Packaging and Safety

A look at Old and New Battery Packaging

Early batteries of the 1700s and 1800s were mostly encased in glass jars, and as the batteries grew in size, sealed wooden containers were used. With the need for portability, sealed cylindrical cells appeared that eventually led to some standardization in size format. To satisfy our curiosity, let’s explore the history of these battery norms.

In the early days, standardization involved primary cells mostly in carbon-zinc; alkaline came only in the early 1960s. With the advent of the sealed nickel-cadmium batteries in the 1950s and 1960s, new sizes appeared, many of which were derived from the standard “A” and “C” sizes established at the end of the 1800s. Manufacturers of lithium-ion departed from the conventional sizes and invented their own formats of cylindrical, prismatic and pouch shapes. Looking at the many formats of cell phone batteries alone, one realizes that standardizing has begun to drift. Table 1 summarizes historic and current battery sizes.

Size	Dimensions	History
F cell	33 x 90 mm	Introduced in 1896 for lanterns; later used for radios; only available in nickel-cadmium today
E cell	N/A	Introduced ca. 1905 to power box lanterns and hobby applications. Discontinued ca. 1980
D cell	34 x 61mm	Introduced in 1898 for flashlights and radios; still current
C cell	25.5 x 50mm	Introduced ca. 1900 to attain smaller form factor
B cell	20.1 x 56.8mm	Introduced in 1900 for portable lighting, including bicycle lights in Europe; discontinued in in North America in 2001
A cell	17 x 50mm	Only available for nickel-cadmium; also come in half sizes
AA cell	14.5 x 50mm	Known as penlight batteries; introduced as two side-by-side cells for pocket lights ca. 1907; used as spy tool during WWI; single AA cells were first sold in 1915 and became popular in 1947
AAA cell	10.5 x 44.5mm	Developed in 1954 to reduce size for Kodak and Polaroid cameras; became popular in the mid 1970s in alkaline
AAAA cell	8.3 x 42.5mm	Offshoot of 9V battery and available since 1990s; used for laser pointers, LED penlights, computer styli, headphone amplifiers
4.5V battery	65 x 61 x 21mm	Three cells form a flat pack; short terminal strip is positive, long strip is negative; common in Europe
9V battery	48.5 x 26.5 x 17.5mm	Introduced in 1956 for transistor radios; contains six prismatic or AAAA cells
18650	18 x 65mm	Developed in the mid 1990s for lithium-ion-ion; commonly used in laptop battery packs
26650	26 x 65	Larger Li-ion battery for industrial applications
26700	26 x 70	Same as 26650 with slightly larger diameter

Table 1: Common old and new battery norms. Some sizes come in fractural lengths mostly in nickel-based chemistries.

When first invented, a battery was perceived as being “big” and this reflects in the sizing convention. While “F” was chosen as a medium-size battery in the late 1800s, our forefathers did not anticipate the need for miniature batteries. Running out of letters towards smaller sizes brought on the awkward designations of AA, AAA and AAAA. It’s also interesting that many sizes never took off and “A” was replaced with the thinner “AA.”

A successful standard for a cylindrical cell is the 18650. Developed in the mid 1990s for lithium-ion, these cells are used for laptops, electric bicycles and even electric vehicles, such as the Tesla Roadster car.

Since the introduction of the 9V battery in 1956, no new consumer format has emerged. Meanwhile, portable devices have lowered the operating voltages and 9V is overkill. The 9V battery is expensive to manufacture and has a low specific energy. A 3.6V battery alternative should be offered in primary and secondary versions with different chemistries. Similar in size to the 9V, this pack would be protected with unique battery terminals that would only allow charging secondary batteries. A code would apply the appropriate algorithm.

Starter batteries for cars also follow battery norms, which consist of the North American BCI, the European DIN and the Japanese JIS standards. These batteries are similar in footprint to allow easy interchange. In an effort to standardize, all American car manufacturers are in the process of converting to the American DIN size batteries. Most manufacturers of deep-cycle and stationary batteries produce their own sizes and the replacement must be sourced from the original maker. Standardizing of the electric vehicle may be too early. Forcing the issue could follow the failed attempt to standardize laptop batteries in the 1990s.

Types of Battery Cells

Early batteries were in jars, but mass production changed the packaging to the cylindrical design. The year 1896 pioneered the large F cell for lanterns; the D cell followed in 1898, the C cell in 1900, and the popular AA was introduced in 1907. Read more about Standardizing Batteries into Norms. Design criteria and cost considerations required new battery formats that offer distinct advantages over the cylindrical design.

Cylindrical Cell

The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder has the ability to withstand internal pressures without deforming. Figure 1 shows a cross section of a cell.

Figure 1: Cross section of a lithium-ion cylindrical cell The cylindrical cell design has good cycling ability, offers a long calendar life, is economical but is heavy and has low packaging density due to space cavities. Courtesy of Sanyo

Typical applications for the cylindrical cell are power tools, medical instruments and laptops. Nickel-cadmium offers the largest variety of cell choices, and some popular formats have spilled over to nickel-metal-hydride. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats.

The established standards for nickel-based batteries did not catch on with lithium-ion and the chemistry has established its own formats. One of the most popular cell packages is the 18650, as illustrated in Figure 2. Eighteen denotes the diameter and 65 is the length of the cell in millimeters. The Li-manganese version 18650 has a capacity of 1,200–1,500mAh; the Li-cobalt version is 2,400–3,000mAh. The larger 26650 cells have a diameter of 26mm with a length of 65mm and deliver about 3,200mAh in the manganese version. This cell format is used in power tools and some hybrid vehicles.

Figure 2: Popular 18650 lithium-ion cell The metallic cylinder measure 18mm in diameter and 65mm the length. The larger 26650 cell measures 26mm in diameter.  Courtesy of Cadex

Lead acid batteries come in flooded and dry formats; portable versions are packaged in a prismatic design resembling a rectangular box made of plastic. Some lead acid systems also use the cylindrical design by adapting the winding technique, and the Hawker Cyclone is in this format. It offers improved cell stability, higher discharge currents and better temperature stability than the conventional prismatic design.

Cylindrical cells include a venting mechanism that releases excess gases when pressure builds up. The more simplistic design utilizes a membrane seal that ruptures under high pressure. Leakage and subsequent dry-out may occur when the membrane breaks. The re-sealable vents with a spring-loaded valve are the preferred design. Cylindrical cells make inefficient use of space, but the air cavities that result with side-by-side placement can be used for air-cooling.

Button Cell

Smaller devices required a more compact cell design, and in the 1980s the button cell met this need. The desired voltage was achieved by stacking the cells into a tube. Early cordless telephones, medical devices and security wands at airports used these batteries.

Although small and inexpensive to build, the stacked button cell fell out of favor, and newer designs reverted to more conventional battery configurations. A drawback of the button cell is swelling if charged too rapidly. Button cells have no safety vent and can only be charged at a 10- to 16-hour charge. However, newer designs claim rapid charge capability. Most button cells in use today are non-rechargeable and can be found in medical implants, watches, hearing aids, car keys and memory backup. Figure 3 illustrates the button cells with accompanying cross section.

Figure 3: Button cells
Button cells, also known as coin cells, offer small size and ease of stacking but do not allow fast charging. Most commercial button cells are non-rechargeable.

Courtesy of Sanyo and Panasonic

Prismatic Cell

Introduced in the early 1990s, the prismatic cell satisfies the demand for thinner sizes and lower manufacturing costs. Wrapped in elegant packages resembling a box of chewing gum or a small chocolate bar, prismatic cells make optimal use of space by using the layered approach. These cells are predominantly found in mobile phones with lithium-ion. No universal format exists and each manufacturer designs its own. If the housing design allows a few millimeters extra in a cell phone or laptop, the manufacturer designs a new pack for the sake of higher capacity. High volume justifies this move.

Prismatic cells are also making critical inroads into larger formats. Packaged inwelded aluminum housings, the cells deliver capacities of 20 to 30Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Figure 4shows the prismatic cell.

Figure 4: Cross section of a prismatic cell The prismatic cell improves space utilization and allows flexible design but it can be more expensive to manufacture, less efficient in thermal management and have a shorter cycle life than the cylindrical design. Courtesy of Polystor Corporation

The prismatic cell requires a slightly thicker wall size to compensate for the decreased mechanical stability from the cylindrical design, resulting in a small capacity drop. Optimizing use of space makes up this loss. Prismatic cells for portable devices range from 400mAh to 2,000mAh.

Pouch Cell

In 1995, the pouch cell surprised the battery world with a radical new design. Rather than using a metallic cylinder and glass-to-metal electrical feed-through for insulation, conductive foil tabs welded to the electrode and sealed to the pouch carry the positive and negative terminals to the outside. Figure 5 illustrates such a pouch cell.

Figure 5: The pouch cell The pouch cell offers a simple, flexible and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life. Courtesy of Cadex

The pouch cell makes the most efficient use of space and achieves a 90 to 95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some alternative support in the battery compartment. The pouch pack finds applications in consumer, military, as well as automotive applications. No standardized pouch cells exist; each manufacturer builds the cells for a specific application.

Pouch packs are commonly Li-polymer. Its specific energy is often lower and the cell is less durable than Li-ion in the cylindrical package. Swelling or bulging as a result of gas generation during charge and discharge is a concern. Battery manufacturers insist that these batteries do not generate excess gases that can lead to swelling. Nevertheless, excess swelling can occur and most is due to faulty manufacturing, and not misuse. Some dealers have failures due to swelling of as much as three percent on certain batches. The pressure from swelling can crack a battery cover, and in some cases break the display and electronic circuit board. Manufacturers say that an inflated cell is safe. While this may be true, do not puncture a swollen cell in close proximity to heat or fire; the escaping gases can ignite. Figure 6 shows a swelled pouch cell.

Figure 6: Swelling pouch cell Swelling can occur as part of gas generation. Battery manufacturers are at odds why this happens. A 5mm (0.2”) battery in a hard shell can grow to 8mm (0.3”), more in a foil package. Courtesy of Cadex

To prevent swelling, the manufacturer adds excess film to create a “gas bag” outside the cell. During the first charge, gases escape into the gasbag, which is then cut off and the pack resealed as part of the finishing process. Expect some swelling on subsequent charges; 8 to 10 percent over 500 cycles is normal. Provision must be made in the battery compartment to allow for expansion. It is best not to stack pouch cells but to lay them flat side by side. Prevent sharp edges that could stress the pouch cell as they expand.

Summary of Packaging Advantages and Disadvantages

• A cell in a cylindrical metallic case has good cycling ability, offers a long calendar life, is economical to manufacture, but is heavy and has low packaging density.
   
• The prismatic metallic case has improved packaging density but can be more expensive to manufacture, is less efficient in thermal management and may have a shorter cycle life.
   
• The prismatic pouch pack is light and cost-effective to manufacture. Exposure to high humidity and hot temperature can shorten the service life. A swelling factor of 8–10 percent over 500 cycles is normal.

Serial and Parallel Battery Configurations

Battery packs achieve the desired operating voltage by connecting several cells in series, with each cell adding to the total terminal voltage. Parallel connection attains higher capacity for increased current handling, as each cell adds to the total current handling. Some packs may have a combination of serial and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve 14.4V and two strings of these 4 cells in parallel (for a pack total of 8 cells) to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning 4 cells are in series and 2 strings of these in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short. The foil also shields against heat transfer should one cell get hot.

Most battery chemistries allow serial and parallel configuration. It is important to use the same battery type with equal capacity throughout and never mix different makes and sizes. A weaker cell causes an imbalance. This is especially critical in a serial configuration and a battery is only as strong as the weakest link.

Imagine a chain with strong and weak links. This chain can pull a small weight but when the tension rises, the weakest link will break. The same happens when connecting cells with different capacities in a battery. The weak cells may not quit immediately but get exhausted more quickly than the strong ones when in continued use. On charge, the low cells fill up before the strong ones and get hot; on discharge the weak are empty before the strong ones and they are getting stressed.

Single Cell Applications

The single-cell design is the simplest battery pack. A typical example of this configuration is the cellular phone battery with a 3.6V lithium-ion cell. Other uses of a single cell are wall clocks, which typically use a 1.5V alkaline cell, as well as wristwatches and memory backup.

The nominal cell voltage of nickel is 1.2V. There is no difference between the 1.2V and 1.25V cell; the marking is simply preference. Whereas consumer batteries use 1.2V/cell as the nominal rating, industrial, aviation and military batteries adhere to the original 1.25V. The alkaline delivers 1.5V, silver-oxide 1.6V, lead acid 2V, primary lithium 3V, Li-phosphate 3.3V and regular lithium-ion 3.6V. Li-manganese and other lithium-based systems sometimes use 3.7V. This has nothing to do with electrochemistry and these batteries can serve as 3.6V cells. Manufacturers like to use a higher voltage because low internal resistance causes less of a voltage drop with a load. Read more: Confusion with Voltages

Serial Connection

Portable equipment needing higher voltages use battery packs with two or more cells connected in series. Figure 3-8 shows a battery pack with four 1.2V nickel-based cells in series to produce 4.8V. In comparison, a four-cell lead acid string with 2V/cell will generate 8V, and four Li-ion with 3.6V/cell will give 14.40V. If you need an odd voltage of, say, 9.5 volts, you can connect five lead acid, eight NiMH/NiCd), or three Li-ion in series. The end battery voltage does not need to be exact as long as it is higher than what the device specifies. A 12V supply should work; most battery-operated devices can tolerate some over-voltage.

Figure 1: Serial connection of four NiCd or NiMH cells 
Adding cells in a string increases the voltage; the current remains the same.

Courtesy of Cadex

A higher voltage has the advantage of keeping the conductor size small. Medium-priced cordless power tools run on 12V and 18V batteries; high-end power tools use 24V and 36V. The car industry talked about increasing the starter battery from 12V (14V) to 36V, better known as 42V, by placing 18 lead acid cells in series. Logistics of changing the electrical components and arcing problems on mechanical switches derailed the move. Early hybrid cars run on 148V batteries; newer models have batteries with 450–500V. Such a high-voltage battery requires 400 nickel-based cells in series. Li-ion cuts the cell count by three.

High-voltage batteries require careful cell matching, especially when drawing heavy loads or when operating in cold temperatures. With so many cells in series, the possibility of one failing is real. One open cell would break the circuit and a shorted one would lower the overall voltage.

Cell matching has always been a challenge when replacing a faulty cell in an aging pack. A new cell has a higher capacity than the others, causing an imbalance. Welded construction adds to the complexity of repair and for these reasons, battery packs are commonly replaced as a unit when one cell fails. High-voltage hybrid batteries, in which a full replacement would be prohibitive, divide the pack into blocks, each consisting of a specific number of cells. If one cell fails, the affected block is replaced.

Figure 2 illustrates a battery pack in which “cell 3” produces only 0.6V instead of the full 1.2V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack and the runtime will be severely shortened. The remaining three cells are unable to deliver their stored energy when the equipment cuts off due to low voltage. The cause of cell failure can be a partial short cell that consumes its own charge from within through elevated self-discharge, or a dry-out in which the cell has lost electrolyte by a leak or through inappropriate usage.

Figure 2: Serial connection with one faulty cell
Faulty “cell 3” lowers the overall voltage from 4.8V to 4.2V, causing the equipment to cut off prematurely. The remaining good cells can no longer deliver the energy.

Courtesy of Cadex

Parallel Connection

If higher currents are needed and larger cells with increased ampere-hour (Ah) ratings are not available or the design has constraints, one or more cells are connected in parallel. Most chemistries allow parallel configurations with little side effect. Figure 3 illustrates four cells connected in parallel. The voltage of the illustrated pack remains at 1.2V, but the current handling and runtime are increased fourfold.

Figure 3: Parallel connection of four cells With parallel cells, the current handling and runtime increases while voltage stays the same. Courtesy of Cadex

A high-resistance cell, or one that is open, is less critical in a parallel circuit than in serial configuration, however, a weak cell reduces the total load capability. It’s like an engine that fires on only three cylinders instead of all four. An electrical short, on the other hand, could be devastating because the faulty cell would drain energy from the other cells, causing a fire hazard. Most so-called shorts are of mild nature and manifest themselves in elevated self-discharge. Figure 4 illustrates a parallel configuration with one faulty cell.

Figure 4: Parallel/connection with one faulty cell A weak cell will not affect the voltage but will provide a low runtime due to reduced current handling. A shorted cell could cause excessive heat and become a fire hazard. Courtesy of Cadex

Serial/Parallel Connection

The serial/parallel configuration shown in Figure 5 allows superior design flexibility and achieves the wanted voltage and current ratings with a standard cell size. The total power is the product of voltage times current, and the four 1.2V/1000mAh cells produce 4.8Wh. Serial/parallel connections are common with lithium-ion, especially for laptop batteries, and the built-in protection circuit must monitor each cell individually. Integrated circuits (ICs) designed for various cell combinations simplify the pack design.

Figure 5: Serial/ parallel connection of four cells This configuration provides maximum design flexibility. Courtesy of Cadex

Simple Guidelines for Using Household Primary Batteries

• Keep the battery contacts clean. A four-cell configuration has eight contacts (cell to holder and holder to next cell); each contact adds resistance.
   
• Never mix batteries; replace all cells when weak. The overall performance is only as good as the weakest link in the chain.
   
• Observe polarity. A reversed cell subtracts rather than adds to the cell voltage.
   
• Remove batteries from the equipment when no longer in use to prevent leakage and corrosion. While spent alkaline normally do not leak, spent carbon-zinc discharge corrosive acid that can destroy the device.
   
• Don’t store loose cells in a metal box. Place individual cells in small plastic bags to prevent an electrical short. Don’t carry loose cells in your pockets.
   
• Keep batteries away from small children. If swallowed, the current flow of the battery can ulcerate the stomach wall.The battery can also rupture and cause poisoning.
   
• Do not recharge non-rechargeable batteries; hydrogen buildup can lead to an explosion. Perform experimental charging only under supervision.

Simple Guidelines for Using Household Secondary Batteries

• Observe polarity when charging a secondary cell. Reversed polarity can cause an electrical short that can lead to heat and fire if left unattended.
   
• Remove fully charged batteries from the charger. A consumer charger may not apply the optimal trickle charge and the cell could be stressed with overcharge.

Confusion with Voltages

A battery is an electrochemical device that produces a voltage potential when placing different metals in acid solutions. The open circuit voltage (OCV) attained varies according to the metals and acid solutions (electrolyte) used. Applying a charge or discharge places the battery in the closed circuit voltage (CCV) condition; charging raises the voltage and discharging lowers it. This voltage behavior is governed by the internal battery resistance; a low resistance produces less fluctuation under load or charge than a high one. Charging and discharging agitates the battery and a full stabilization takes up to 24 hours. Temperature also has a role; cold rises the voltage and heat lowers it.

Manufacturers rate a battery by assigning a nominal voltage and with a few exceptions, these voltages follow an agreed convention. Rating some Li-ion higher than the standard 3.6V/cell may help in product marketing but for the user, a chemistry-specific voltage counts. Here are the nominal voltages of the most common batteries in brief. 

Lead Acid

The nominal voltage of lead acid is 2.00 volts per cell, however when measuring the open circuit voltage (OCV), the voltage of a fully charged battery should be 2.10V/cell. Keeping lead acid below 2.10V/cell will cause the buildup of sulfation.

Nickel-based

In consumer applications, NiCd and NiMH are rated at 1.2V/cell, industrial, aviation and military batteries adhere to the original 1.25V. There is no difference between the 1.2V and 1.25V cell; the marking is simply preference.

Lithium-ion

The nominal voltage of lithium-ion had been 3.60V/cell. This is a practical figure because it represents three nickel-based batteries connected in series (3 x 1.2V = 3.6V). Some cell manufacturers mark their Li-ion products as 3.70V/cell or higher. This poses a marketing advantage because of higher watt-hours on paper (multiplying voltage times current equals W). It also creates unfamiliar references of 11.1V and 14.8V when connecting three and four cells in series. Let this higher voltage not cause confusion; equipment manufacturers will always adhere to the nominal cell voltage of 3.60V for most Li-ion systems, and the standard designation of 10.8V and 14.4V will always work.

How did this higher voltage creep in? To calculate the nominal voltage, we take a fully charged battery that measures 4.20V and then fully discharge it to 3.00V at a rate of 0.5C while plotting the average voltage. For Li-cobalt, the average voltage comes to 3.6V/cell. Performing the same discharge on a fully charged Li-manganese with a lower internal resistance will result in a higher average voltage. Pure spinel has one of the lowest internal resistances, and the plotted voltage on a load moves up to between 3.70 and 3.80V/cell. This higher midpoint voltage does not change the full-charge and end-of-discharge voltage threshold.

The phosphate-based lithium-ion deviates from others in the Li-ion family and the nominal cell voltages are specified at between 3.20 and 3.30V. Because of the voltage difference, the two lithium-ion families are not interchangeable. New lithium-based batteries will have other voltages and specialty chargers may be needed.

Primary Batteries

The alkaline delivers 1.5V, silver-oxide 1.6V and primary lithium 3V. The 9-volt battery has six cells in series. Do not charge primary batteries because overcharge can produce explosive gases. See Reusable Alkaline. 

Protection Circuits

Batteries can release high power, and most packs include protection to safeguard against malfunction. The most basic safety device in a battery is a fuse that opens on high current. Some devices open permanently and render the battery useless; others are more forgiving and reset. The Polyswitch™ is such a re-settable device. It creates a high resistance on excess current and reverts back to the low ON position when the condition normalizes. A third method is a solid-state switch that measures the current and disconnects on excessive load conditions. All switching devices have a residual resistance during normal operation, which causes a slight increase in overall battery resistance and a subsequent voltage drop.

Intrinsically Safe Batteries

Intrinsically safe (IS) batteries contain protection circuits that prevent the formation of high currents, which could lead to excess heat, sparks and explosion. Authorities mandate IS batteries for two-way radios, gas detectors and other electronic instruments operating in hazardous areas such as oil refineries, chemical plants and grain elevators. There are several levels of intrinsic safety, each serving a specific hazard level, and the requirements vary from country to country. The provisions are in addition to the protection circuit for lithium-ion, and the approval standards are rigorous. This results in a high price for the battery.

Making Lithium-ion Safe

Battery packs for laptops and other portable devices contain many levels of protection to assure safety under (almost) all circumstances when in the hands of the public. The safety begins with the battery cell, which includes: [1] a built-in temperature switch called PTC that protects against high current surges, [2] a circuit interrupt device (CID) that opens the electrical path if an over-charge raises the internal cell pressure to 1000 kPa (145psi), and [3]a safety vent that releases gas in the event of a rapid increase in cell pressure.

In addition to these internal safeguards, an external electronic protection circuit prevents the charge voltage of any cell from exceeding 4.30V. Furthermore, a fuse cuts the current if the skin temperature of any cell approaches 90°C (194°F). To prevent the battery from over-discharging, a control circuit cuts off the current path at about 2.20V/cell.

Each cell in a string needs independent voltage monitoring. The higher the cell count, the more complex the protection circuit becomes. Four cells in series had been the practical limit for consumer applications. Today, new chips accommodate 5–7, 7–10 and 13 cells in series. For specialty applications, such as the hybrid or electric vehicle delivering several hundred volts, specialty protection circuits are made, which sharply increases the overall cost of the battery. Monitoring two or more cells in parallel to get higher current is less critical than controlling voltages in a string configuration.

Protection circuits can only shield abuse from the outside, such as an electrical short or faulty charger. If, however, a defect occurs within the cell, such as contamination caused by microscopic metal particles, the external protection circuit has little effect and cannot arrest the reaction. Reinforced and self-healing separators are being developed for cells used in electric powertrains, but this makes the batteries large and expensive. While a Li-ion for a laptop provides a capacity of 170–200Wh/kg, the EV Li-ion has only 100–110Wh/kg.

The gas released by venting of a Li-ion cell as part of pressure buildup is mainly carbon dioxide (CO₂). Other gases that form through abusive heating are vaporized electrolyte consisting of ethylene and/or propylene. Burning gases include combustion products of the organic solvents.

Li-ion commonly discharges to 3.0V/cell. This is the threshold at which most portable equipment stops working. The lowest “low-voltage” power cut-off is 2.5V/cell, and during prolonged storage, the self-discharge causes the voltage to drop further. This causes the protection circuit to turn off and the battery goes to sleep as if dead. Most chargers ignore Li-ion packs that have gone to sleep and a charge is no longer possible.

While in the ON position, the internal protection circuit has a resistance of 50 to 100mOhm. The circuit typically consists of two switches connected in series; one is responsible for the high cut-off, and the other for the low cut-off. The protection circuit of some smaller cellular batteries can be relaxed, and some get away with only the cell’s intrinsic protection and/or an external fuse. The absence of a full protection circuit saves money, but a new problem arises. Here is what can happen.

Some low-cost chargers rely solely on the battery’s protection circuit to terminate charge current. Without a functioning voltage termination switch in the battery, the cell voltage can rise too high and overcharge the battery. Heat buildup and bulging are early indications of pending failures before potential disintegration occurs. Figure 1 shows a battery that has fragmented while charging in a car.

Figure 1:  Exploded cellular phone Generic cell phone disintegrated while charging in the back of a car.Combination of unsafe battery and charger can have a lethal effect. Manufacturers advise only to use approved batteries and chargers. By owner’s permission

A concern also arises if static electricity or a faulty charger has destroyed the battery’s protection circuit. This can fuse the solid-state switches into a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but fail to provide the required safety.

Low price makes generic replacement batteries from Asia popular with cell phone users. While the quality and performance of these batteries is improving, some do not provide the same high safety as the original branded version. A wise shopper spends a little more and replaces the battery with an approved model.

I receive many questions on www.BatteryUniversity.com from visitors wanting to know why the aftermarket does not provide low-cost laptop batteries as readily as cellular batteries. This is mainly due to safety. While a 1,400mAh cellular battery stores only 4Wh of energy, a laptop battery holds about 60Wh, 15 times more. Many manufacturers of consumer batteries protect the batteries with a security inscription that very few can break. Although aftermarket batteries are available, many do not offer all the functions of the branded version. Typical problems are fuel-gauge errors and not being able to charge correctly.

Manufacturers of lithium-ion batteries do not mention the word “explosion” and refer to “venting with flame” or “rapid disassembly.” Although seen as a slower and more controlled process than explosion, venting with flame, or rapid disassembly, can nevertheless be violent and inflict injury to those in close proximity. The court hears many legal cases involving laptops and other batteries that are said to have caused property damage, fire and personal injury. This is also a large concern in the aviation industry. Most of the batteries for consumer products are shipped by air just in time for improved inventory control.

Simple Guidelines for Using Lithium-ion Batteries

• Exercise caution when handling and testing lithium-ion batteries.
   
• Do not short-circuit, overcharge, crush, drop, mutilate, penetrate with foreign objects, apply reverse polarity, expose to high temperature or disassemble packs and cells.
   
• Use only lithium-ion cells with a designated protection circuit and approved charger.
   
• High temperature during charge or discharge may hint of pending failure. Discontinue using the battery and/or charger.
   
• The electrolyte is highly flammable and battery rupture can cause physical injury.
   
• Use a foam extinguisher, CO₂, dry chemical, powdered graphite, copper powder or soda(sodium carbonate) to extinguish a lithium-ion fire. Only pour water to prevent the fire from spreading.
   
• If the fire of a burning lithium-ion battery cannot be extinguished, allow the pack to burn out on its own in a controlled and safe way.

                              

*   IATA (International Air Transport Association) works with airlines and air transport industry to promote safe, reliable, secure and economical air travel.

Safety Concerns with Li-ion

Modern batteries contain highly reactive chemicals that will react at elevated temperature by default. The objective is to operate in a stable environmental bandwidth.

Safety is a sensitive issue that gets much media and legal attention, especially with Li-ion batteries. Any energy storage device carries a risk, and in the 1800s steam engines exploded and people got hurt. Carrying highly flammable gasoline in cars was a hot topic in the early 1900s. Battery makers are obligated to meet safety requirements, but less reputable firms may cheat — it’s “buyers be beware!” Most OEMs use only Li-ion batteries that comply with one or several safety standards.

Lithium-ion has a high specific energy and even though safe, high usage by millions of consumers is bound to generate failures. In 2006, a one-in-200,000 breakdown triggered a recall of almost six million lithium-ion packs. Heat-related battery failures are taken very seriously, and manufacturers choose a conservative approach. Let’s examine this closer.

Sony, the maker of the lithium-ion cells in question, points out 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. Battery manufacturers strive to minimize the presence of such particles; however, complex assembly techniques make the elimination of all metallic dust nearly impossible. Cells with ultra-thin separators of only 20–25µm are more susceptible to impurities than the older designs with lower Ah ratings. Whereas the 1,350mAh cell in the 18650 package could tolerate the nail penetration test, the high-density 2,400mAh becomes a bomb when performing the same test. New safety standards are more lifelike and the UL1642 Underwriters Laboratories (UL) test no longer mandates nail penetration for safety acceptance of lithium-based batteries.

Li-ion using conventional metal oxides is nearing its theoretical limit on specific energy. Rather than optimizing runtime, battery makers are improving manufacturing methods to enhance safety and increase the calendar life. The real problem lies in rare occasions when an electrical short develops inside the cell. In such a case, the external protection peripherals are ineffective to stop the thermal runaway, once in progress. The batteries recalled in 2006 passed the UL safety requirements — yet they failed in normal use.

Let’s examine the inner workings of the cell closer. A mild short will only cause elevated self-discharge and the heat buildup is minimal because the discharging power is very low. If, however, enough microscopic metallic particles converge on one spot, a sizable current begins to flow between the electrodes of the cell, and the spot heats up.

Uneven separators may also trigger cell failure. Poor conductivity due to dry area increases the resistance, which can generate local heat spots that weaken the integrity of the separator. Heat is always an enemy of the battery. When fully charged, elevated temperature causes a harmful reaction between the positive and negative electrodes and the electrolyte. As a small water leak in a faulty hydro dam can develop to a torrent and take a structure down, so also can heat buildup damage the insulation layer in a cell and cause an electrical short. The temperature can quickly reach 500°C (932°F), at which point the cell catches fire or explodes. This thermal runaway that occurs is known as “venting with flame.” “Rapid disassembly” is the preferred term by the battery industry.

If the battery gets very hot, immediately remove the device from proximity to flammable materials and bring it to a non-combustible surface. If at all possible, put a disintegrating laptop or cell phone outdoors and let it burn out. If the fire occurs in an airplane, FAA tells flight attendants not to use fire extinguishers but specify water or pop (soda). Water cools the adjacent material and prevents the fire from spreading. Many research laboratories and factories also use water to put out battery fires. Allow good ventilation while the battery burns itself out. Li-ion contains no lithium metal and does not react with water. A fire with batteries containing lithium metal requires a different extinguishing method.

During a thermal runaway, the high heat of the failing cell may propagate to the next cells, causing them to become thermally unstable also. A chain reaction can occur in which each cell disintegrates on its own timetable. A pack can thus be destroyed in a few seconds or over several hours as each cell is being consumed one by one. To increase safety, packs should include dividers to protect the failing cell from spreading to the neighboring one. (In the Tesla Roadster car, each cell is encased in its own metal compartment.) Figure shows a laptop that was damaged by a faulty Li-ion battery.

Figure: Suspected Li-ion battery destroys laptop The owner says the laptop popped, hissed, sizzled and began filling the room with smoke. Courtesy of Shmuel De-Leon

While Li-ion is being scrutinized for safety, other chemistries also have problems. Nickel- and lead-based batteries cause fires too, and some are being recalled. The reasons of these failures are faulty separators resulting from aging, rough handling, excessive vibration and high-temperature.

Let me assure you that lithium-ion batteries are safe and that heat-related failures are rare. While the safety measures are especially critical for larger multi-cell batteries, small packs for cell phones need fewer safety precautions. 

Building a Lithium-ion Pack

Reputable battery manufacturers do not supply lithium-ion cells to uncertified battery assemblers. This precaution is reasonable when considering the danger of explosion and fire when charging and discharging a Li-ion pack beyond safe limits without an approved protection circuit.

Authorizing a battery pack for the commercial market and for air transport can cost $10,000 to $20,000. Such a high price is troubling when considering that obsolescence in the battery industry is common. Manufacturers often discontinue a cell in favor of higher capacities. The switch to the improved cell will require a new certification even though the dimensions of the new cell are the same as the previous model.

Cell manufacturers must comply with their own vigorous cell testing and we ask, “Why are additional tests required when using an approved cell?” The cell approvals cannot be transferred to the pack because the regulatory authorities do not recognize the safety confirmation of the naked cell. The finished battery must be tested separately to assure correct assembly and is registered as a standalone product. Read aboutSafety Concerns with Li-ion.

As part of the test, the finished battery must undergo electrical and mechanical assessment to meet theRecommendations on the Transport of Dangerous Goods on lithium-ion batteries for air shipment, rules set by the United Nations (UN). The electrical test stresses the battery by applying high heat, followed by a forced charge, abnormal discharge and an electrical short. During the mechanical test, the battery is crush-tested and exposed to high impact, shock and vibration. The UN Transport test also requires altitude, thermal stability, vibration, shock, short circuit and overcharge checks. The UN Transport works in conjunction with the Federal Aviation Administration(FAA), the US Department of Transport (US DOT) and the International Air Transport Association (IATA).*

The authorized laboratory performing the tests needs 24 battery samples consisting of 12 new packs and 12 specimens that have been cycled for 50 times. IATA wants to assure that the batteries in question are airworthy and have field integrity. Cycling them for 50 times before the test satisfies this requirement.

The high certification costs make many small manufacturers shy away from using Li-ion for low-volume products; they choose nickel-based systems instead. While strict control is justified, an uncertified Li-ion kept in the hands of the expert and out of aircraft would be acceptable, but controlling such movement in the public domain is next to impossible. This makes it hard for the hobbyist who wants to win a race with a high-powered Li-ion battery but is bogged down by many rules.

With recurring accidents while transporting lithium-based batteries by air, regulatory authorities will likely tighten the shipping requirements further. However, anything made too cumbersome and difficult will entice some battery manufacturers to trick the system, defeating the very purpose of protecting the traveling public. Read about How to Transport Batteries.
                                   

*              IATA (International Air Transport Association) works with airlines and air transport industry to promote safe, reliable, secure and economical air travel.