What is a battery?
When most people think of batteries, the first things that come to mind are most likely the small disposable batteries we keep in our remote controls, toys, flashlights, etc. With the advent of cell phones, portable computers, and tablets, however, most of us are now familiar, for better or worse, with rechargeable batteries, which not only commonly power small computing devices, but are increasingly finding their way into other electronic devices such as watches, headphones, computer mice and trackpads, flashlights and portable speakers. Rechargeable batteries are even powering many of today’s cars—a shift enabled by increases in battery performance and reductions in the cost of batteries.
Any battery relies on a voltage being developed across the terminals of one or more cells as the result of a chemical reaction--the cell being the smallest unit of electrochemistry in the battery, which may consist of a single cell or multiple cells connected in series or parallel arrangement. So, like our bodies, batteries are a chemical system, and we know from experience that chemical systems are sensitive to temperature—in fact, chemical systems like our bodies can only function in a narrow range of temperature. Consider that the range from 0°F to 100°F is only ±10% in absolute temperature, yet our bodies, and our batteries, struggle to operate outside of this relatively narrow range of temperature. Chemical reactions are also known to proceed at varying rates—for example, exploding dynamite is a chemical reaction that proceeds very quickly, while a burning match is a similar reaction that proceeds relatively slowly. Batteries that rely on a slow chemical reaction can only deliver energy slowly, while batteries supporting faster reactions can deliver energy much more quickly. The ability to deliver power quickly is referred to as the power density of the battery.
An ideal rechargeable battery would have a terminal voltage that is fixed over all states of charge, and it would have the ability to deliver all of its stored energy instantly, at any temperature. It would also be able to be charged and discharged an infinite number of times without losing capacity or the ability to deliver energy quickly. In reality, battery voltage tends to decrease at low states of charge, and limitations in reaction rate and the physical construction of the battery restrict its ability to deliver stored energy quickly. Real batteries also lose capacity after a number of charge/discharge cycles, and their characteristics degrade outside of a narrow temperature range.
Since no battery is perfect, different types of batteries have been developed which have characteristics best suited to a particular application. A cell phone or laptop, where portability and small size/low weight are important will typically have the highest energy density battery available, where energy density is the amount of energy per unit volume or unit weight of the battery. Such batteries may not have the best power density or the best performance at low temperatures—these are design tradeoffs required to achieve the highest energy density. A power tool, by contrast, requires a battery with high power density to enable the delivery of the high currents associated with motors under load. Such batteries will have somewhat lower energy density because of design tradeoffs.
Lithium-Ion (Li-Ion) batteries
Li-Ion batteries were developed in the 1980s and found application in portable electronic devices starting in the early 1990s, replacing rechargeable nickel-cadmium and nickel-metal-hydride batteries. Li-Ion batteries offered a number of significant advantages over these more established chemistries, such as higher terminal voltage, lighter weight, lack of memory effect, and good cycle life. Li-Ion batteries generate voltage by taking advantage of the potential difference between two oxidation states of lithium. A Li-Ion battery consists of a graphite anode and a cathode comprised of some combination of nickel, manganese, and cobalt oxides, the ratio of which depends on the battery application. Separating the anode and cathode is a separator made of polyethylene or polypropylene, saturated with an electrically conductive electrolyte made of lithium salt in combination with an organic solvent. In addition, a variety of microscopic coatings are applied to the anode, cathode, and separator to improve cycle performance, provide long-term stability to the chemical system, and enhance safety under abuse conditions. Current is delivered to and from the battery terminals via copper and aluminum current collectors, on which the anode and cathode are deposited in thin films during the battery manufacturing process.
During charging, electron current causes lithium ions to flow from the cathode, through the separator, into the graphite anode, forming lithiated carbon through a process called intercalation. During charging, the battery voltage will rise to about 4.2V / cell, after which the charger limits the voltage and begins reducing current to a prescribed level where the battery is fully charged. On discharge, the process is reversed, and electron current is able to flow through the load. During discharge, the battery voltage will drop to about 2.8V / cell, at which time discharge must be stopped. The charge/discharge process may be repeated hundreds or thousands of times, however, degradation mechanisms will set in, limiting the life of the battery, which we’ll discuss in more detail later. Generally, when discussing the state of degradation mechanisms of a battery we refer to that as the state of health of the battery.
Li-Ion battery management
While Li-Ion batteries offer many advantages, one disadvantage is that they require sophisticated electronic controls to maintain the safety and performance of the chemical system. We discussed above that the charger limits the cell voltage to about 4.2V / cell, however, if the charger should fail, and the cell voltage is allowed to rise above this limit, the constituent materials of the cell will begin to degrade; if the cell voltage rises further, a thermal runaway condition could ensue, whereby the organic electrolyte ignites and cell catches fire, posing a threat to nearby persons or property. If the cell is of the type using a hard cylindrical or rectangular pressure vessel, thermal runaway could cause the cell to explode, however, all cells contain a mechanical venting means which is designed to prevent an explosion. One function of the battery management system (BMS) is to disconnect the battery from the charger in the event of a charger failure. Disconnection of the charger typically happens slightly above the maximum cell charging voltage. The BMS may alert the host device controller that the charger has failed so that the user can have the device or charger repaired.
On the discharge side, the cell voltage should not be allowed to drop below about 2.0V/cell, otherwise, other degradation mechanisms may occur. For example, at low cell voltages, copper from the current collector may begin to dissolve into the electrolyte, and on subsequent charging the copper may cause shorts across the separator, posing a safety hazard. Another function of the BMS is to disconnect the load from the battery at voltages below about 2.0V / cell. Such a function may also include a permanent failure that prevents any further charging of the battery pack. Ensuring that the cell voltage stays within these prescribed limits on both the high and low ends requires that the battery management system contains bi-directional switches or relays which can disconnect either the charger or the load as needed. The circuitry controlling these switches must operate continuously and draw very little current from the battery while operating.
In addition to bi-direction switches and voltage control means, the BMS performs other important safety functions designed to protect the battery and the user under fault or abuse conditions. Typically, the BMS will monitor current into and out of the battery and disconnect the battery in the event either of these currents exceeds prescribed limits. The BMS will also contain one or more temperature sensors that will stop charging above or below certain prescribed temperature limits.
Another function of the BMS is to provide state-of-charge and state-of-health information to the user. The “% remaining” indicator that one sees on a cell phone, laptop, or electric vehicle is driven by calculations performed on measurements made by the BMS. State-of-health may be reported in a variety of ways: Total battery cycles, present battery capacity, battery capacity relative to new, or simply a “replace battery” service indicator.
Battery Charging and Preserving State-of-Health
The Li-Ion battery charger may be contained in the host device, or it may be an external device that takes power from the grid, solar panels, or another power source for charging the battery. The battery charger must be able to limit the initial current provided to the battery at the start of charge, the final voltage the battery reaches, and the final current at which to stop charging referred to as the termination current. The specific value of these parameters of initial current, final voltage, and termination current vary with the specific type of battery chemistry, the environmental conditions during charging, as well as the state of health of the battery. Clearly, Li-Ion battery charging is a complex process involving a number of dynamic variables. Charging incorrectly is one of the main causes of battery degradation; conversely, intelligent charging is one of the best ways to maintain the best battery state of health.
As mentioned above, there are several mechanisms by which battery performance degrades over time. One such mechanism is related to cycling, namely that when lithium ions are intercalated into the anode on charging, the structure of the anode changes on a microscopic level, such that when the ions are returned to the cathode on discharge, the anode material does not return to the exact physical state prior to that cycle. After many cycles, the anode may undergo a significant enough change in a physical state that it cannot accept as many lithium ions as when new. If the cell is not contained in a pressure vessel, the cell may undergo swelling as it cycles for this reason—a phenomenon referred to as plate swelling.
Another degradation mechanism relates to side reactions that can occur in the cell which may produce gas or other impurities that compromise the cell’s function. Side reactions are generally worse at high cell voltage and at high temperatures; the longer these conditions persist, the more gas and impurities are created. These mechanisms can be viewed as being proportional to a time integral function of temperature and voltage, with the effects being exponential above a certain threshold. For example, cell side reactions may be significantly worse when going between 50°C and 60°C than in going from 40°C to 50°C. A cell generating internal gas from side reactions may undergo gaseous swelling, a process that further degrades the cell by forcing the anode, cathode, and separator further apart, reducing the ability of the cell to deliver power. Gaseous swelling can also occur if the cell’s hermetic seal is compromised allowing outside air and humidity into the cell. Breach of a seal could result from mechanical shock or vibration, physical abuse, or from a defect introduced during cell or battery manufacturing.
We’ve been referring to cell degradation mechanisms, but it should be understood that as the battery is a collection of cells, the failure of one or more cells will likely result in the failure of the entire battery pack. An exception might be a very large battery containing hundreds of cells in which case a single cell failure might only produce a minor effect on overall battery performance.
So, how do we make batteries last longer? By avoiding those conditions that result in battery degradation. Plate swelling due to cycling may be the most difficult to avoid, however, the microscopic changes in anode structure are more likely to occur as we try to pack more and more lithium ions into the anode or extract more and more lithium ions on discharge, so avoiding full charge and full discharge conditions is the best way to minimize this degradation. Side reactions and associated gas swelling are aggravated at high temperatures and high states of charge, so it’s best to avoid these conditions, or at least minimize the amount of time the battery is in a high temperature plus high state-of-charge condition. Batteries should also be protected from mechanical shock and other stresses that might compromise the hermetic seal of the cells.
As designers of battery systems, we recognize that the end-users of batteries don’t want to think about battery degradation, or what they might have to do to minimize it. They want their devices to “just work”, and to continue working for as many years as possible. As such, we build intelligence into the chargers, battery management systems, and host devices to address these known degradation mechanisms and minimize them. For example, an intelligent adaptive charger would work in tandem with information provided by the BMS regarding temperature and state of health to tailor the charging parameters, and even the algorithm to match the conditions of that particular battery. Intelligent systems can even be designed to adapt to user behaviors, for example, if you leave the house in your EV every day at 8 a.m., the charger may decide to complete the charging process at 7:30 a.m. rather than at 2 a.m. to minimize the time the battery is at a full state of charge. Or a battery backup system for your home might be aware of an impending storm and top off the backup battery at the appropriate time while maintaining a lower state of charge at other times to preserve state-of-health.
Batteries are becoming an increasingly important technology—emerging as the preferred power source not just for our phones and portable computers, but for power tools, automobiles, campers, and for backing up our homes when the grid goes out. Designers of battery systems, armed with a keen understanding of cell chemistry, are developing architectures that are increasingly intelligent and situationally aware, enabling them to extend the service life of batteries well beyond what would have been possible a few short years ago. Extending service life and preserving the state of health of batteries will be key factors in minimizing waste as we transition away from fossil-fuel combustion to more renewable sources of energy.