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In this 2017 update of articles from 2015, and 2016, the technical experts at Robson Forensic highlight the safety protections built into Lithium-ion batteries. Failure of these battery safety features can lead to fire and explosions. Our experts are frequently engaged to investigate battery fires in a range of consumer products, including cell phones, laptops, RC models, e-cigarettes, vape devices, and other portable electronic devices.

Safe Battery Design to Prevent Fires & Explosions

The past decade has seen significant advances in lithium battery technology. Lithium-ion batteries are the preferred power source for many portable electronics because of their high energy density, long cycle life, and higher operational voltage as compared to NiCd and NiMH systems.

However, Lithium-ion batteries are sensitive to overcharge, over-discharge, over-current, short-circuiting, and physical abuse which has attracted much attention recently. Unlike other battery types, Li-ion battery electrolyte is a volatile, extremely flammable solvent that will burn quite vigorously and easily.

The high energy density can undergo thermal runaway. Basically, if the battery cell gets too hot, lithium ions will begin reacting with oxygen stored as metal oxides and release even more heat, which accelerates the reaction further. What inevitably results is a battery that self-ignites, sprays its highly flammable electrolyte out which promptly ignites in the fresh supply of oxygen.

Lithium-ion batteries require very carefully designed battery management to prevent catastrophic failure. This includes mechanisms that protect them from short circuit, over-charge, under-charge, over-discharge, over-temperature, over-current, pressure build-up, and physical abuse and disable them before dangerous conditions arise. However, the protection devices are ineffective to stop a thermal runaway once in progress which could result in an explosion or fire. Therefore, battery cell protections are critical in Lithium batteries.

Short Circuit or Excessive Current Protection

One type of safety device is a Positive Thermal Coefficient (PTC), which is a re-settable device that provides protection against short circuits. PTC is a temperature sensitive current limiting switch which will have a very low resistance at normal temperature and allows current to pass. As electric current increases, temperature increases, and the resistance increases dramatically and stops the current flow. Once the short circuit or overcurrent is removed and conditions normalize, the PTC will cool down and reset and allow current to flow again.

Overcharge/Over-voltage and Over Discharge/Under-voltage Protection

Overcharge Condition

If a Lithium Ion battery is overcharged, the battery becomes unstable and produces carbon dioxide (CO2), the cell pressure rises and if the charge is allowed to continue, the pressure will rise further which could result in swelling, rupturing, explosion and fire. Overcharge conditions can be caused by things such as a faulty charger, a non-compatible charger, or inadequate protection within the battery.

Over-discharging Condition

Over-discharging or storage in a discharged state for extended periods can result in a breakdown of the electrode materials. The cathode undergoes an irreversible chemical reaction and during subsequent charging, the separator can be damaged and create a dead short, which could result in swelling, rupturing, explosion and fire. Over-discharge conditions can be caused by usage past the minimum voltage level and by inadequate protection within the battery.

Over-current Conditions (charge or discharge)

Over-current conditions occur when a cell is charged or discharged at a much higher current than it’s designed for. Over-current conditions can be caused by an oversized load, a short circuit or non-compatible charger and by inadequate protection within the battery.

Short Circuit Conditions

Accidental short circuits occur when the positive and negative terminals are shorted or connected.

Short circuits can result in swelling, rupturing, explosion and fire. Short circuits can occur when exposed terminals come into contact with metal objects or from poor wiring practices, or from abuse. Short circuit conditions can cause problems when there is inadequate protection within the battery.

The Protection Circuit Module (PCM) is an electronic circuit that can protect the battery against overcharge, over discharge, short circuit, and over current. All of these factors are important in obtaining the maximum safety and optimal performance from a Lithium battery. The PCM monitors the voltage and current and disconnects the battery cell when conditions reach predetermined limits.

Over Pressurization Protection

The electrochemical process inside a battery can give rise to the generation of gases, particularly during conditions of over charge. This is called gassing. Since battery cells are sealed, gassing will cause a pressure build-up within the cell, which will usually be accompanied by a rise in temperature, until the cell ruptures or explodes. To overcome this problem two different protection methods are used.

A Current Interrupt Device (CID) is a mechanical pressure switch that will permanently disconnect the battery connection when the battery cell experiences excessive internal pressure. High pressure in the battery cell is often due to overcharging. If the pressure switch pops it will disconnect the external plus pole and the internal positive pole on the battery cell and electric current will stop flowing.

Sealed battery cells typically incorporate some type of vent to allow a controlled release of gas in the event of excessive cell pressure. The release of chemicals into the atmosphere could be dangerous. This is the last line of defense for a pressurized cell. Cells are not meant to vent excessively under normal operating conditions.


Mechanical Protection

The mechanical protection provided by the case of the battery cell is intended to be resistant to shocks and vibrations and a large range of temperatures. The battery is designed to be efficiently protected from reasonable environmental stresses (e.g. external impacts, thermal shocks) while minimizing size and weight.

Separators play a key role inside all batteries. Their main function is to keep the positive and negative

electrodes apart to prevent electrical short circuits and at the same time allow rapid transfer of ions.

The strong demand for higher capacity cells has resulted in reduced thickness of the separators.

The thicker the separator, the greater the mechanical strength and the lower the probability

of punctures during cell assembly. Thinner separators take up less space and permit increased capacity and rate, but also provide less insulation between electrodes and less mechanical robustness. If the separator loses mechanical integrity, then the electrodes can come into direct contact, react chemically, and result in thermal runaway. Ultra-thin separators put more stringent requirements on the manufacturing process because thin separators are more susceptible to impurities of metallic dust particles than the older designs with thicker separators.

Procedures and Discipline

Battery manufacturers typically provide instructions with batteries. Investigation of battery incidents includes review of the practices of the user for possible misuse, determination of foreseeability of misuse, and protections for foreseeable misuse. Some typical instructions are listed below.

  • Never use a battery in a way it was not intended.
  • Only use charger specifically designed for type battery being charged.
  • Follow manufacturers recommendations for operating limits and conditions.
  • Store the batteries in a cool place.
  • Prevent the shorting the battery terminals. (Don’t put battery in your pocket with keys or coins).
  • Never charge with a modified charger.
  • Never attempt to charge a battery which has been physically damaged.
  • Never charge a battery at a higher current than specified.
  • Never leave the battery in the charger once it is fully charged.
  • Never disassemble a battery.

Battery Failure - Fire / Explosion

Lithium ion batteries can become thermally unstable, which can lead to uncontrolled thermal runaway in which the cell gets increasingly hotter until it bulges, hisses and vents flaming gases, or it explodes. Once in progress, the protection devices discussed throughout this article are incapable of stopping a thermal runaway. There is an additional risk that high heat from failing cells may propagate to adjacent cells, causing a chain reaction of failure. Investigation of battery incidents includes consideration for what protections there are for preventing an incident and for what protections there are for guarding against injury.

Many major battery brands use comprehensive designs, incorporating necessary protections and rigid quality control practices. However, our own casework involving lithium battery fires and explosions demonstrates that some manufacturers fail to provide adequate design protections and/or proper manufacturing processes.

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INVESTIGATING BATTERY FIRES & EXPLOSIONS

The electrical engineers at Robson Forensic regularly investigate battery fires and explosions to determine their cause. Batteries have inherent risks and LiPo batteries can have catastrophic results when they fail. Investigations of these incidents involving batteries include determination of cause, determination of whether there is or is not a manufacturing or design defect including review of safety devices, and determination of whether the instructions or warnings are adequate or have been followed.

For more information submit an inquiry or visit our Electrical Engineering practice page.

 

Featured Expert

Kenneth J. Kutchek, P.E.

Electrical Engineer & Controls Expert

Ken is an electrical engineer with more than 25 years of hands-on experience in industrial automation, manufacturing automation, process control, industrial controls, software development and workplace safety. He applies his expertise to forensic casework involving injuries and fires of electrical origin, as well as various issues involving industrial systems performance.

Ken developed this article in concert with other members of the electrical engineering practice group. Please contact Robson Forensic to discuss your case and determine which of our experts is best qualified to assist with the specific issues in your case.