summary
Batteries come in a wide variety of chemical compositions and sizes, making it difficult to determine the right battery for a specific application. This article introduces different standards to help engineers select the right battery for their application scenarios. Furthermore, it discusses five common primary battery chemical compositions currently on the market and their potential suitable applications. This article will primarily focus on healthcare applications and provide applicable guidelines for various products using primary batteries.
Introduction
Choosing the right primary battery may require trade-offs between several conflicting requirements. You want a battery with sufficient capacity to power the device continuously for an extended period, and an output voltage range that matches the power requirements of integrated circuits. Typically, you'd prefer a smaller battery size to minimize the overall product size. Additionally, cost, availability, and shelf life must be considered. As engineers, we must also consider the environmental impact of our design decisions. The batteries we choose for our products may end up in landfills, sitting there for years. To help designers make the right selection, we will focus on the chemistry of commonly used alkaline batteries, lithium metal batteries, silver oxide batteries, and zinc-air batteries, and evaluate their use in the design of disposable electrocardiogram (ECG) chest patches.
Primary and secondary batteries
The main difference between primary and secondary batteries is that primary batteries are non-rechargeable, while secondary batteries are rechargeable. The electrochemical reactions that occur in a primary battery are irreversible. Once the anode is oxidized, the battery can no longer generate electrical energy. In a rechargeable battery, however, the anode can be deoxidized. Therefore, the battery can be recharged and reused. Secondary batteries are generally more expensive than primary batteries and are typically not used in disposable systems. Primary batteries have a longer shelf life due to their lower self-discharge current, but rechargeable secondary batteries can provide more power, especially in high-current applications.
The environmental impact of different types of batteries is complex. On the one hand, rechargeable batteries are reusable and do not require frequent replacement, meaning they generate less waste. On the other hand, rechargeable batteries contain potentially hazardous substances. Primary batteries also contain hazardous substances, but at much lower concentrations. When comparing these two types of batteries individually, a rechargeable battery emits more greenhouse gases and generates more hazardous waste than a primary battery. However, after 20 charge cycles, a rechargeable battery generates 90% less waste than a disposable primary battery, and is therefore considered more environmentally friendly.
Medical Application Standards
Batteries used in medical applications must meet stringent safety and performance standards. The ANSI/AAMI ES 60601-1 standard for medical electrical equipment specifies several regulatory standards that batteries must comply with, including IEC 60086-4 and IEC 60086-5 for primary batteries, and UL 2054 for household and commercial batteries. In addition, there are application-specific standards, such as ISO 20127 for electric toothbrushes.
The FDA also has specific requirements for lithium-ion batteries, including that they must be manufactured in UL-certified facilities and that each battery must be traceable for failure analysis. In addition to correctly selecting the battery chemistry, the battery manufacturer must be carefully vetted to ensure compliance with relevant FDA and IEC application regulations.
Voltage range
Primary batteries typically offer two voltage ranges: 1.5 V and 3.3 V. Which voltage range to choose depends on the specific application. Buck converters are generally more efficient than boost converters. Battery regulators typically use buck-boost converters to maximize the battery's voltage range. However, buck-boost converters usually have four (instead of two) switches, making them larger and requiring more external components than buck converters.
alkaline
Alkaline batteries offer significant advantages, such as being suitable for powering analog circuits like TV remotes or clocks, and are the most commonly used type of primary battery. Compared to other battery chemistry, these batteries have a higher internal resistance, which increases as the battery discharges. Due to this characteristic, alkaline batteries are generally unsuitable for digital circuits requiring higher loads or with different duty cycles and operating modes. As the physical size of alkaline batteries decreases, their internal resistance also increases. Therefore, high-current applications (such as toys with numerous LEDs and speakers) may require the use of size D cells, where clocks can be powered by coin cells. Alkaline batteries are generally considered safe to use and store, with minimal concerns about explosions or leaks. Regulatory standards for alkaline batteries differ from those for lithium-ion batteries.
Alkaline batteries are generally not used in medical devices because they have limited power output and a short lifespan compared to other battery chemistry compositions. In medical applications, alkaline batteries can be used in low-cost blood glucose meters, thermometers, and other devices with infrequent or non-critical functions.
Several types of lithium-based primary batteries are available on the market, all of which use lithium as the anode material and a metal as the cathode. These batteries are commonly referred to as lithium metal batteries. The two most widely used lithium metal primary batteries are lithium manganese dioxide (LiMnO2) and lithium disulfide (LiFeS2).
LiMnO2 batteries have a nominal output voltage of 3 V and low internal resistance, making them ideal for digital applications requiring different load profiles and duty cycles. LiFeS2 batteries have a nominal output voltage of 1.5 V and similar internal resistance to LiMnO2 batteries. LiFeS2 batteries can often directly replace alkaline batteries in devices requiring 1.5 V.
Lithium metal batteries are prone to leakage and explosion, thus requiring special handling and transportation restrictions. However, compared to alkaline batteries, lithium metal batteries have many advantages: twice the capacity of alkaline batteries in similar form factors, longer lifespan, and lighter weight.
Therefore, lithium metal batteries are gradually replacing alkaline batteries in many applications. Lithium metal batteries are also used in critical medical devices such as implantable devices like continuous glucose monitors, infusion pumps, and defibrillators. 4
silver oxide battery
Another common type of primary cell is the silver oxide (Ag-O) battery, which uses silver as the cathode and zinc as the anode. Silver oxide batteries have a similar nominal output voltage (1.55 V) to alkaline batteries, but with higher capacity and a flatter discharge curve, making them suitable for digital applications. Because of the presence of silver in the cathode, large-size silver oxide batteries are very expensive; therefore, they are primarily used in button cells or coin cells.
Previous practical applications have shown that silver oxide batteries are prone to leakage, so mercury was added to the batteries to counteract corrosion. In recent years, battery manufacturers have found alternative methods to minimize corrosion without using mercury, making silver oxide batteries more environmentally sustainable. Compared to lithium-ion batteries, silver oxide batteries are generally safer and have longer operating times, and the discharge curves of the two types of batteries are similar. However, the higher cost due to the use of a silver cathode limits their application in low-cost scenarios. Because the silver coating can reduce the risk of infection from implanted devices, Ag-O battery chemistry is increasingly being used in implanted devices.
Zinc-air batteries
Zinc-air batteries have a unique battery chemistry compared to previous batteries. They use a zinc anode, surrounding air as the cathode, and an electrolyte paste in between. The battery has a typical button cell shape with an opening in the casing to allow air in. Before use, this opening is sealed to prevent air from entering the battery. Once the seal breaks, oxygen enters from the cathode, and electrons begin to flow from the zinc anode through the electrolyte paste to the cathode. Because the cathode of a zinc-air battery is not metallic (as is the chemistry of other batteries), zinc-air batteries are lightweight and cost-effective. They also retain charge and have a relatively flat discharge rate. The output voltage range of zinc-air batteries is 0.9 V to 1.4 V.
Zinc-air batteries have limited use in medical devices because they must be exposed to the environment to function. Many medical devices require a degree of isolation from the environment, which zinc-air batteries cannot provide. Due to their lightweight construction and long lifespan, they are primarily used in hearing aid batteries.
Application Examples
Above, we outlined common battery chemistry compositions and their functionalities. Now, we'll introduce them one by one through application examples. In this example, we consider an ECG chest patch with an expected runtime of 5 days. This wearable patch is designed to be disposable, completely sealed (non-replaceable battery), waterproof, and features Bluetooth® communication for wireless transmission of ECG data. The patch will also include a MAX30208 temperature sensor (for recording patient body temperature) and an ADXL367 accelerometer (for monitoring patient activity). It can be used in hospital environments, outpatient clinics, and patients' homes. In this application, we'll use a MAX30001 as the ECG analog front-end (AFE) and a MAX32655 as the microcontroller unit (MCU). The power management scheme will be selected based on the battery.
Based on these requirements, we can decide which type of battery to use. Wearable products require a compact design, meaning the battery should be small and lightweight, thus coin cell batteries are the appropriate choice. Lithium disulfide batteries can be ruled out first, as coin cell batteries do not use this type. Since the patch is disposable, rechargeable or secondary batteries cannot be used. Furthermore, the battery is completely sealed, so zinc-air batteries are not an option. Bluetooth communication support is also required, and the MAX32655 has different operating modes. Based on these requirements, alkaline batteries, due to their high internal resistance, are clearly unsuitable for this application. We are left with only lithium manganese and silver oxide primary battery chemistry.
The lithium manganese battery has a nominal output voltage of 3.0V and a higher specific energy than the silver oxide battery. We can easily power it with a 235 mAh (silver oxide) CR2032 battery. The silver oxide battery has a nominal output voltage of 1.55V, and the largest existing coin cell we could find is the 200 mAh SR44W battery. Looking at the design requirements, the patch needs to operate for 5 days. By constructing load profiles (discussed in more detail in the previous article 6), we found that the patch is expected to consume 45 mA of current per day, or 225 mA over 5 days. The requirement for a higher capacity battery diminishes the competitive advantage of the silver oxide coin cell, therefore we will choose the lithium manganese battery for this application.
in conclusion
When selecting a battery for a specific application, careful consideration must be given to its size, compatibility, and functionality. By understanding the advantages and disadvantages of each battery's chemical composition, a suitable battery for the system design requirements can be chosen.
