) considers to be a reasonable level of reassurance to the consumer.

Assessing written assurance evidence

In cases where the applicant is seeking approval via written assurance of reasonable performance, the evidence provided may be in the form of illustrative evidence of the degradation rate under certain conditions, such as under an accelerated testing regime.

This must be supported by an explanation of the way in which this evidence demonstrates long-term maintenance of the performance.

The VCA may consider the following elements as indicators of quality sufficient to offer reasonable performance during normal use. In this context, they will consider what is appropriate to the vehicle and range/performance stated by the manufacturer.

The applicant may wish to identify the possible failure modes and mitigating actions that have been taken.

As a guide to the level of detail expected, we would not expect the evidence submitted to require more than 10 pages. Annexes can be provided.

The VCA and/or supporting independent technical experts may request additional information to inform assessment of the application.

What will be assessed: written assurance

The following types of indicators or factors which the panel and independent technical experts will take into account when assessing this evidence are:

Battery cell

Data at the cell level can include the published test data of cell manufacturers.

Transportation

Conforms to UN transport standard ST/SG/AC.10/27.

Cycling tests

Provide appropriate evidence, to represent the final application of:

• c-rating
• depth of discharge
• influence of temperature on characteristics

Self discharge rates

Values and method of establishing them.

Impedance

Measurement method and evolution with both elapsed time and use.

Capacity

Measurement method and evolution with both elapsed time and use.

Battery pack

Evidence needs to clearly show how the cells have been combined to produce performance in a battery pack that meets the stated specification.

Mechanical integrity

• resistance to stresses caused by dimensional changes of cells in operation
• what vibration, strength and damage testing has been performed?
• strategy for preventing and containing effects of cell failures

Thermal integrity

Provide a summary of your thermal management strategy and how it is achieved, such as for example cell-to-cell temperature variation and overall pack temperature control.

What cooling strategy has been put in place to support the operation of the battery in the UK environment?

Cycling tests

• are the tests appropriate to the claimed usage of the vehicle?
• are the cycling profiles, ratings and temperatures of the tests appropriate?

Environmental resistance

• what is the IP rating of the pack?
• resistance to dusts and liquid and gaseous contaminants
• resistance to extremes of temperature

Battery management system (BMS)

How does the BMS ensure that the individual cells are not required to operate outside of its specification?

Vehicle

The integration of a battery pack into the operational context of a vehicle can impact on the battery degradation rate. How has this been considered by the manufacturer?

In addition to the points below, the issues considered at the pack level should also be addressed at the vehicle level.

Onboard diagnostics

What error and information messages will be provided to the driver on the status of the battery?

What error messages will be recorded for maintenance?

Dirt, dust, water ingress and mechanical stress

What measures have been taken to assess and control these factors?

Fire performance

Is there a report that confirms appropriate testing has been carried out?

To note: similar guidance will be supplied on preparing fuel cell degradation evidence as more fuel cell vehicles come to market.