Chapter 3: The Battery Pack
In the 1700 and 1800s, cells were encased in glass jars. Later, larger batteries were developed that used wooden containers. The inside was treated with a sealant to prevent electrolyte leakage. With the need for portability, the cylindrical cell appeared. After World War II, these cells became the standard format for smaller, rechargeable batteries.
Downsizing required smaller and more compact cell design. The button cell, which gained popularity in the 1980s, was a first attempt to achieve a reasonably flat geometry, or obtain higher voltages in a compact profile by stacking. The early 1990s brought the prismatic cell, which was followed by the modern pouch cell.
This chapter addresses the cell designs, pack configurations and intrinsic safety devices. In keeping with portability, this book addresses only the smaller cells used for portable batteries.
The Cylindrical Cell
The cylindrical cell continues to be the most widely used packaging style. The advantages are ease of manufacture and good mechanical stability. The cylinder has the ability to withstand high internal pressures. While charging, the cell pressure of a NiCd can reach 1379 kilopascals (kPa) or 200 pounds per square inch (psi). A venting system is added on one end of the cylinder. Venting occurs if the cell pressure reaches between 150 and 200 psi. Figure 3-1 illustrates the conventional cell of a NiCd battery. [3.1]
The cylindrical cell is moderately priced and offers high energy density. Typical applications are wireless communication, mobile computing, biomedical instruments, power tools and other uses that do not demand ultra-small size.
NiCd offers the largest selection of cylindrical cells. A good variety is also available in the NiMH family, especially in the smaller cell formats. In addition to cylindrical formats, NiMH also comes in the prismatic cell packaging.
The Li-ion batteries are only available in limited cells sizes, the most popular being the 18650. ‘Eighteen’ denotes the diameter in millimeters and ‘650’ describes the length in millimeters. The 18650 cell has a capacity of 1800 to 2000mAh. The larger 26650 cell has a diameter of 26 mm and delivers 3200mAh. Because of the flat geometry of the Li-ion polymer, this battery chemistry is not available in a cylindrical format.
Most SLA batteries are built in a prismatic format, thus creating a rectangle box that is commonly made of plastic materials. There are SLA batteries, however, that take advantage of the cylindrical design by using a winding technique that is similar to the conventional cell. The cylindrical Hawker Cyclone SLA is said to offer improved cell stability, provide higher discharge currents and have better temperature stability than the conventional prismatic design.
The drawback of the cylindrical cell is less than maximum use of space. When stacking the cells, air cavities are formed. Because of fixed cell size, the pack must be designed around the available cell size.
Almost all cylindrical cells are equipped with a venting mechanism to expel excess gases in an orderly manner. Whereas nickel-based batteries feature a resealable vent, many cylindrical Li-ion contain a membrane seal that ruptures if the pressure exceeds 3448 kPa (500 psi). There is usually some serious swelling of the cell before the seal breaks. Venting only occurs under extreme conditions.
The Button Cell
The button cell was developed to miniaturize battery packs and solve stacking problems. Today, this architecture is limited to a small niche market. Non-rechargeable versions of the button cell continue to be popular and can be found in watches, hearing aids and memory backup.
The main applications of the rechargeable button cell are (or were) older cordless telephones, biomedical devices and industrial instruments. Although small in design and inexpensive to manufacture, the main drawback is swelling if charged too rapidly. Button cells have no safety vent and can only be charged at a 10 to 16 hour charge rate. New designs claim rapid charge capability. [3.2]
The Prismatic Cell
The prismatic cell was developed in response to consumer demand for thinner pack sizes. Introduced in the early 1990’s, the prismatic cell makes almost maximum use of space when stacking. Narrow and elegant battery styles are possible that suit today’s slim-style geometry. Prismatic cells are used predominantly for mobile phone applications. Figure 3-3 shows the prismatic cell.
Prismatic cells are most common in the lithium battery family. The Li-ion polymer is exclusively prismatic. No universally accepted cell size exists for Li-ion polymer batteries. One leading manufacturer may bring out one or more sizes that fit a certain portable device, such as a mobile phone. While these cells are produced at high volume, other cell manufacturers follow suit and offer an identical cell at a competitive price. Prismatic cells that have gained acceptance are the 340648 and the 340848. Measured in millimeters, ‘34’ denotes the width, ‘06’ or ‘08’ the thickness and ‘48’ the length of the cell. [3.3]
Some prismatic cells are similar in size but are off by just a small fraction. Such is the case with the Panasonic cell that measures 34 mm by 50 mm and is 6.5 mm thick. If a few cubic millimeters can be added for a given application, the manufacturer will do so for the sake of higher capacities.
The disadvantage of the prismatic cell is slightly lower energy densities compared to the cylindrical equivalent. In addition, the prismatic cell is more expensive to manufacture and does not provide the same mechanical stability enjoyed by the cylindrical cell. To prevent bulging when pressure builds up, heavier gauge metal is used for the container. The manufacturer allows some degree of bulging when designing the battery pack.
The prismatic cell is offered in limited sizes and chemistries and runs from about 400mAh to 2000mAh and higher. Because of the very large quantities required for mobile phones, special prismatic cells are built to fit certain models. Most prismatic cells do not have a venting system. In case of pressure build-up, the cell starts to bulge. When correctly used and properly charged, no swelling should occur.
The Pouch Cell
Cell design made a profound advance in 1995 when the pouch cell concept was developed. Rather than using an expensive metallic cylinder and glass-to-metal electrical feed-through to insulate the opposite polarity, the positive and negative plates are enclosed in flexible, heat-sealable foils. The electrical contacts consist of conductive foil tabs that are welded to the electrode and sealed to the pouch material. Figure 3-4 illustrates the pouch cell.
The pouch cell concept allows tailoring to exact cell dimensions. It makes the most efficient use of available space and achieves a packaging efficiency of 90 to 95 percent, the highest among battery packs. Because of the absence of a metal can, the pouch pack has a lower weight. The main applications are mobile phones and military devices. No standardized pouch cells exist, but rather, each manufacturer builds to a special application.
The pouch cell is exclusively used for Li-ion and Li-ion polymer chemistries. At the present time, it costs more to produce this cell architecture and its reliability has not been fully proven. In addition, the energy density and load current are slightly lower than that of conventional cell designs. The cycle life in everyday applications is not well documented but is, at present, less than that of the Li-ion system with conventional cell design.
A critical issue with the pouch cell is the swelling that occurs when gas is generated during charging or discharging. Battery manufacturers insist that Li-ion or Polymer cells do not generate gas if properly formatted, are charged at the correct current and are kept within allotted voltage levels. When designing the protective housing for a pouch cell, some provision for swelling must be made. To alleviate the swelling issue when using multiple cells, it is best not to stack pouch cells, but lay them side by side. [3.4]
The pouch cell is highly sensitive to twisting. Point pressure must also be avoided. The protective housing must be designed to protect the cell from mechanical stress.
Series and Parallel Configurations
In most cases, a single cell does not provide a high enough voltage and a serial connection of several cells is needed. The metallic skin of the cell is insulated to prevent the ‘hot’ metal cylinders from creating an electrical short circuit against the neighboring cell.
Nickel-based cells provide a nominal cell voltage of 1.25V. A lead acid cell delivers 2V and most Li-ion cells are rated at 3.6V. The spinel (manganese) and Li-ion polymer systems sometimes use 3.7V as the designated cell voltage. This is the reason for the often unfamiliar voltages, such as 11.1V for a three cell pack of spinel chemistry.
Nickel-based cells are often marked 1.2V. There is no difference between a 1.2 and 1.25V cell; it is simply the preference of the manufacturer in marking. Whereas commercial batteries tend to be identified with 1.2V/cell, industrial, aviation and military batteries are still marked with the original designation of 1.25V/cell.
A five-cell nickel-based battery delivers 6V (6.25V with 1.25V/cell marking) and a six-cell pack has 7.2V (7.5V with 1.25V/cell marking). The portable lead acid comes in 3 cell (6V) and 6 cell (12V) formats. The Li-ion family has either 3.6V for a single cell pack, 7.2V for a two-cell pack or 10.8V for a three-cell pack. The 3.6V and 7.2V batteries are commonly used for mobile phones; laptops use the larger 10.8V packs.
There has been a trend towards lower voltage batteries for light portable devices, such as mobile phones. This was made possible through advancements in microelectronics. To achieve the same energy with lower voltages, higher currents are needed. With higher currents, a low internal battery resistance is critical. This presents a challenge if protection devices are used. Some losses through the solid-state switches of protection devices cannot be avoided.
Packs with fewer cells in series generally perform better than those with 12 cells or more. Similar to a chain, the more links that are used, the greater the odds of one breaking. On higher voltage batteries, precise cell matching becomes important, especially if high load currents are drawn or if the pack is operated in cold temperatures.
Parallel connections are used to obtain higher ampere-hour (Ah) ratings. When possible, pack designers prefer using larger cells. This may not always be practical because new battery chemistries come in limited sizes. Often, a parallel connection is the only option to increase the battery rating. Paralleling is also necessary if pack dimensions restrict the use of larger cells. Among the battery chemistries, Li-ion lends itself best to parallel connection.
Most battery packs include some type of protection to safeguard battery and equipment, should a malfunction occur. The most basic protection is a fuse that opens if excessively high current is drawn. Some fuses open permanently and render the battery useless once the filament is broken; other fuses are based on a Polyswitch™, which resembles a resettable fuse. On excess current, the Polyswitch™ creates a high resistance, inhibiting the current flow. When the condition normalizes, the resistance of the switch reverts to the low ON position, allowing normal operation to resume. Solid-state switches are also used to disrupt the current. Both solid-state switches and the Polyswitch™ have a residual resistance to the ON position during normal operation, causing a slight increase in internal battery resistance.
A more complex protection circuit is found in intrinsically safe batteries. These batteries are mandated for two-way radios, gas detectors and other electronic instruments that operate in a hazardous area such as oil refineries and grain elevators. Intrinsically safe batteries prevent explosion, should the electronic devices malfunction while operating in areas that contain explosive gases or high dust concentration. The protection circuit prevents excessive current, which could lead to high heat and electric spark.
There are several levels of intrinsic safety, each serving a specific hazard level. The requirement for intrinsic safety varies from country to country. The purchase cost of an intrinsically safe battery is two or three times that of a regular battery.
Commercial Li-ion packs contain one of the most exact protection circuits in the battery industry. These circuits assure safety under all circumstances when in the hands of the public. Typically, a Field Effect Transistor (FET) opens if the charge voltage of any cell reaches 4.30V and a fuse activates if the cell temperature approaches 90°C (194°F). In addition, a disconnect switch in each cell permanently interrupts the charge current if a safe pressure threshold of 1034 kPa (150 psi) is exceeded. To prevent the battery from over-discharging, the control circuit cuts off the current path at low voltage, which is typically 2.50V/cell.
The Li-ion is typically discharged to 3V/cell. The lowest ‘low-voltage’ power cut-off is 2.5V/cell. During prolonged storage, however, a discharge below that cut-off level is possible. Manufacturers recommend a ‘trickle’ charge to raise such a battery gradually back up into the acceptable voltage window.
Not all chargers are designed to apply a charge once a Li-ion battery has dipped below 2.5V/cell. A ‘wake-up’ boost will be needed to first engage the electronic circuit, after which a gentle charge is applied to re-energize the battery. Caution must be applied not to boost lithium-based batteries back to life, which have dwelled at a very low voltage for a prolonged time.
Each parallel string of cells of a Li-ion pack needs independent voltage monitoring. The more cells that are connected in series, the more complex the protection circuit becomes. Four cells in series is the practical limit for commercial applications.
The internal protection circuit of a mobile phone while in the ON position has a resistance of 50 to 100 mOhm. The circuit normally consists of two switches connected in series. One is responsible for high cut-off, the other for low cut-off. The combined resistance of these two devices virtually doubles the internal resistance of a battery pack, especially if only one cell is used. Battery packs powering mobile phones, for example, must be capable of delivering high current bursts. The internal protection does, in a certain way, interfere with the current delivery.
Some small Li-ion packs with spinel chemistry containing one or two cells may not include an electronic protection circuit. Instead, they use a single component fuse device. These cells are deemed safe because of small size and low capacity. In addition, spinel is more tolerant than other systems if abused. The absence of a protection circuit saves money, but a new problem arises. Here is what can happen:
Mobile phone users have access to chargers that may not be approved by the battery manufacturer. Available at low cost for car and travel, these chargers may rely on the battery’s protection circuit to terminate at full charge. Without the protection circuit, the battery cell voltage rises too high and overcharges the battery. Apparently still safe, irreversible battery damage often occurs. Heat buildup and bulging is common under these circumstances. Such situations must be avoided at all times. The manufacturers are often at a loss when it comes to replacing these batteries under warranty.
Li-ion batteries with cobalt electrodes, for example, require full safety protection. A major concern arises if static electricity or a faulty charger has destroyed the battery’s protection circuit. Such damage often causes the solid-state switches to fuse in a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but does not provide the required safety. If charged beyond safe voltage limits with a poorly designed accessory charger, the battery may heat up, then bulge and in some cases vent with flame. Shorting such a battery can also be hazardous.
Manufacturers of Li-ion batteries refrain from mentioning explosion. ‘Venting with flame’ is the accepted terminology. Although slower in reaction than an explosion, venting with flame can be very violent and inflicts injury to those in close proximity. It can also damage the equipment to which the battery is connected.
Most manufacturers do not sell the Li-ion cells by themselves but make them available in a battery pack, complete with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond its safe limits. Most battery assembling houses must certify the pack assembly and protection circuit intended to be used with the manufacturer before these items are approved for sale.