Echelon Utilization of Power Batteries: Residual Value Assessment, Testing Standards, and Second-Lif

Echelon utilization—the practice of repurposing retired electric vehicle power batteries into second-life applications such as energy storage and backup power—has become a critical strategy for maximizing battery lifecycle value and supporting global carbon neutrality goals. This article provides a comprehensive analysis of echelon utilization technology, covering residual capacity testing, safety evaluation methods, applicable standards such as GB/T 34015, UL 1974, and IEC 63330, and the major application scenarios for second-life power batteries.

Technical Overview

Echelon utilization, also known as cascaded utilization or second-life application, refers to the systematic repurposing of power batteries that have been retired from electric vehicles but still retain substantial usable energy storage capability. When a traction battery degrades to approximately 70%–80% of its original rated capacity, it no longer meets the range and power demands of automotive service, yet it remains fully suitable for applications with lower energy density and power requirements.

The lifecycle value chain of a power battery typically consists of three stages: first life in an electric vehicle (typically 5–8 years), second life in echelon applications such as stationary energy storage (typically 5–10 additional years), and final material recovery through recycling. By extending the usable service life of battery assets, echelon utilization reduces the levelized cost of energy storage, delays the flow of end-of-life batteries to recycling facilities, and lowers the environmental footprint associated with raw material extraction.

The technical feasibility of echelon utilization depends on rigorous assessment of the residual value of retired batteries, including state of health (SOH), residual capacity, safety margins, and cell-to-cell consistency. Because retired battery packs arrive from diverse vehicle platforms, service histories, and degradation profiles, a standardized testing and grading workflow is essential. Key international and national standards governing this field include the GB/T 34015 series for residual energy testing, UL 1974 for battery repurposing evaluation, IEC 63330 for repurposing requirements, GB/T 33598 for pack dismantling, and GB/T 36276 for energy storage battery requirements.

Testing Samples

Retired Battery Cells

Individual cylindrical, prismatic, or pouch cells extracted during the dismantling of retired EV battery packs. Cell-level samples are used for precise residual capacity measurement, internal resistance characterization, self-discharge screening, and safety verification before regrouping.

Retired Battery Modules

Module-level samples, typically consisting of multiple cells connected in series or parallel with original structural framing and sensing hardware. Module testing verifies residual energy, consistency within the module, and structural integrity after vehicle service.

Retired Battery Packs

Complete pack assemblies evaluated for feasibility of direct reuse without dismantling. Pack-level assessment covers electrical performance, insulation resistance, thermal management function, and the data records of the battery management system (BMS).

Battery Management Systems

BMS hardware and firmware from retired packs, evaluated for data integrity, historical usage records (mileage, cycle counts, fault logs), and functional reliability in second-life configurations.

Testing Items

The evaluation of retired power batteries for echelon utilization comprises the following core testing categories:

• Appearance and structural inspection – visual examination for deformation, leakage, corrosion, terminal damage, and electrolyte residue; dimensional verification after vehicle service.
• Residual capacity and energy testing – determination of remaining discharge capacity and energy under specified rate and temperature conditions, in accordance with GB/T 34015.1.
• State of health (SOH) assessment – quantification of remaining capacity relative to rated capacity, calculated as SOH = current capacity / rated capacity × 100%.
• Internal resistance testing – measurement of AC impedance (typically at 1 kHz) and DC internal resistance using pulse current methods such as hybrid pulse power characterization (HPPC).
• Self-discharge testing – evaluation of charge retention over defined standing periods to identify cells with elevated self-discharge caused by internal micro-shorts.
• Rate capability and power characteristics – verification of discharge capability at different C-rates to match batteries with target applications.
• Cycle life verification – accelerated cycle testing to estimate remaining service life under second-life duty profiles.
• Consistency screening – statistical grading of capacity, voltage, and internal resistance distributions to enable matched regrouping.
• Safety testing – overcharge, over-discharge, external short circuit, crush, vibration, thermal abuse, and insulation resistance testing per GB 38031, UL 1974, or IEC 62619 protocols.
• Environmental adaptability – temperature cycling, humidity exposure, and transport safety validation including UN 38.3 type assessments.

Testing Methods

Pretreatment and Grading

Retired packs first undergo safe discharge pretreatment in accordance with GB/T 34015.3, followed by non-destructive dismantling of the pack into modules or cells as described in GB/T 33598. BMS historical data is extracted to stratify incoming batteries by chemistry, service history, and suspected degradation level, reducing subsequent testing burden.

Residual Capacity Measurement

Cells or modules are conditioned at a controlled ambient temperature of 25 °C ± 5 °C and charged using the manufacturer-specified procedure. Residual capacity is determined by constant-current discharge at a defined rate (commonly 1I3 or 0.5I1) to the specified cut-off voltage, following the procedures of GB/T 34015.1. Duplicate cycles are performed to confirm measurement repeatability, and residual energy in watt-hours is integrated from the discharge curve.

SOH and Consistency Analysis

Measured capacities are normalized against rated capacity to compute SOH values. Statistical methods, including capacity binning and internal resistance distribution analysis, are applied to sort cells into matched groups. Cells exhibiting anomalous impedance rise, abnormal self-discharge, or capacity outliers are rejected and routed to material recovery.

Safety Verification

Candidate cells and regrouped modules undergo abuse testing per UL 1974, IEC 62619, or GB 38031 protocols, including overcharge, external short circuit, and thermal stability tests. Insulation resistance and withstand voltage tests confirm dielectric integrity before second-life deployment. Only batteries passing all safety criteria are authorized for echelon applications.

System-Level Validation

Reassembled second-life battery systems are validated against energy storage requirements such as GB/T 36276, covering integrated performance, thermal management behavior, communication protocols, and protection functions under simulated duty cycles.

Applications

Second-life power batteries are deployed across a growing range of stationary and mobile low-power scenarios:

• Grid-scale energy storage – peak shaving and valley filling, renewable energy time-shifting, and grid frequency regulation supporting solar and wind integration.
• Telecommunication backup power – direct-current backup supplies for 5G base stations and communication rooms, one of the largest current markets for echelon batteries.
• Commercial and residential energy storage – distributed storage systems that reduce electricity costs through time-of-use arbitrage and increase self-consumption of rooftop photovoltaic generation.
• Low-speed electric vehicles – power sources for electric scooters, sightseeing vehicles, golf carts, forklifts, and automated guided vehicles (AGVs).
• Charging station buffering – energy buffering at EV charging stations to reduce peak grid demand and defer distribution infrastructure upgrades.
• Emergency and off-grid power – mobile power supplies, street lighting, and backup systems for critical facilities in areas with unreliable grid coverage.

FAQ

At what capacity is a power battery retired from vehicle service?

Industry practice generally retires traction batteries when capacity declines to approximately 70%–80% of the original rated value, at which point vehicle range and power performance no longer meet user requirements even though substantial storage capability remains.

How long can a second-life battery continue to operate?

Depending on the SOH at entry and the duty severity of the application, echelon batteries typically provide an additional 5–10 years of service in stationary storage roles, where charge rates, discharge depths, and thermal conditions are more benign than in vehicle use.

Which standards govern echelon utilization testing?

Principal standards include the GB/T 34015 series (residual energy testing and discharge pretreatment), GB/T 33598 (dismantling requirements), UL 1974 (evaluation for repurposing batteries), IEC 63330 (repurposing of secondary batteries), GB/T 36276 (storage batteries), and IEC 62619 (safety of industrial lithium systems). Testing should be performed by accredited third-party laboratories compliant with ISO/IEC 17025.

What are the main safety risks of second-life batteries?

The primary risks stem from non-uniform degradation, hidden internal shorts, lithium plating accumulated during first-life fast charging, and poor cell consistency after regrouping. These risks are mitigated through rigorous incoming screening, SOH stratification, matched regrouping, comprehensive abuse testing, and second-life BMS supervision with appropriate derating margins.

Is echelon utilization economically viable?

Economic viability depends on the balance between acquisition and testing costs and the value of delivered storage capacity. As testing automation, BMS data traceability, and standardization improve, the cost advantage of echelon batteries relative to new cells in stationary storage continues to strengthen, particularly when combined with evolving battery passport and traceability regulations.