8th July 2020
The Unique Challenges of Lithium-Ion Battery Safety [Part 1]
The global lithium-ion (Li-ion) battery market is expected to grow at a CAGR of 16.4%, from USD 44.2 billion in 2020 to USD 94.4 billion in 2025 [Source]. Factors driving the li-ion battery market growth include increasing demand for – electric vehicles, lithium-ion batteries for industrial applications, connected and IoT-based devices, and automation and battery-operated material-handling equipment in industries.
Lithium-ion batteries have been in use for over 20 years. From small smartphones to high capacity batteries that power electric automobiles, the high energy density of Li-ion cells makes them more versatile and cost-effective than any other battery technology. The benefit, however, does not warrant them to be risk-free. To understand the unique challenges related to this chemistry, we need to dive deep into the science of battery technology.
What makes Li-ion Batteries unsafe?
Batteries used in smartphones are small and usually have one li-ion cell. Laptops use larger batteries that consist of 3 – 12 li-ion cells. The batteries used in electric cars and airplanes have hundreds of cells. Li-ion cells are produced in a variety of form factors – cylindrical, prismatic, pouch. Cell cases can be hard or soft, whereas the electrodes inside the cell can be wound or stacked. The cases can also include a variety of protection mechanisms.
The protection electronics in a battery pack prevent a variety of abuse scenarios like:
- external short circuit
- charging outside of the accepted
- temperature range
- over-discharge, and fast charging of over-discharged cells
- deep discharging
A Li-ion battery contains two electrodes – the cathode and the anode. A semi-permeable sheet – the ion-conductor and electron-insulator nonaqueous electrolyte, separates these electrodes. As you begin to charge a li-ion battery, li-ion moves from the cathode through the micro-perforations in the separator to the anode. The reverse happens when the battery discharges, the li-ions flow from anode to cathode, powering an electronic device.
When the semi-permeable separator fails and lets the anode and cathode touch, it causes a short circuit. That’s when the battery starts to overheat. The separator is the predominant cause of severe li-ion battery damages.
Let's take a look at the events that cause the separator to fail:
1. Thermal Runway
Li-ion batteries are temperature and voltage-sensitive. The safe zone is the temperature ranging from 10 °C to +55 °C. The primary hazard in a lithium-ion battery is the electrolyte, which at its core, is flammable. As the battery temperature rises to above ~80°C, the exothermic reaction rate inside the battery increases.
There are various causes of overheating, such as overcharging, exposure to over/under temperature, external or internal short circuits. An internal shorting is one of the primary reasons for thermal runaway.
If this goes unchecked, it could further lead to an exponential rise in cell temperature, ultimately losing stability. This leads to all remaining thermal and electrochemical energy being released to the environment.
Read: To know more about the importance of Effective Thermal Management for battery life and performance, read our whitepaper on the ‘Need for Thermal Management for Lithium-Ion Battery Packs.’
2. Voltage Limits
Lithium-ion cells are vulnerable to stress by voltage ranges outside of safe ones between 2.5 and 3.65/4.1/4.2 or 4.35V – (depending on the chemistry of the cell). Exceeding this voltage range leads to premature ageing and in safety risks, thanks to the reactive components within the cells.
When stored for long periods, the tiny current draw of the protection circuitry may drain the battery below its shutoff voltage. Regular chargers may then be of no use since the battery management system may record this battery (or charger) ‘failure.’ Many sorts of lithium-ion cells can’t be charged safely below 0 °C, as this will end in lithium plating and can cause complications like internal short-circuit paths.
3. Internal Short Circuits
Rough handling and overcharging can cause internal short circuits. As the battery becomes stuffed with lithium ions, it expands. Too much lithium can mechanically stress the battery and compromise the internal insulation. In some cases, overcharging can result in electron-conducting metallic deposits between the electrodes.
4. Charging Problems
The battery should be charged only at a temperature of +5°C to +45°C. Li-ion batteries, when overheated or overcharged, could suffer severe damages like thermal runaway and cell rupture. Overcharge will end in the decomposition of cathode materials, thus the oxidation of the electrolyte.
Over-discharge will cause the decomposition of SEIs on the anode and copper foil oxidation. Charging a battery cell too quickly could also result in a short circuit.
5. Faulty Chargers
Faulty chargers can affect the security of the battery as they will destroy the battery’s protection circuit. While charging at temperatures below 0 °C, the negative electrode of the cells gets plated with pure lithium, which may compromise the entire pack’s security.
Safety is an essential criterion for electric-cars and large scale applications. It is often manifested by stability on abuse, including mechanical, electrical, and thermal abuse, and is still a complicated issue involving lithium batteries.
Cell design and fabrication also have a significant influence on the cell’s electrochemical and safety performances. Enhancements are still needed for high energy density lithium batteries. The electrolyte is undoubtedly the most effective method to settle the safety issues affecting lithium batteries. The cold temperature causes the performance of all batteries to drop.
Extremely low temperature also makes charging more difficult, especially with Li-ion, as charging is more delicate than discharging. Many EV batteries include a heating blanket to protect the battery when charging at cold temperatures.
Core Functions of a BMS
The Battery Management System functions beyond the battery safety goal. As the first layer of defense, the BMS acts as a watchtower inside the battery pack. It plays a crucial role in detecting potential threats, protecting the battery pack from severe damage, and enhancing the battery’s safety, reliability, and longevity.
It is a sensor and a controller that measures and computes data to ensure that the battery is operating in a safe area. The BMS essentially avoids the power distribution to be interrupted abruptly and erratically, safeguarding the battery pack from severe damage.
The core functions of a BMS for Li-ion batteries are to protect the battery from overcharge, over-discharge, overcurrent, over and under-voltage, and thermal runaway.
The BMS provides critical information with regards to:
- Thermal Protection
- Over-current Protection
- Cell Balancing
- SoC and SoH calculation
- Over & Under Voltage Protection (Each Cell)
- Communication with all battery components
- Data acquisition and analysis
- Prolong battery life
Ensuring battery safety at the component level, integration level, battery level, vehicle level is an absolute must to make sure that the end user is safe.
Here's how the BMS helps the user overcome safety challenges:
At the cell and battery level —
- Pre-emption – avoid thermal runaway
- Detection – warning of potential failure
- Intervention – stop thermal runaway
- Containment – minimize damage
At the vehicle level —
- Safe driving
- Preventive Protection
- Adaptive Protection
- Secure and rescue
In response to the need to eliminate battery failures, various operations in the BMS are implemented to monitor the battery’s performance and health to prevent unfortunate mishaps drawn towards fire hazards or explosions. As the “brain” of any battery pack, the BMS is the key to the reliable and efficient functioning of the battery and the choice of BMS defines the performance and life of your battery.
Battery Management as a Platform
Battery Power is finding new applications each day. To meet this demand, we’ve realized the need to build not just application-specific systems but also an advanced modular platform. We’ve built ION’s battery management platform on our proprietary Fusion Firmware code. Using the platform, we have designed three BMS variants – CT, LT, and XT to cater to a wide range of applications.