Battery management systems (BMS) are the cornerstone of modern electric vehicle and energy storage safety. They monitor cell voltages, regulate charging and discharging, balance cell loads, and protect packs from operating outside safe temperature ranges. For most of the industry, the battery management system is the battery’s guardian, the last line of defence against catastrophic failure.

However, a growing body of evidence suggests that the conventional BMS, for all its sophistication, has critical blind spots. It cannot detect the earliest signs of thermal runaway, sense moisture accumulating inside a pack, or record the shock load a battery absorbed in the event of a collision.

These are the gaps that, in real-world conditions, allow battery fires to go undetected until they are already disastrous, and that leave engineers, fleet operators, and insurers without the data they need to make informed decisions about battery health. Understanding where conventional battery management systems fall short and what it takes to address those shortfalls is essential for anyone engineering or operating lithium-ion battery packs today.

What Is a Battery Management System?

A battery management system is an electronic control system embedded within a battery pack. Its primary functions are to measure and report cell level voltage and temperature, prevent cells from being overcharged or overdischarged, balance the charge state across individual cells, and provide an overall state-of-charge (SoC) and state-of-health (SoH) estimate to the host vehicle or system.

In a modern electric vehicle, the battery management system communicates over a CAN bus, feeding data to the vehicle’s powertrain controller and in many implementations to cloud telematics systems. It is a mature, well understood technology. Most regulatory frameworks for EV safety assume the battery management system as a given.

The problem is not that battery management systems are poorly engineered. The problem is that they were designed around a relatively narrow set of measurable parameters of voltage and temperature. Regrettably voltage and temperature alone do not tell the full story of what is happening inside a battery pack.

The Limitations of Conventional Battery Management Systems

Sparse Temperature Monitoring

Most battery management system architectures place temperature sensors at intervals across the pack typically one sensor per module, or even one per several cells. This means that an anomalous cell in between two sensor points may experience a significant temperature rise before any measurement captures it. In a densely packed lithium-ion module, the distance between a malfunctioning cell and the nearest temperature sensor can be enough to allow a thermal event to develop undetected.

Voltage Masking

Detecting early cell degradation through voltage measurements is inherently difficult in parallel cell configurations. When one cell in a parallel group begins to fail, the remaining cells prop up the terminal voltage of the group as a whole, masking the anomaly. By the time a voltage deviation becomes visible at the battery management system, the failing cell may already be in an advanced state of degradation. The battery management system, in effect, can be deceived by the very architecture of the pack it is protecting.

No Gas Detection

One of the most significant limitations of conventional battery management systems is that they have no means of detecting the gases that battery cells emit when they begin to vent. Cell venting sometimes called off-gassing is the earliest observable sign of thermal runaway. In the minutes before a cell reaches the temperatures that trigger a fire or explosion, it releases Volatile Organic Compounds (VOCs) and, in some failure scenarios, hydrogen. These gases are measurable and distinguishable. A conventional battery management system cannot measure them.

Independent testing by Sandia National Laboratories found that VOC detection can provide more than seven minutes of warning ahead of thermal runaway, a window that temperature sensors alone simply cannot offer.

That seven minute window matters enormously. It is the difference between a controlled system shutdown, a safe vehicle evacuation, or a circuit isolation and a fire that cannot be stopped. The Global Technical Regulation on Electric Vehicle Safety (GTR20) specifically addresses the need for early warning systems that allow safe evacuation of occupants. A temperature only battery management system cannot reliably meet that requirement.

No Moisture or Humidity Monitoring

Water ingress is a serious threat to battery pack longevity and safety. Moisture inside a pack compromises cell insulation, promotes corrosion, and increases the risk of short circuits. In liquid-cooled packs, condensation can form on internal components when the coolant temperature drops below the dew point of the air inside the enclosure, a scenario that is particularly likely during cold starts or after aggressive cooling cycles. Conventional battery management systems have no means of detecting moisture or tracking dew point, leaving this failure mode entirely unmonitored.

No Impact or Shock Recording

When an electric vehicle is involved in a collision, the mechanical integrity of the battery pack is immediately in question. Current battery management systems provide no record of the shock loads the pack has experienced. Engineers tasked with assessing post incident battery condition must rely on visual inspection and, where available, vehicle telemetry data, neither of which provides a reliable picture of whether the pack’s internal structure has been compromised. Without that data, the options are limited: scrap the pack or accept unknown risk.

The Risks These Gaps Create

Taken individually, each of these battery management systems limitations represents a monitoring gap. Taken together, they represent a systemic vulnerability in the safety architecture of lithium-ion battery packs, one that has real consequences for EV manufacturers, fleet operators, second life battery integrators, and end users.

Thermal runaway events in electric vehicles receive disproportionate media attention relative to their statistical frequency. But the impact of that attention on EV adoption is measurable. Every high profile battery fire reinforces public scepticism about electric vehicle safety. The industry’s ability to demonstrate that thermal events are detectable before they become catastrophic and that the data exists to respond appropriately is not merely a technical question. It is a commercial and reputational one.

For the growing second life battery market, the gaps are equally consequential. An EV battery destined for repurposing in a stationary energy storage system carries with it an unknown history. Without a detailed record of the shocks it has absorbed, the humidity conditions it has operated in, and the thermal events (however minor) it may have experienced, the risk profile of that battery is largely guesswork. This creates hesitancy across the supply chain and limits the economic viability of circular battery models.

Closing the Gaps: The Role of Cell Guard

Metis Engineering’s Cell Guard sensor was developed specifically to address the monitoring blind spots that conventional battery management systems leave open. Rather than replacing the battery management system, Cell Guard operates alongside it providing a complementary layer of environmental and physical data that voltage and temperature measurements alone cannot deliver.

Cell Guard is a compact, CAN based sensor designed to be installed near the breather port of a battery pack or energy storage enclosure. From that position, it continuously monitors Volatile Organic Compounds (VOCs), absolute pressure, air temperature, relative humidity, dew point, and in its expanded configuration hydrogen concentration. An optional three-axis accelerometer adds the ability to record shock loads up to 24G, including the magnitude and duration of any impact.

Each of these parameters directly addresses a specific gap in conventional battery management system capability. VOC detection provides the earliest possible warning of cell venting, triggering an alert while thermal runaway is still preventable. Humidity and dew point monitoring flags moisture ingress and the condensation risk that liquid-cooled packs face. Shock data provides a verifiable record of the mechanical stresses the pack has experienced, information that is invaluable for post incident assessment, battery resale, and repurposing decisions.

Cell Guard detected a thermal runaway event in under 60 seconds in independent testing and provided actionable warning data more than seven minutes before competing sensor technologies using hydrogen detection alone.

This independent validation, conducted by Sandia National Laboratories and published in the Journal of the Electrochemical Society, confirms what Metis Engineering designed Cell Guard to demonstrate, that VOC detection is a significantly more effective early warning method than hydrogen sensing, precisely because VOCs are emitted during the cell venting phase before hydrogen generation, and well before temperatures reach the threshold for disastrous failure.

Cell Guard is also designed for practical integration. Its configurable CAN bus speed and address, Molex Nano-Fit connector, and compact form factor allow it to be installed in new battery packs or retrofitted into existing architectures without disrupting pack design or airflow. It operates in a low-power monitoring mode, transmitting on CAN only when a pre-set threshold is exceeded, minimising parasitic draw while ensuring that alerts are generated without delay when they matter.

Applications Across the Battery Lifecycle

The use cases for Cell Guard span the full battery lifecycle, from first installation through to second life deployment. In electric vehicles including passenger cars, commercial buses, eVTOL aircraft, marine vessels, and micromobility platforms Cell Guard provides the early warning capability that enables safe evacuation and controlled system shutdown before thermal runaway takes hold.

In stationary energy storage systems, whether grid scale, commercial, or residential, Cell Guard provides continuous environmental oversight across variable temperature installations where the consequences of undetected moisture or gas accumulation are equally serious.

For the second life battery market, the accelerometer data and environmental records that Cell Guard accumulates throughout a battery’s operational life are directly relevant to the emerging concept of the battery passport, a traceable, verifiable record of battery condition that underpins confident repurposing and resale decisions. Allye Energy, which integrates repurposed EV battery packs into grid scale energy storage systems, has deployed Cell Guard with accelerometer precisely to address this requirement.

Cell Guard is tested and certified to ISO Automotive Standards, including ISO7637-2, ISO16750-2, and ISO16750-4, and is assembled in the UK under ISO9001-certified quality conditions. It is currently deployed in ASIL B applications across a range of OEM and Tier 1 programmes.

A More Complete Picture of Battery Health

Battery management systems are an indispensable part of EV and ESS safety architecture. However, the assumption that a battery management system alone is sufficient to protect a lithium-ion battery pack from all foreseeable failure modes is no longer tenable, not given what we know about how thermal runaway progresses, how moisture damage develops, and how mechanical stress accumulates over a battery’s working life.

Cell Guard is not a replacement for the battery management system. It is the complementary layer that closes the gaps the battery management systems cannot address providing engineers, fleet operators, and system integrators with the data they need to detect problems earlier, respond faster, and make better informed decisions about battery health from manufacture through to end of life.

For organisations engineering or operating lithium-ion battery systems, the question is no longer whether supplementary monitoring is necessary. It is which parameters to monitor, and how to integrate that capability most effectively into an existing system architecture. Cell Guard answers both questions.

Find out more

To learn more about Cell Guard or to request a product sample, visit Cell Guard – Metis Engineering or contact the Metis Engineering team at info@metisengineering.com