Background

The Office for Product Safety and Standards (OPSS) commissioned research to improve the evidence base on the causes of the safety risks and hazards associated with PLEV batteries and chargers. This report includes:

• a literature study of scientific papers and reports of real-world evidence of PLEV fires in the UK and other major markets
• a review of existing UK legislation and applicable standards
• the findings of detailed inspection and testing of several e-bike and e-scooter batteries available on the UK market, covering a broad price range

For convenience, the executive summary and conclusions of the report are provided below.

Read the full report.

Acknowledgements

This report was prepared for OPSS by Warwick Manufacturing Group (WMG), a department of the University of Warwick, Coventry, CV4 7AL, UK.

The views expressed in this report are those of the authors, not necessarily those of OPSS or the Department for Business and Trade, nor do they reflect Government policy.

The following organisations were consulted as part of this project:

• Bicycle Association
• Dyson Ltd.
• Electrical Safety First
• Exponent, Inc.
• Halfords Group plc
• London Fire Brigade
• National Fire Chiefs’ Council
• Pure Electric Ltd.
• Swifty Scooters Ltd.
• UL Solutions

We are grateful to all respondents for their time and insights.

Executive summary

The Personal Light Electric Vehicle (PLEV) market has grown rapidly in the last decade and is an important part of the electrification and decarbonisation of urban transport systems, particularly for short journeys. Electric propulsion systems are now used in a diverse range of light vehicles, from pedal-assistance in on-road and off-road electric bicycles and 3- and 4-wheeled urban delivery vehicles (e-bikes), to numerous other vehicles such as hoverboards and electric scooters (e-scooters), many of which are currently not legal for road use. The battery energy storage systems for PLEVs sold in the UK predominantly use the Lithium-ion cell chemistry, which is also widespread in other market sectors such as personal electronic devices, electric passenger cars and grid energy storage. All these markets benefit from the high energy density and power density offered by Lithium-ion cells, and the rapid growth in the global market for Lithium-ion cells has resulted in manufacturing economies of scale which have significantly improved their affordability. From 2013 to 2023, the price of Lithium-ion batteries has fallen by 82%. However, Lithium-ion batteries can undergo severe failures, known as thermal runaway, wherein cells undergo a highly exothermic chemical reaction which can result in venting of large volumes of hot, toxic, flammable gas. When the gas ignites, it can cause fire and even explosions.

Like personal electronic devices, PLEV batteries are commonly charged indoors. However, PLEV batteries are much larger than those in most other consumer battery-powered devices and contain significantly more energy. PLEV batteries typically contain 30-100 cylindrical cells. When thermal runaway starts in a single cell within a battery, it can propagate to other cells in the battery. The resulting gas, fire and explosions can be extremely hazardous to the health of those in buildings where these incidents occur, resulting in severe injury or death.

This report was commissioned to improve the evidence base available to OPSS on the causes of the safety risks and hazards associated with PLEV batteries and chargers. The work has included a literature study of scientific papers and reports of real-world evidence of PLEV fires in the UK and other major markets. Stakeholders from emergency services, safety groups, PLEV manufacturers and retailers, standards organisations and battery experts have been consulted. A review of existing UK legislation and applicable standards has been conducted, and a battery assessment procedure was defined, prior to detailed inspection and testing of several e-bike and e-scooter batteries available on the UK market, covering a broad price range. This has resulted in suggestions for actions by government, standards bodies and other groups, aimed at reducing the frequency and severity of PLEV fires.

PLEV fires: real-world statistics and stakeholder evidence

Fire services across the UK have responded to PLEV fires, but London Fire Brigade (LFB) has likely the largest evidence and incident database and has documented the increase in the frequency, from two events in 2017 to at least 178 incidents in London in 2023. Since 2020, the increase has been dominated by e-bikes, rather than e-scooters. Unfortunately, despite the efforts of LFB and other fire services, the quality of incident data is constrained by the degree of destruction caused by the fires, which means that it is often impossible to determine key details such as the identity of the product or whether the battery was on charge at the time.

Stakeholders consulted for this report have highlighted perceived shortcomings in UK incident reporting systems and battery standards, the importance of cell and battery manufacturing quality, and the need to target consumer advice towards the most frequent PLEV users. PLEV supply-chain organisations who were consulted are generally supportive of improved safety requirements that are enforced to ensure a consistently high standard across the market.

Causes of PLEV Lithium-ion battery fires

When riding a PLEV, the battery is progressively discharged to a lower state of charge (SoC) as energy is drawn from the pack to power the motor. Subsequent charging increases the SoC. Typically, the charger will continue to charge the battery up to 100% SoC (fully charged) unless the user switches off or disconnects the charger earlier, after which the battery will then remain at 100% SoC until the PLEV is next used. The scientific literature on Lithium-ion cell thermal runaway demonstrates that the likelihood and severity are both increased when the cell is at a higher SoC, and increased still further if the cell is overcharged, overheated, or mechanically damaged. The hazards to human health are exacerbated when the battery is indoors, where routes of escape from flames and ejecta may be limited, and a room can quickly fill with hot, toxic and suffocating gases.

When cells are combined into a battery, the overall likelihood and severity of a thermal incident is determined by a complex range of factors, including the effectiveness of passive and active protection systems in the cells and the battery. The battery management system (BMS) plays a critical role to monitor the state of the individual cells, and ensure that their voltage, current and temperature limits are not exceeded.

Legislation and standards

A review of the product safety legislation has confirmed that many PLEV products, including complete e-bikes and e-scooters and conversion kits that have a motor, are covered by the Supply of Machinery (Safety) Regulations 2008 (SMSR). The SMSR contains a detailed list of essential health and safety requirements that manufacturers must comply with, including avoidance of hazards such as high temperature, fire and explosion, which are directly relevant to PLEV safety. The SMSR also requires the manufacturer to provide documentation (e.g. assembly instructions, declaration of incorporation), which can assist authorities with enforcement and can guide professionals and consumers on correct use of the product. However, separately sold batteries, often used in conversion kits, are covered by the General Product Safety Regulations 2005 (GPSR) or the Electrical Equipment (Safety) Regulations 2016 (EESR). This requires all consumer products placed on the market to be safe, but does not contain detailed health and safety requirements, or require such extensive documentation. Regulations set the legal framework which require that only products which comply with the essential requirements, as set out in the regulation, are placed on the market, these regulations place obligations on manufacturers, and others in the supply chain. Standards are a voluntary means by which those who manufacture products can assess designs. Test reports against the requirements detailed in standards are often used as evidence to support claims of compliance made for those products.

A review of the standards applicable to e-bikes and e-scooters has shown differences between the two: The latest e-bike standard specifies that, where a manufacturer wishes to claim compliance with the standard for the e-bike, they will also need to ensure that the e-bike battery conforms with the most rigorous applicable UK battery standard, but the standard for e-scooters has less stringent battery safety requirements. There is no dedicated standard to cover conversion kits, as the existing standards cover complete e-bikes or e-scooters and there is no legal or industry definition of a conversion kit for which a standard could be created. The existing standards for batteries can be used for conversion kit batteries, but the mix-and-match nature of conversion kits requires a re-examination of the relevant risks.

A review of the battery standards has highlighted several suggestions for improvement, relating mainly to the severity of test conditions and the ability of the battery to remain safe in the event of single-point failures in the protective systems and components.

Product inspection and testing

Several e-bike, e-scooter and conversion kit batteries, covering a range of price points, have been investigated through a process of teardown analysis, which involves disassembly and inspection of the design and manufacturing quality, and abuse testing, which tests the battery’s ability to protect itself from reasonably foreseeable misuse.

Teardown of some products has shown examples of poor manufacturing processes and quality, absence of essential safety features such as temperature sensors, and poor design choices that increase the likelihood of water ingress and cell overheating. The batteries also have widely differing sophistication in the electronic components used in their BMSs. In some cases, the hardware restricts the ability for the battery manufacturer to configure the BMS for the operating limits of individual cell types, meaning that inappropriate generic limits are used.

The abuse testing has shown a clear correlation between the price-per-unit-of-energy of PLEV batteries and the safety outcomes. The tests have shown that many BMSs do not prevent the battery from exceeding the current and temperature limits stated on the cell manufacturers’ datasheets. It is even possible that the cell limits may be exceeded when the battery is used with the vehicle system and charger for which it was intended. Where an unsuitable charger or a modified PLEV drive motor is used, the risk of the batteries being electrically or thermally abused is far higher. When such reasonably foreseeable misuse occurs, the tests have revealed over-heating of protective components, rendering those protections ineffective, in a way that is not apparent to the consumer. As a result, several batteries tested went into thermal runaway, leading rapidly to fire, explosions and clouds of toxic gas.

When such abuse occurs, the tests have demonstrated over-heating of protective components, rendering those protections ineffectual, in a way that is not apparent to the consumer. The over-heated components were seen to directly heat neighbouring cells and they failed to prevent over-voltage or over-current, which then resulted in the cells becoming overcharged. As a result, several batteries tested went into thermal runaway, leading rapidly to fire, explosions and clouds of toxic gas that would be extremely hazardous to anyone in the same space.

However, the tests also showed that the PLEV batteries with a higher price-per-unit-of-energy, which had better designed safety circuits, more sophisticated electronics and were better manufactured, successfully prevented thermal runaway by a combination of passive and active protection systems. Nevertheless, in the event of single-point failures in the BMS, many of the tested batteries remain reliant on passive safety devices in the cells, which only activate when the cell is already in a significantly overcharged condition, wherein the likelihood of thermal runaway is significantly increased.

The testing demonstrated that over-charged cells can enter thermal runaway sometime after the charging has stopped. This is because the damage that had already been done to the cells causes gradual self-heating, which eventually caused the cells to reach the critical temperature for thermal runaway to occur. Prior to thermal runaway, there may be no outward indication that the battery is compromised.

Greater use of secondary or “redundant” active safety systems, particularly in the charging circuit, would reduce the susceptibility to thermal runaway in the event of single-point failures. It is important that standards should include tests that introduce or mimic single-point failures, to ensure that the secondary active safety systems effectively prevent thermal runaway.

Conclusions

Real-world data

Personal Light Electric Vehicle (PLEV) fires have become a significant UK-wide problem, with severe incidents causing particular challenges for fire and rescue services in the UK. While UK-wide data has not been collated for long enough or with sufficient detail to be analysed, London Fire Brigade (LFB) data from 2017 to 2023 shows a rapid increase in the annual number of incidents. The annual number of e-scooter incidents has not increased since 2021, but the number of e bike incidents has continued to climb. WMG analysed the e-bike data to ascertain how many incidents involved conversion kits compared to Original Equipment Manufacturer (OEM)-made e-bikes. Of the 56% of incidents where this could be ascertained, over three quarters were conversion kits. The data on brand and model type of e-bikes and e-scooters has been used to inform some of the products purchased for teardown and abuse testing in this project.

Almost all incidents recorded by LFB occurred indoors, either in domestic or commercial properties. The LFB data on whether a battery was being charged at the time of the fire are not definitive, due to the high proportion of incidents where this could not be determined. However, there is evidence to suggest that some consumers use incompatible chargers, rated at up to twice the voltage of the battery involved in the fire.

The OPSS Product Safety and Consumers Wave 5 survey data (UK Government, Dec 2023) imply that issues around PLEV fire safety are more widespread than the LFB data suggest: Of the 7% of respondents who owned or had access to a PLEV, 22% had experienced a safety issue, and 14% of these involved fire, explosion, smoke or over-heating.

The survey also showed 35% of PLEV owners had purchased a separate battery or charger, and the proportion of such PLEV owners who had experienced a safety incident (41% of separate battery purchasers and 48% of separate charger purchasers) was much higher, compared to safety incidents reported by owners who had not bought a separate charger or battery (10%). This suggests that mixing batteries and chargers that are not supplied together increases the likelihood of safety incidents.

The UK is not alone. New York City Fire Department has recorded a rapid annual increase in incidents since 2019, and in early 2024, the New York City Council voted to pass new safety regulations which require businesses that sell e-bikes and e-scooters to post safety information about Lithium-ion battery storage in stores and online and prohibits the sale of batteries that are not certified as meeting the relevant UL standards.

Scientific data on causes of Lithium-ion cell thermal runaway

Thermal runaway is a highly exothermic (heat-generating) chemical process. Once it starts, it proceeds rapidly and is almost impossible to halt with any amount of cooling. To reach thermal runaway, some part of a cell must reach an abnormally high temperature of around 150-180 °C. In a battery, thermal runaway invariably starts in a single cell and can propagate to other cells, multiplying the severity of the incident. In laboratory conditions, thermal runaway can be started by three main causes:

1. Mechanical abuse, such as crushing or penetration of cells.
2. Thermal abuse, meaning over-heating.
3. Electrical abuse by over-charging (over-voltage or charging over-current) or over-discharging (under-voltage or discharging over-current). Over-charging is generally much more likely to cause thermal runaway than over-discharging.

Defects within a cell, resulting from manufacture or the effects of long-term use, are also known causes, but are much more challenging to reproduce in the laboratory.

Considering the high severity of thermal runaway, even with a low or limited likelihood of it occurring, cells and batteries must be designed based on the assumption that single-cell thermal runaway will happen. It is essential that the battery as a whole should be designed and manufactured to:

1. Minimise the likelihood of thermal runaway.
2. Ensure detection of the potential causes of thermal runaway.
3. Act to prevent thermal runaway when potential causes are detected.
4. Mitigate against single points of failure.
5. Mitigate against the severity of thermal runaway when it does occur.

Point 1 above relies firstly on manufacturing quality and secondly on ensuring that the cells are only used within the voltage, current and temperature limits stated by the cell manufacturer.

Point 2 above requires the BMS to be equipped with appropriate sensors for voltage, current and temperature, and that its software is appropriately calibrated and validated to ensure that, when the cell manufacturer’s stated limits are exceeded, this is detected.

Point 3 above requires that, upon detection of abnormal conditions, the BMS can reliably protect the cells and other components. This requires circuitry which can limit or interrupt the charge or discharge current, including prevention of reverse current flow in charge and discharge circuits unless the battery can operate safely in such conditions.

Point 4 above requires tolerance to single-point failures in hardware and software: There should be secondary or “redundant” features to ensure that the protection system is failure-tolerant. These may include passive protection components in individual cells (e.g. current interrupt device) or at battery-level (e.g. fuse), but these generally cannot offer the effectiveness of prevention that the active systems in the BMS can provide across a range of single-point failures. It is also essential that single-point failures shall not cause cascading failures to other critical components.

Point 5 above requires design measures in the battery to prevent, minimise or slow down propagation from cell to cell, including effective venting of the gas and other ejecta from the initiating cell and features to protect other cells from the electrical, thermal and mechanical effects that can cause a cascading failure. The ejecta from a single cell can cause damage to property and human health, but this will generally be far less than if multiple cells enter thermal runaway.

If the whole battery is at a high State of Charge (SoC), or worse still, overcharged, then the likelihood, rapidity and severity of thermal propagation are substantially increased.

Literature review

A review of public domain sources has shown the easy availability of guidance on how to bypass BMS protection mechanisms, to liberate more performance from a battery, or to recover a deeply discharged battery. Such tampering has been highlighted as a serious safety concern. Prevention of tampering is a major technical challenge, but notably one of the batteries purchased in this project had protections which prevented it from being charged other than when connected to the originally supplied charger. Discharge using laboratory equipment was also prevented. No publicly available information was found on how to defeat this system. The work did not include an exhaustive attempt to defeat these protections, but this product demonstrates that anti-tampering measures can be effective against reasonable attempts at misuse.

The literature review included a review of statements made by stakeholders in the USA to the US Consumer Product Safety Commission (CPSC). Several stakeholders pushed for mandatory certification of PLEV batteries. A bike manufacturer asserted that markets that require certification to minimum safety standards have seen far fewer issues with fires and general battery safety with e-bikes and went on to opine that the battery should protect itself and should not depend on the charger. This view is shared by WMG.

ISO 4210-10, a technical specification for e-bike safety including details for battery-to-charger communication protocols intended to ensure that over-charging cannot occur, was mentioned in the CPSC hearing, and has been promoted in the UK by the safety charity Electrical Safety First. It has not been adopted as standard in Europe or the UK, but in WMG’s view, an appropriate battery-charger protocol would have significant safety benefits.

Singapore is a market that has recently implemented laws requiring type-approval and registration of e-bikes, which must be fitted with a tamper-proof seal. Harsh financial penalties and/or jail terms apply to individuals found not to comply. Related rules also apply to e-scooters. In early 2023, reports indicated that the number of PLEV fire incidents had started to fall. Singapore’s fire service attributed the fall to the new laws.

Stakeholder input

Stakeholders from the PLEV supply-chain, safety experts, fire services, and standards organisations have been consulted for this report. The fire and rescue services highlighted the perceived shortcomings in UK incident reporting and the challenge for firefighters to gather incident data. This is not their primary role, they are not trained as experts in forensic examination of incident scenes and the physical evidence is often so damaged by the fire that it is difficult to obtain meaningful data. Nevertheless, while data collection itself will not reduce the number of incidents, it is an important tool to monitor progress to reduce the number of incidents, and to identify products which are unsafe and consumer usage patterns that correlate with occurrence of fires.

Stakeholders also highlighted shortcomings with some battery standards, the importance of manufacturing quality, and the need to target consumer advice at the most frequent PLEV users, such as gig-economy riders and couriers. The supply-chain organisations, including UK manufacturers and retailers, are supportive of improved safety requirements that are enforced to ensure a consistently high standard for all market players.

The stakeholders highlighted the international dimension of the PLEV market, and the similar thermal incidents that occur in other market sectors which use Lithium-ion cells and batteries. PLEVs should not be seen in isolation, and measures to tackle the fire safety of products with Lithium-ion batteries should be coordinated at international level, as has been done in, for example, the automotive sector.

Combining real-world evidence with scientific data

The scientific literature shows a clear correlation between high SoC and increased likelihood and severity of thermal runaway. Taken together with the real-world evidence, we conclude that charging is likely to be a significant contributor to thermal incidents.

There are also numerous other factors which can cause thermal runaway, including manufacturing defects, mechanical damage, and over-heating.

The state of the art in cell manufacturing quality-control enables almost all defects to be avoided or detected by in-process checks but cannot completely eliminate the possibility of defects in cells. Many types of cell manufacturing defects can result in internal short-circuits. However, they may only manifest themselves after some period of use, for example because of shock and vibration causing slight movement of internal parts or the microstructural changes resulting from repeated charge and discharge. If undetected during manufacture, they could over time contribute to causing thermal runaway. It is therefore essential that all Lithium-ion cells for PLEVs are manufactured to consistently high quality standards to reduce defect occurrence to the lowest possible level.

Mechanical damage, caused by severe shock, vibration, or impact could result in thermal runaway. Stakeholder input confirmed that consumer use of some PLEVs, particularly e scooters, can include such mechanical stresses, but while manufacturers have evidence of mechanical damage to cells inside customer batteries, there is no clear evidence that these have resulted in severe real-world thermal incidents.

Mechanical damage or poor design or manufacture can result in moisture entering inside the outer casing of the battery. Long-term exposure to environmental factors such as Ultraviolet (UV) radiation from sunlight can also compromise the sealing of the battery housing. Stakeholder experience has shown that moisture ingress can result in corrosion of electrical components, particularly in the BMS, but also the cells. These can potentially lead to thermal incidents. Most product validation and certification tests fail to detect these long-term effects, because the tests are conducted on new batteries. Accelerated environmental ageing tests, followed by water ingress tests, can improve detection.

Over-heating could be caused by certain consumer actions, such as leaving a battery close to a domestic heat source, or tampering, such as altering the motor power with a consequent increase in discharge current, or by using an inappropriate charger which provides excessive charging current. While the first of these causes is beyond the reasonable control of the battery itself, the latter two can be addressed by appropriate BMS design. Stakeholder experience and WMG’s product teardowns show that some products are not fitted with temperature sensors, which are essential for the BMS to detect and act upon over-heating.

This includes preventing over-voltage (meaning that the battery reaches > 100% SoC), but also over-current, which can happen at any SoC, especially in colder conditions where the charging current that the cells can safely accept is substantially reduced. The battery must also prevent other potential causes that are well established from scientific knowledge, such as over-heating. The BMS plays a key role in implementing such active protection.

When cells are combined in a battery, the overall likelihood and severity of a thermal incident is determined by a complex range of factors, including those stated above and the effectiveness of passive and active protections in the cells and the battery. The BMS should play a critical role to monitor the state of the individual cells, and ensure that their voltage, current and temperature limits are not exceeded.

Concluding remarks

In WMG’s view, battery safety should not need to be a factor in consumers’ PLEV purchase decisions: Safety should be inherent to all products offered for sale, irrespective of their price and other attributes, as is required by existing UK legislation. To achieve this, PLEV batteries must be designed to protect themselves against reasonably foreseeable misuse and manufactured to consistently high quality levels. However, evidence of the growing number of serious PLEV fires in the UK shows that some manufacturers are failing to achieve the level of safety required in UK consumer product legislation.

Product testing for this report showed that the PLEV batteries with a higher price-per-unit-of-energy, which had better designed safety circuits, more sophisticated electronics and were better manufactured, successfully prevented thermal runaway by a combination of passive and active protection systems.

However, product testing has also shown that some PLEV BMS fail to respect the current and temperature limits specified for the cells that they use. Both the cells and the BMS protective circuits are thereby susceptible to damage, which can lead to thermal runaway, especially if the battery is used with an incompatible charger. These susceptibilities can and should be addressed by improved BMS design and testing.

Inspection of the general design and manufacturing quality of the tested products has shown examples of other deficiencies, such as lack of waterproofing and poor weld quality, which manufacturers should also address.

Enforcement of legislation and market surveillance may currently be compromised by a lack of consistency in the consumer safety legislation which applies to PLEV products, particularly separately-sold batteries. Further inconsistency and shortcomings in the supporting standards also undermine the need for clarity, uniformity and technical robustness to help manufacturers to comply with legislative requirements.

This report has made a large number of suggestions for actions which can improve PLEV safety, spanning the following areas:

• consistency in the legislation applicable to PLEV batteries
• consistency in the standards covering all PLEV batteries
• numerous detailed improvements to standards, ranging from cell production quality to the abuse testing methodology and functional safety
• collection of incident data
• consumer advice
• increased obligations and penalties for companies selling PLEVs and their batteries

If these suggestions are acted on by the relevant parties in government, standards bodies, manufacturers and other stakeholders, WMG believes that the unacceptably high level of PLEV fires can be reduced over time. There is, however, no quick fix, due to the large number of products already in the hands of consumers and the lead-time for changes to legislation, and for updates to standards, and for manufacturers to develop, validate, productionise and introduce new products to the market.