Basic Battery Safety: Very Different Chemistries, Very Different Concerns

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It’s almost Halloween, and children’s thoughts turn to candy and treats, parent’s thoughts turn to preparing costumes and perhaps those dental cavities, and electrical engineers’ thoughts turn to… batteries.

Why batteries? Engineers know that batteries are an integral part of many Halloween lights, displays, and exhibits. They are used in countless applications, come in innumerable shapes and forms, and are generally readily available, so it’s easy to take them for granted as easy-to-use sources of electrical power.

But the reality is that there are many subtleties in these ubiquitous, passive components. Depending on the battery function, size and, most critically, its chemistry, they can be safe and easy to use, or a serious chemical or fire risk. (Note that the terms “battery” and “cell” are often used interchangeably. Strictly speaking, a cell is a single metal-electrolyte assembly, and multiple cells are often used to build a battery that provides higher voltage. If there is confusion, look carefully at the context; in many cases, any terminology misuse is not an issue in most discussions.)

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Figure 1: Behind this simple schematic symbol for “battery” lies a complex world of chemistry, materials science, and various types and degrees of risk. (Source: Author-Rendered)

The first step is to always bear in mind that batteries are electrochemical storehouses of energy. They source power—the rate at which this energy is used—based on interaction between their conducting surfaces (lead, silver, copper, lithium, and many other metals) and an interposed electrolyte, which is a “brew” of complex chemicals and substances. As a result, there are two potential sources of problems with batteries: the fact that they are dense storehouses of energy (as is gasoline!), and the materials used (the conductors and electrolyte).

Batteries are divided into two basic groups: primary and secondary. Primary batteries cannot be recharged; once they are depleted, they are no longer useful (there are some claims that they can be recharged to some extent, but the recharged capacity is usually far less than a new primary cell offers and may actually be risky). Secondary batteries are intended to be recharged, which brings another area of concern into the battery world—that of the dangers from improper recharging.

Chemistry Tells the Tale

There are many battery designs and chemistries in widespread use, plus variations on each of them. In addition, there are lesser-known technologies used in specialty applications. Among the more commonly used batteries are those based on lead, silver-oxide, alkaline, and lithium (carbon-zinc and nickel-cadmium have largely been superseded). Each type offers a different set of tradeoffs among key performance attributes: output voltage, density of energy (W-hr) capacity (assessed by both volume and weight), installation issues, operating and storage temperature range, ease of management when charging for secondary cells, potential hazards, and cost, of course, to cite just a few of the many factors.

Let’s look at some of their key characteristics with respect to safety:

Lead-acid batteries are used primarily in internal-combustion vehicles to start the car and power the electronics; also used for back-up power and storage installations. While the lead is a potential risk if ingested, and the sulfuric acid is certainly a hazard (some batteries put the acid in a gel form to reduce possibilities of leakage and spilling), this battery is not used for “personal” applications due to the relativity heavy weight and low energy density. This chemistry can tolerate overcharging, but gives off potentially explosive hydrogen and oxygen gas at high charging rates so the area must have some ventilation.

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Figure 2: The basic structure of the lead-acid battery, which has been in use for well over one hundred years in its basic form, yet has evolved to a much higher level of performance, reliability, capacity, and form factor. (Source: Progressive Dynamics, Inc.)

Otherwise, the charging process is relatively simple when compared to other types of secondary batteries. The battery is charged by a voltage source that should stop when the battery reaches full charge. A small trickle-charge can be used to keep the charge level up (“topped off”) to compensate for unavoidable self-discharge even when there is no load on the battery.

The chemistry of silver-oxide batteries uses silver as one electrode, zinc as the other, and an electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). Batteries using this design can range from tiny button cells, often used in personal devices such as small clocks, timers, and meters or large situations such as powering torpedoes or subsection of spacecraft. Most silver-oxide batteries are primary devices, but there are some versions that can be used as secondary supplies in non-consumer applications. These batteries have a high energy density by both weight and volume, but the ones used in mass-market applications are fairly small, so the overall energy storage is also very modest.

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Figure 3: Button and coin batteries are the most common form factor for silver-oxide-based batteries, and pose only a modest disposal hazard, unless the case is breached. They come in dozens of not-quite-identical sizes (diameter and thickness), which can sometimes be frustrating when a replacement is needed. (Source: Wikipedia)

The silver and zinc are not risk concerns, but the electrolyte is if the case is corroded or damaged (the latter is an infrequent situation, unless it has been mechanically abused). Swallowing this battery can be harmful. The contents of an open battery can cause serious chemical burns of mouth, esophagus, and gastrointestinal tract; skin or eye contact with the contents of an opened battery can cause irritation and/or chemical burns.

Alkaline batteries are the most-common battery chemistry in consumer use, in standard sizes designated AAA, AA, C, D, and 9 V as well as button and coin cells, and offer a very attractive balance between energy capacity and price. Most alkaline batteries are disposable primary batteries and not intended for recharge (although there are some specialized secondary versions that support recharging). The alkaline battery uses zinc and manganese along with an alkaline electrolyte of potassium hydroxide, housed in a steel-alloy case.

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Figure 4: Consumers are very familiar with the standard form factor and sizing of alkaline batteries, as well as their wide availability. (Source: Duracell, Inc.)

There are two risks with alkaline batteries, aside from problems of bursting caused by trying to recharge primary ones. Over time, the case can corrode, and the electrolyte can leak; the electrolyte is a hazard to skin and especially to the eyes. The bigger issue is that these batteries can explode if dropped into a fire. For this reason, it’s important to dispose of them in a non-incinerator trash stream.

There are dozens of variations of lithium-based battery chemistries, each offering subtle tradeoffs in density, cost, self-discharge rates, performance under lighter and heavier loads, cost, and other factors. In recent years, due to their high energy density by weight and volume, lithium batteries have become very popular as rechargeable batteries in higher-end consumer products such as smartphones, laptop PCs, and others. They have also found major roles in large-scale commercial and industrial applications such as the main power-storage subsystem for the Boeing 787 Dreamliner, with a capacity of hundreds of kW-hr. While lithium batteries are sometimes used as primary cells, they have found widest acceptance as rechargeable sources of power.

Regardless of the specific chemistry, all lithium batteries are significant fire risks, whether charged or discharged. When charged, they are intense, compact stores of energy which can self-ignite due to an internal fault, unusually a consequence of contamination by microscopic metal particles in the ultra-thin separate layer. This causes a high-resistance short-circuit, which can heat up as the cell’s energy goes through it, and the heat may initiate thermal runaway and full-scale ignition. Even worse, the overheated cell can cause adjacent cells to also overheat and ignite as well. There are documented cases of laptop PCs that are not even plugged self-igniting and going up in flames, for example (and the Boeing 787 has somewhat similar problems, but on a much larger scale).

Not only is there the obvious danger due to fire, but lithium fires cannot be put out with water; special foams and chemicals must be used. As a result, many airlines and cargo carriers have banned the shipment of charged lithium batteries due to the self-ignition risk. Of course, lithium batteries should never be incinerated. They can also ignite if their discharge/charge rate is too high or they are overcharged, so careful management of that cycle is essential.

With lithium-based batteries and multi-cell packs, safety is achieved by a multi-level approach in manufacturing and use. First, the manufacturing process must be pristine and carefully controlled, of course. Second, the chemistry can be adjusted to reduce the stored energy density to a lower level, although this compromises a major virtue of this chemistry.

The next safety layers involve the special protection built into the cell. A PTC (positive temperature coefficient) device opens and blocks current flow if there are current surges or if the internal temperature rises due to overcharging or excessive discharge rate; other charge and discharge monitoring ICs are also built into the battery pack. Note that avoiding the initiation of thermal runaway is key, since once it starts, it cannot be stopped electrically via any sort of safety circuit.

For example, for internal monitoring, the bq76920, bq76930, and bq76940 series of ICs from Texas Instruments are designed to handle Li-ion battery packs having three to fifteen cells in series. An internal A/D converter (ADC) measures cell voltage, die temperature, and external thermistor, while a separate ADC measures pack current (coulomb counting). Hardware protection features include monitoring of overcurrent in discharge, short circuit in discharge, overvoltage, and undervoltage conditions. When a fault condition is detected, the IC terminates charging/discharging and sends an interrupt to the system microcontroller.

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Figure 5: The BQ76920, BQ76930 and BQ76940 ICs (for three to fifteen cells) from Texas Instruments are designed to be built into a battery pack, and contain multiple sophisticated functions to monitor and alert if there is any problem during the charging/discharging of lithium-based cells. (Source: Texas Instruments)

Batteries are so commonplace and versatile that it is easy to be lulled into complacency when using them. But as chemical-based components that also incorporate some potentially hazardous materials, each battery type has risks.

In some cases, the source of danger is outside the domain of the electrical engineer; in others, it is within that domain and the use of specialized ICs and system topologies is mandatory to allow batteries to be effectively and safely deployed. The experience of billions of batteries in regular, widespread use shows that their risks can be contained and managed.

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