Rechargeable batteries provide untethered power, enabling convenience, dependability, less waste, and mobility.
While batteries provide untethered power, enabling convenience, dependability, and mobility, environmental responsibility suggests that rechargeable batteries have the same benefits, but can save money while reducing the amount of waste. This article examines the benefits offered by rechargeable battery technology, enabling us to make life better.
The technological hardware we use is greatly enabled by the physical phenomena of electrical charge in motion—electricity. Alternating current (AC), which moves bidirectionally, is useful for sending power over vast distances because of its simplicity in changing voltages through the use of transformers. On the other hand, direct current (DC) is a unidirectional current where electricity moves from a negative terminal in the direction of the positive terminal. One of the unique features of DC is that it can get stored for later usage.
Batteries are electrochemical cells capable of storing and delivering DC power. They come in various shapes and sizes and electrical capacities that get designed around their intended usage. Batteries might get classified as either primary or secondary. Primary batteries are not rechargeable, they are designed for a singular usage, whereas secondary batteries are rechargeable and available to be used repeatedly.
Batteries: unplug and recharge
Energy storage available in the form of DC batteries offers a significant advantage over AC power. It allows users to be highly mobile. By eliminating the need to be directly connected to an AC power source, batteries enable users untethered freedom to use electronic devices on the go. Unplugging leads to higher productivity, flexibility, and efficiency. It allows users to be untethered from physical locations such as power plants, factories, offices, and homes, and be out and about. After all, few of us today want to be tied to an electrical outlet all day.
How far out and about, one might ask. Well, pretty far. At the time of this writing, NASA astronauts Christopher J. Cassidy and Robert L. Behnken were aboard the International Space Station (ISS), working diligently to replace various DC batteries (Figure 1). NASA reported the astronauts were “swapping five aging nickel-hydrogen (NiH2) batteries with two new lithium-ion (Li-ion) batteries” and later moved on to swap one more NiH2 battery for a Li-ion battery on the Starboard-6 truss structure worksite. Rechargeable batteries allow us to be unplugged even into the vast domains of space.
Besides allowing us to go unplugged, secondary batteries are rechargeable and can be recharged repeatedly. One important ramification of this is that battery costs must be reevaluated over time. In many cases, primary batteries are less costly to purchase. However, when the amount of current consumption is calculated over time, the need only to buy a secondary cell initially and then get many low-cost recharges out of it will make the economic benefits of rechargeable batteries stand out as very advantageous. The principle of the total cost of ownership (ToC) should be examined based on the context of the specific application.
Another side benefit of being rechargeable is the impact that this has on environmental issues. We desire to live in a world whereby humankind stewards its resources well and works to mitigate harm to the environment. There is general agreement globally that every business endeavor should commit to and work to achieve initiatives that promote greater environmental responsibility. Although the ecological impact is not often priced into a ToC calculation, it should be (Figure 2).
Because batteries are electrochemical devices, there should be an impetus to reduce the consumption of its resultant chemical waste. Primary units, as mentioned, are suitable for one-time usage. They must be disposed of after they lose their charge, so rechargeable batteries reduce chemical and solid waste.
Capacity retention and number of charges
How long can a battery go before it needs to be replaced (primary cell) or recharged (secondary cell)? Battery life is determined by a battery’s capacity to hold an electric charge. This battery capacity gets measured in Ampere hours (Ah). Because rechargeable batteries are electrochemical devices, capacity is limited in a primary aspect based upon how large the physical cell will be. In small devices or portable applications, it can be expressed in milliamp-hours (mAh).
Inherently large loads, items drawing large currents, will discharge a battery quicker. An adjustment factor, called consumption rate, is generally applied to battery life calculations. Consumption rate helps account for the reality that the battery will ultimately decrease in its ability to deliver current at some point before it gets fully discharged. Mouser Electronics provides a useful battery life calculator for engineers to make quick calculations.
Battery Life = Battery Capacity (Ah) / Load Current (A) × Consumption Rate (0.7 default)
Runtime = 10 × Ampere Hours (Ah) / Load in Watts (W)
Secondary batteries provide unplugged freedom and the ability to be recharged, but how many times can they get recharged before they break down into failure? To be most advantageous, the number of recharges available should exceed the expected number of discharges that will occur. As is the case of the secondary batteries aboard the ISS, there is a limit. These have to be some of the most highly-designed cells in usage, yet they ultimately wear out and need to be replaced.
Almost everyone is familiar with rechargeable batteries in a specific form available for commercial and reusable use in products employed in our everyday lives. Often this takes the form of rechargeable batteries used in cameras, remote controls, flashlights, smoke detectors, remote-control toys and hobbies, and clocks. Often what is employed is a standardized rechargeable battery in the AA or AAA category. These types of readily used rechargeable batteries are recognized to be able to be charged 1,000 (103) times before they wear out.
Have you ever pulled out two fresh batteries, plugged them in, and found them not ready to function? This effect can be caused by an issue known as capacity retention. It is analogous to me with college Algebra and Latin. At one time, I knew them well. I was fully charged. But alas, over time, it has slowly slipped away. I still maintain some, but it is a shell of itself.
Likewise, batteries do not just sit at 100 percent charge once they reach it. Instead, they generally lose a very minute amount of charge over time, resulting in a shrinkage of their total available charge. In common parlance, this is known as the battery’s shelf life. How long can it stay in storage before it ultimately trickles away, and its charge becomes worn out? For primary cells, this means they are now useless. For a rechargeable cell, they simply must be recharged, and then they will be fresh.
Common types of rechargeable batteries
Mouser Electronics provides various batteries and battery technologies from several manufacturers. Different types of materials are employed to make rechargeable batteries; some of the most common include lead-acid (Pb-acid), Li-ion, lithium iron phosphate (LiFePO4), nickel cadmium (NiCd), and nickel metal hydride (NiMH). Let’s quickly take a look at each of these.
If you own or ride in an automobile, you should be familiar with this type of rechargeable battery. Lead-acid batteries are the default type of battery found in most gas combustion automobiles. Lead-acid batteries have a high power for mass performance that makes them suitable for high amperage demand applications; that includes turning over an automotive starter that requires lots of energy.
Li-ion batteries work well with automotive, aerospace, communication, medical, military, and industrial applications. These batteries are commonly found in items such as portable devices, power tools, and electric vehicles (EVs). They provide excellent energy density in lightweight packages. They were also the subject of much scrutiny when it was observed that incorrect storage could lead to dangerous situations in confined spaces such as airplanes.
Lithium iron phosphate battery
A type of Li-ion batteries, LiFePO4 offers a longer number of full charge/discharge cycles, known as lifecycle. In comparison to a lead-acid battery, Li-ion batteries have less mass and many times higher lifecycle. Suitable applications include medical, solar and wind, mobility, transportation, sports and recreation, and utility services.
Nickel cadmium battery
The employment of cadmium brings higher costs relative to nickel-metal or Li-ion, and the chemical is toxic. NiCd batteries are well-suited for applications that expose the cells to deep discharges such as in harsh environments. NiCd batteries also enjoy the benefit of offering a higher lifecycle.
Nickel metal hydride battery
Newer and less toxic than NiCd, NiMH batteries have good charge capacity. Commonly found at checkout counters around the world operating at 1.2V, they are excellent in high-current drain applications. They supply a constant voltage level until they are fully discharged, not being susceptible to the voltage dropping found in some other types of batteries.
Protecting Li-ion batteries and packs
To ensure long life and reliability, individual battery cells and battery packs must be electrically protected. Bourns has been doing just that for decades, while it’s also expanding its portfolio. Bourns’ broad portfolio allows designers to select the right circuit protection component to meet their increasingly complex, demanding, and compact battery pack requirements.
High-voltage Li-ion battery packs offer significant advantages such as low weight and high energy density. However, they require battery management systems (BMS) to operate within safe limits. BMS connect to the Li-ion battery packs and perform four major functions:
Bourns offers a broad portfolio of components suitable for BMS (Figure 3). These components include isolation transformers, signal transformers, power inductors, common-mode chokes, TVS diodes, fuses, current-sensing resistors, and transient blocking units (TBUs).
Figure 3 A battery management system showing the key components used in the design. Source: Bourns
Battery pack protection
Mobile devices, including smartphones, tablets, and single-lens digital cameras, primarily employ Li-ion battery packs. Larger physical items such as EVs, industrial machinery, and robotics also use battery packs.
Battery cells have inherent electrical, environmental, and mechanical challenges. When overcharged or overheated, a battery cell can rupture, combust, or explode. Even if overcharging or overheating does not result in a fire, the battery can still be compromised and can be more susceptible to further damage from physical factors, including vibration, impact, and exposure to heat.
Charge and temperature
During charging and discharging cycles, battery cells can face over-current, over-voltage, and over-temperature conditions. The charging process for Li-ion batteries consists of two phases: constant current and constant voltage. In the constant current charging phase, the charge current is applied to the battery until the voltage limit per cell is reached. Li-ion batteries cannot accept a higher voltage charge than specified without being damaged.
The constant voltage phase then begins as the applied current declines to a few percent of the constant charge current. During this time, the maximum cell voltage is applied to the battery. For multi-cell battery packs, a balancing phase occurs between the constant current and constant voltage phases to ensure a consistent charge among cells. In such packs, the voltage applied in the constant voltage stage is the product of the number of cells and the maximum voltage per cell.
Protection electronics are employed outside the cells to protect from overcharge, undercharge, and external temperatures. Circuit protection solutions for battery packs comprise several devices, which are crucial design considerations during the charging and discharging of the battery pack.
Two prevalent over-voltage and over-current protection methods in cell designs utilize battery management ICs and field-effect transistors (FETs). Most battery packs, battery cells, and specifically single-cell Li-ion battery pack designs will need a second level of protection. Bourns multifuse polymer PTC (PPTC) devices or the company’s miniature resettable TCO devices, also known as mini-breakers, are over-temperature protection solutions. Also, dual battery management ICs and FETs provide a two-fold level of over-voltage and over-current protection. The protection design can include a current-sense resistor (CSR) to monitor the voltage and current.
Enhanced battery performance management that relies on tight and low temperature coefficient of resistance (TCR) can be achieved with current sense resistors to measure drift and resistance accurately. Bourns has a comprehensive line of current-sense resistors such as the Bourns CRM2512 chip resistors. These are high-power current sense chip resistors with thick film technology. These chip resistors are available with a rating of 2 W in a standard 2512 chip format. The CRM2512 with wide resistance range is suitable in power supply circuits, including current sensing and current limiting designs.
This article was originally published on EDN.
Paul Golata, a technology specialist, is responsible for steering the marketing direction for solid-state lighting and other advanced technology-related products at Mouser Electronics.