Why India Needs a Common Method to Measure EV Battery Health

A recent study highlights a fundamental problem in electric mobility: battery health is widely reported, but the methods used to measure it are not sufficiently standardised.

Two electric vehicles can report identical battery health and still have batteries in materially different conditions. More troublingly, different assessment methods applied to the same battery can produce different estimates of its health.

For an industry increasingly dependent on battery performance, this is more than a technical inconvenience. It affects vehicle valuation, warranty decisions, financing, insurance, maintenance and the eventual reuse or recycling of batteries.

As India's electric vehicle market expands, the absence of a widely accepted, reproducible method for measuring battery State of Health (SOH) deserves greater attention.

A 2025 research paper in npj Clean Energy by Philip Bilfinger and colleagues examines this problem and proposes a controlled, vehicle-level charging procedure for measuring energy- and capacity-based battery health. Its implications extend beyond the laboratory.

A percentage without a common definition

State of Health is often expressed as a percentage of a battery's original capacity or energy. A reported figure of 85 per cent sounds precise, but it raises questions. Is it based on electrical charge capacity or usable energy? Was it measured directly or estimated by the vehicle's battery management system? What was the reference value? At what temperature and under what operating conditions was it measured?

Unless these questions have consistent answers, two SOH figures may not be comparable.

Capacity measures the amount of electrical charge a battery can transfer. Energy also accounts for voltage and is more directly related to the work a battery can perform. Both are valuable indicators, but neither is a complete description of battery condition.

A battery may retain much of its capacity while experiencing increased internal resistance, reduced power capability, growing differences between cells or changes in thermal behaviour. One percentage cannot represent every dimension of battery health.

A practical route towards standardisation

The researchers at the Technical University of Munich propose measuring capacity and energy through a controlled charging procedure, with defined conditions for charging power, battery temperature, preparation and voltage limits. The paper proposes a measurement duration of at least 15 hours at low charging power, with a controlled battery temperature around 20°C, within a tolerance of 5°C.

They evaluate the approach using production electric vehicles, including models from Volkswagen, Porsche, Tesla and Cupra. Results reported for Cupra vehicles illustrate how a fixed battery-voltage window can provide more consistent comparisons than relying only on displayed state-of-charge limits.

The wider lesson is that the procedure behind a measurement matters as much as the algorithm used to calculate it.

The proposed method also has limitations. A lengthy controlled charging session can be difficult to accommodate in a high-utilisation fleet. Cell balancing, software changes and battery configuration can affect measurements. Appropriate reference values and voltage windows must be established. This is a research proposal, not an internationally adopted universal test.

Why this matters for India

Consider a used electric vehicle approaching a change of ownership. Its battery is an important part of its residual value. If its reported health is based only on an undisclosed manufacturer-specific calculation, a buyer may have little basis for comparing it with another vehicle.

A financier assessing the asset needs evidence for estimating residual value. An insurer needs information that can be interpreted consistently. A fleet operator needs to distinguish ordinary ageing from emerging technical concerns.

Independent, reproducible measurements will not remove every uncertainty. They can, however, provide a common reference point for transactions that otherwise depend on incompatible definitions and assumptions.

Battery health and battery circularity

A battery no longer suitable for demanding vehicle operation may still be useful in a less demanding application. But deciding between continued use, repair, remanufacturing, repurposing and recycling requires more than a capacity percentage.

Power capability, resistance, cell consistency, thermal behaviour, safety history and the requirements of the proposed next application also matter.

Remaining Useful Life (RUL) adds another layer of uncertainty. Unlike a present-condition measurement, it is a forecast. It depends on how the battery has aged, how it will be used in future and what threshold defines the end of its useful service.

Standardising SOH measurement is necessary, but not sufficient, for a credible second-life battery market. A wider assessment must connect battery identity, lifecycle history, present condition, application suitability and confidence in the evidence.

From measurement standards to market confidence

India already has a regulatory framework addressing battery waste management, producer responsibility, refurbishment and recycling. A complementary priority is the technical infrastructure that supports decisions before a battery becomes waste.

A practical starting point would be common definitions for capacity-based and energy-based SOH, agreed reference values, documented test conditions and protocols for independent assessment. Manufacturers could provide access to the minimum information needed for verification without exposing proprietary control algorithms or commercially sensitive operating records.

Different vehicle classes and battery chemistries may require adapted procedures. The objective is not an identical test in every circumstance, but results whose meaning, limitations and basis of comparison are clear.

The study by Bilfinger and colleagues contributes an important piece of this larger effort: making vehicle-level health measurements more reproducible. For India, the opportunity is to develop interoperable methods suited to domestic conditions, commercial fleets and an emerging battery-circularity market.

An EV battery's reported health should not depend on an unexplained algorithm or an undisclosed test procedure. Before batteries can be confidently valued, financed, insured, repurposed or recycled, the market needs agreement on what battery health means and how it should be measured.

A common methodology is not merely technical documentation. It is infrastructure for trust in the electric mobility economy.

The views expressed are personal and do not necessarily represent those of the author’s employer.