Lithium-ion Safety Concerns - Battery University

When Sony introduced the first lithium-ion battery in , they knew of the potential safety risks. A recall of the previously released rechargeable metallic lithium battery was a bleak reminder of the discipline one must exercise when dealing with this high energy-dense battery system.

Pioneering work for the lithium battery began in , but is was not until the early 's when the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the eighties. These early models were based on metallic lithium and offered very high energy density. However, inherent instabilities of lithium metal, especially during charging, put a damper on the development. The cell had the potential of a thermal run-away. The temperature would quickly rise to the melting point of the metallic lithium and cause a violent reaction. A large quantity of rechargeable lithium batteries had to be recalled in after the pack in a cellular released hot gases and inflicted burns to a man's face.

Because of the inherent instability of lithium metal, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density, the lithium-ion system is safe, providing certain precautions are met when charging and discharging. Today, lithium-ion is one of the most successful and safe battery chemistries available. Two billion cells are produced every year.

Lithium-ion cells with cobalt cathodes hold twice the energy of a nickel-based battery and four-times that of lead acid. Lithium-ion is a low maintenance system, an advantage that most other chemistries cannot claim. There is no memory and the battery does not require scheduled cycling to prolong its life. Nor does lithium-ion have the sulfation problem of lead acid that occurs when the battery is stored without periodic topping charge. Lithium-ion has a low self-discharge and is environmentally friendly. Disposal causes minimal harm.

Long battery runtimes have always been the wish of many consumers. Battery manufacturers responded by packing more active material into a cell and making the electrodes and separator thinner. This enabled a doubling of energy density since lithium-ion was introduced in .

The high energy density comes at a price. Manufacturing methods become more critical the denser the cells become. With a separator thickness of only 20-25µm, any small intrusion of metallic dust particles can have devastating consequences. Appropriate measures will be needed to achieve the mandated safety standard set forth by UL . Whereas a nail penetration test could be tolerated on the older cell with a capacity of 1.35Ah, today's high-density 2.4Ah cell would become a bomb when performing the same test. UL does not require nail penetration. Lithium-ion batteries are nearing their theoretical energy density limit and battery manufacturers are beginning to focus on improving manufacturing methods and increasing safety.

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Recall of lithium-ion batteries

With the high usage of lithium-ion in cell phones, digital cameras and laptops, there are bound to be issues. A one-in-200,000 failure rate triggered a recall of almost six million lithium-ion packs used in laptops manufactured by Dell and Apple. Heat related battery failures are taken very seriously and manufacturers chose a conservative approach. The decision to replace the batteries puts the consumer at ease and lawyers at bay. Let's now take a look at what's behind the recall.

Sony Energy Devices (Sony), the maker of the lithium-ion cells in question, says that on rare occasions microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit within the cell. Although battery manufacturers strive to minimize the presence of metallic particles, complex assembly techniques make the elimination of all metallic dust nearly impossible.

Figure 1: Lithium-ion battery damages a laptop.
Safety issues are enticing battery manufacturers to change the manufacturing process. According to Sony, contamination of Cu, Al, Fe and Ni particles during the manufacturing process may cause an internal short circuit.

A mild short will only cause an elevated self-discharge. Little heat is generated because the discharging energy is very low. If, however, enough microscopic metal particles converge on one spot, a major electrical short can develop and a sizable current will flow between the positive and negative plates. This causes the temperature to rise, leading to a thermal runaway, also referred to 'venting with flame.'

Lithium-ion cells with cobalt cathodes (same as the recalled laptop batteries) should never rise above 130°C (265°F). At 150°C (302°F) the cell becomes thermally unstable, a condition that can lead to a thermal runaway in which flaming gases are vented.

During a thermal runaway, the high heat of the failing cell can propagate to the next cell, causing it to become thermally unstable as well. In some cases, a chain reaction occurs in which each cell disintegrates at its own timetable. A pack can get destroyed within a few short seconds or linger on for several hours as each cell is consumed one-by-one. To increase safety, packs are fitted with dividers to protect the failing cell from spreading to neighboring cells.

Safety level of lithium-ion systems

There are two basic types of lithium-ion chemistries: cobalt and manganese (spinel). To achieve maximum runtime, cell phones, digital cameras and laptops use cobalt-based lithium-ion. Manganese is the newer of the two chemistries and offers superior thermal stability. It can sustain temperatures of up to 250°C (482°F) before becoming unstable. In addition, manganese has a very low internal resistance and can deliver high current on demand. Increasingly, these batteries are used for power tools and medical devices. Hybrid and electric vehicles will be next.

The drawback of spinel is lower energy density. Typically, a cell made of a pure manganese cathode provides only about half the capacity of cobalt. Cell and laptop users would not be happy if their batteries quit halfway through the expected runtime. To find a workable compromise between high energy density, operational safety and good current delivery, manufacturers of lithium-ion batteries can mix the metals. Typical cathode materials are cobalt, nickel, manganese and iron phosphate.

Let me assure the reader that lithium-ion batteries are safe and heat related failures are rare. The battery manufacturers achieve this high reliability by adding three layers of protection. They are: [1] limiting the amount of active material to achieve a workable equilibrium of energy density and safety; [2] inclusion of various safety mechanisms within the cell; and [3] the addition of an electronic protection circuit in the battery pack.

These protection devices work in the following ways: The PTC device built into the cell acts as a protection to inhibit high current surges; the circuit interrupt device (CID) opens the electrical path if an excessively high charge voltage raises the internal cell pressure to 10 Bar (150 psi); and the safety vent allows a controlled release of gas in the event of a rapid increase in cell pressure. In addition to the mechanical safeguards, the electronic protection circuit external to the cells opens a solid-state switch if the charge voltage of any cell reaches 4.30V. A fuse cuts the current flow if the skin temperature of the cell approaches 90°C (194°F). To prevent the battery from over-discharging, the control circuit cuts off the current path at about 2.50V/cell. In some applications, the higher inherent safety of the spinel system permits the exclusion of the electric circuit. In such a case, the battery relies wholly on the protection devices that are built into the cell.

We need to keep in mind that these safety precautions are only effective if the mode of operation comes from the outside, such as with an electrical short or a faulty charger. Under normal circumstances, a lithium-ion battery will simply power down when a short circuit occurs. If, however, a defect is inherent to the electrochemical cell, such as in contamination caused by microscopic metal particles, this anomaly will go undetected. Nor can the safety circuit stop the disintegration once the cell is in thermal runaway mode. Nothing can stop it once triggered.

What every battery user should know

A major concern arises if static electricity or a faulty charger has destroyed the battery's protection circuit. Such damage can permanently fuse the solid-state switches in an ON position without the user knowing. A battery with a faulty protection circuit may function normally but does not provide protection against abuse.

Another safety issue is cold temperature charging. Consumer grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the packs appear to be charging normally, plating of metallic lithium occurs on the anode while on a sub-freezing charge. The plating is permanent and cannot be removed. If done repeatedly, such damage can compromise the safety of the pack. The battery will become more vulnerable to failure if subjected to impact, crush or high rate charging.

Asia produces many non-brand replacement batteries that are popular with cell users because of low price. Many of these batteries don't provide the same high safety standard as the main brand equivalent. A wise shopper spends a little more and replaces the battery with an approved model. Figure 1 shows a cell that was destroyed while charging in a car. The owner believes that a no-name pack caused the destruction.

Technology Profile

Battery packs built to customer specifications using Lithium-Ion and Lithium-Polymer cells have been Designed and Developed at SWE for over 20 years. SWE has invested extensively in acquiring technology and creating intellectual property associated with development of battery packs and battery systems that utilize Lithium-Ion and Lithium-Polymer chemistry. SWE participates with the intellectual community in sharing technical advances via international battery technology conferences and technical journals. SWE has produced a number of patents directly related to the control of Lithium-Ion batteries and has various related trade secrets – some of which are being considered for future patent applications.  SWE has reduced to practice all of this intellectual property in delivered products used in above-ground, down hole and subsea applications.

Introduction To Lithium-Ion

Lithium-ion batteries have advanced to the level where there are very few applications that cannot take advantage of the excellent cycle life, power & energy density and wide operating temperature range inherent in Lithium-ion technology. Lithium-ion batteries are also Environmentally Friendly.

Lithium-ion batteries are common in consumer electronics. They are one of the most popular types of battery for portable electronics, with one of the best energy-to-weight ratios, no memory effect, and a very slow loss of charge when not in use. In addition to consumer electronics, smaller versions of lithium-ion batteries can be found in specialty applications ranging from human implantable cells to various satellites, to hybrid vehicles and military craft.  Lithium-ion batteries are growing in popularity for defense, automotive, and aerospace applications due to their high energy density.

Recent advances in Battery Management System (BMS) electronic technology for the Lithium-Ion battery and new modular design concepts for construction of complex battery systems have resulted in battery systems which are safer, more robust, more flexible, longer life, and easier to charge and maintain. Pack protection circuits (PTCs), shutdown separators, etc. (developed for mass consumer use) provide several layers of safety not available in other chemistries.

All cells sold by SWE are qualified by the manufacturer to UL requirements.

Lithium-Ion Battery Transportation Safety

Until , there were no DOT restrictions on transportation of Li-Ion cells or batteries. However, beginning in /, the DOT required battery packs to pass new DOT tests. This requirement is exempted for prototype battery packs.

Lithium-Ion Battery Features

Lithium-Ion batteries can be customized to customer needs for size, fit, and performance. Lithium-Ion batteries have a high ENERGY DENSITY (weight to size ratio).

VOLTAGE PER CELL: Lithium-Ion batteries have a nominal voltage of 3.7 volts per cell. By using the cells in series, a battery pack can have any voltage possible in 3.7 volt steps. Ex. Lithium-Ion batteries use 3 cells to provide an 11.1 volt battery, 4 cells to provide a 14.8 volts battery or 10 cells to provide 37 volts battery.

CAPACITY: Lithium-Ion cells are place in parallel to provide the amount of amp-hours (Ah) required. The Ahs can range from a few amps to hundred of amps, depending on the application requirement. Ex. Lithium-Ion batteries use three 2.6Ah cells in parallel will produce 7.8 Ah or use ten 2.6Ah cells in parallel to produce 26 Ah. There a number of cells with high Ah rating that can be used to provide you with the CAPACITY that is required for your application.

MAX CHARGE RATE: Lithium-Ion has a nominal Maximum Charge rate of 1C and Lithium-Polymer of 2C. There are cells that have charge rates up to 10C. By selecting the correct cell you can have a Lithium-Ion battery pack that will meet your requirements.

CHARGE TECHNOLOGY: Lithium-Ion batteries using a typical Pack Protect circuit have a complex charging profile and only a charger that was design for that battery should be used. But if you use the SWE BMS (Battery Management System) with the Pulse Charging on board you will need only a simple DC power supply with constant power to charge the battery pack, or connect it directly to a Solar panel with only an isolation Diode. In other words, no special Li-Ion charger is necessary, thereby reducing the cost of the system.

MAX DISCHARGE RATE: Lithium-Ion has a Maximum Discharge rate of 2C and Lithium-Polymer at 3C (Note: there are selections of Lithium-Polymer cells that have discharge rates greater than a 30C rate).

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DISCHARGE TEMPERATURE RANGE: Lithium-Ion and Lithium-Polymer have a limit of discharging from -20C to 60C. SWE has selected Chemistry and empirical data with an increased limit on discharging down to -50°C.

STORAGE: Recommended temperature range for storage: -20°C to 60°C (storage at temperatures below 20°C reduces permanent capacity loss).Recommended voltage range for short term storage is 3.0 to 4.2 V per cell in series.Prolonged storage periods: Store Li-Ion batteries at about 75% capacity (3.85 V to 4.0 V) and at low temperature to reduce permanent capacity loss over long storage periods.

PRECAUTIONARY NOTES: Lithium-Ion cells have very high power and energy density. Exercise common sense precautions when handling or testing. DO NOT replace individual cells or modules, or combine this battery in series or parallel with other batteries as this may present a risk of fire or explosion.

CAUTION: Lithium-Ion batteries may present a risk of fire or chemical burn if mistreated. DO NOT short circuit, overcharge, crush, mutilate, nail penetrate, reverse polarity, disassemble, expose to temperatures above 100°C (212°F), or incinerate.

DISPOSAL: Lithium-Ion batteries do not contain materials that harm the environment; therefore, there are no disposal rules or restrictions. Lithium-Ion batteries can be recycled to recover the relatively expensive cobalt contained in the Cathode. At end of life, dispose of the used battery safely. Keep away from children.  Do not disassemble and do not dispose of in fire.

SWE Li-Ion Battery Management System (BMS)

SWE has a variety of BMS’s to meet customer needs, from a 1 series pack to 10 series pack. The BMS’s can be stacked in Series to meet requirements for voltage and placed in parallel to provide the current requirements.

Battery Pack Protection: The BMS provides the following: Battery Overcharge Protection (Most critical), - Battery Over-discharge Protection, - Discharge Over-current Protection, - Charge Over-current Protection, - Load short-circuit Protection, and Inhibit 0V charging condition.

Charge Management: Charge Temperature Monitoring: - Pulse charging on board and Charge control can be set to a percent of capacity per customer requirement.

Fuel Gauge: Accurate Battery Fuel Gauging, - Cell Temperature monitoring, and Industry standard serial bus Protocols. SMBus, I2C, RS485, and others.

Recommended Max Currents:  8 Amps continuous, up to 20 Amp pulse for 30ms. (Note: 5 series to 10 series BMS an addition 4 current booster PCAs of 16 Amps per can be added allowing a total of 72 Amps per BMS.)

Cell Balancing: Each module contains cell balancing circuits to balance the series connected sections during charge or continuous balancing.

SUMMARY DESCRIPTION: The BMS provides necessary protection against Overcharge, Over-discharge, and abnormal charge, discharge current and short-circuit load.

The integrated Pulse Charge Management enables end-user to utilize a simple DC power supply with constant power to charge the battery pack or connect direct to a Solar panel with only an isolation Diode. In other words, no special Lithium-Ion charger is necessary there-by reducing the cost of the system. Charging is also inhibited outside a pre-set temperature window, typical, 0°C to 45°C charging.

Lithium-Ion batteries do not have to be frequently fully discharged and recharged ("deep-cycled"), but this may be necessary after about every 30th recharge to recalibrate any electronic "fuel gauge" if used. This prevents the fuel gauge from showing an incorrect battery charge.

Lithium-Ion Battery Storage

The speed at which a Lithium-Ion battery ages is governed by temperature and the state-of-charge. Lithium-Ion batteries should be kept cool. Ideally, they are stored in a refrigerator. Aging will take its toll much faster at high temperatures. 

The recommended Battery systems storage temperature is room temperature or colder. The batteries should be monitored and recharged as required. Typically, the battery should be checked every 90 days to determine whether the pack should be recharge or not.

Lithium-Ion batteries should never be depleted to below their minimum voltage, 2.4 V to 3.0 V per cell.

Like all rechargeable batteries, Lithium-Ion batteries should be charged early and often.  However, if they are not used for a long time, they should be brought to a charge level of around 80% or less depending on the energy of the pack and the length of storage time.

A Lithium-Ion battery will lose storage capacity if it is kept at 100% state of charge during storage.

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