How to choose the right battery for healthcare and medical device design

summary

Batteries come in a variety of chemistries and form factors, making it difficult to determine the right battery for a specific application. This article will introduce different criteria to help engineers choose the right battery for their use case. It will also discuss the five common primary cell chemistries on the market today and the applications they may be suitable for. This article will focus on healthcare applications and provide applicable guidelines for the various different products that use primary cells.

Introduction

Selecting the right primary battery can require a trade-off between several conflicting requirements. You want the battery capacity to be large enough to power the device continuously for an extended period of time, and the output voltage range to fit the needs of powering integrated circuits. Typically, you want the battery to be as small as possible to minimize the overall size of the product. Cost, availability, and shelf life also need to be considered. As engineers, we must also consider the environmental impact of our design decisions. The battery we choose for our product may end up in a landfill, where it will remain for many years. To help designers make the right choice, we will focus on the commonly used alkaline, lithium metal, silver oxide, and zinc air battery chemistries and evaluate their use in a disposable electrocardiogram (ECG) chest patch design.

Primary battery and secondary battery

The main difference between primary and secondary batteries is that primary batteries are not rechargeable, while secondary batteries are rechargeable. The electrochemical reaction that occurs in primary batteries is irreversible. Once the anode is oxidized, the battery can no longer generate electricity. In rechargeable batteries, the anode can be deoxidized. Therefore, the battery can be recharged and reused. Secondary batteries are generally more expensive than primary batteries and are not generally 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 question of the environmental impact of different types of batteries is complex. On the one hand, secondary batteries can be reused and do not need to be replaced as often, which means less waste is generated. On the other hand, secondary batteries contain hazardous substances that can be harmful to the environment. Primary batteries also contain hazardous substances, but in much lower concentrations. When comparing the two types of batteries, secondary batteries emit more greenhouse gases and produce more hazardous waste than primary batteries per battery. However, after 20 charging cycles, secondary batteries produce 90% less waste than disposable primary batteries, so they are considered more environmentally friendly. 1

Medical application standards

Batteries for medical applications must meet stringent safety and performance standards. The ANSI/AAMI ES 60601-1 standard for medical electrical devices specifies several regulatory standards that batteries must meet, including IEC 60086-4 and IEC 60086-5 for primary cells and UL2054 for household and commercial batteries. In addition, there are specific standards for different applications, such as ISO 20127 for electric toothbrushes.2

The FDA also has specific requirements for lithium batteries, including that they must be produced in a UL-certified factory and that each battery must be traceable for failure analysis. In addition to choosing the right battery chemistry, the battery manufacturer must also be carefully reviewed to ensure that it complies with applicable FDA and IEC regulatory requirements.

Voltage range

Primary batteries are usually available in two voltage ranges: 1.5 V and 3.3 V. Which voltage range should be chosen depends on the specific application. Buck converters are usually more efficient than boost converters. 3 Battery regulators generally use buck-boost converters to maximize the battery voltage range. However, buck-boost converters usually have four switches instead of two, so they are larger than buck converters and require more external components.

Table 1. Comparison of original batteries

Figure 1. Chemical composition of a galvanic cell.

Alkaline

Alkaline batteries are the most commonly used type of primary cell, as they offer significant advantages, such as being suitable for powering analog circuits such as TV remote controls or clocks. Compared to other battery chemistries, these batteries have a higher internal resistance, which increases as the battery discharges. Because of this characteristic, alkaline batteries are generally not suitable for digital circuits that require higher loads or have different duty cycles and operating modes. As the physical size of the battery decreases, the internal resistance of alkaline batteries also increases. Therefore, higher current applications (such as toys with a large number of LEDs and speakers) may require the use of D cell batteries, where the clock can be powered by a button cell. Alkaline batteries are generally considered safe to use and store, with little concern for explosion or leakage, and are not subject to the same regulatory standards as lithium-ion batteries.

Alkaline batteries are not typically used in medical devices due to their limited power output and short life compared to other battery chemistries. In medical applications, alkaline batteries can be used in low-cost glucometers, thermometers, and other devices that are not frequently used and have non-critical functions.

Figure 2. Lithium-ion primary cells: lithium-manganese dioxide (Li-M or LiMnO2) and lithium disulfide (Li-FeS2).

There are several lithium-based primary cells on the market, all of which use lithium as the anode material and metal as the cathode. These batteries are often called lithium metal batteries. The two most widely used lithium metal primary cells are lithium manganese dioxide (LiMnO2) and lithium disulfide (LiFeS2).

LiMnO2 batteries have a nominal output voltage of 3 V and low internal resistance. They are well suited for digital applications that require 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 for devices that require 1.5 V.

Lithium metal batteries are prone to leakage and explosion, requiring special handling and imposing shipping restrictions. However, lithium metal batteries offer many advantages over alkaline batteries: twice the capacity in a similar form factor, longer life, and less weight.

As a result, lithium metal batteries are gradually replacing alkaline batteries in many applications. Lithium metal batteries are also used in critical medical devices such as continuous glucose monitors, infusion pumps, and implantable devices such as defibrillators.

Silver oxide battery

Another common primary cell is the silver oxide (Ag-O) cell, which uses silver as the cathode and zinc as the anode. Silver oxide cells have a similar nominal output voltage to alkaline cells (i.e., 1.55 V), but have a higher capacity and a flatter discharge curve, making them suitable for digital applications. Due to the presence of silver in the cathode, large-size silver oxide cells can be very expensive, so silver oxide cells are mainly used in button or coin cells.

Figure 3. Silver oxide cells are commonly used as watch batteries.

In the past, silver oxide batteries have been known to leak, so mercury was added to the batteries to counteract corrosion. In recent years, battery manufacturers have found other ways to minimize corrosion without using mercury, making silver oxide batteries more environmentally sustainable. Silver oxide batteries are generally safer and last longer than lithium batteries, and the two batteries have similar discharge curves, but their higher cost due to the use of silver cathodes has limited their use in low-cost scenarios. Ag-O battery chemistries are increasingly used in implantable devices because silver coatings can reduce the risk of infection caused by implanted devices. 5

Zinc-air battery

Zinc-air batteries have a unique battery chemistry compared to previous battery chemistries. Zinc-air batteries use a zinc anode, ambient air as the cathode, and an electrolyte paste in between. The battery is in the typical button cell shape with an opening in the casing to allow air to enter. The opening is sealed until the battery is used, which prevents air from entering the battery. Once the seal is broken, 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 zinc-air batteries is not metal (as in other battery chemistries), zinc-air batteries are lightweight and cost-effective. They also retain a charge and have a relatively flat discharge rate. The output voltage of zinc-air batteries ranges from 0.9 V to 1.4 V.

Figure 4. Hearing aids are typically powered by zinc-air batteries.

Since zinc-air batteries must be exposed to the environment to work, their use in medical devices is limited. Many medical devices require a certain degree of isolation from the environment, which zinc-air batteries do not allow. Due to the lightweight construction and long life of these batteries, they are mainly used in hearing aid batteries.

Application Examples

Now that we have outlined the common battery chemistries and the capabilities they offer, let’s look at each one through an application example. In this example, we consider an ECG chest patch with an expected runtime of 5 days. This wearable patch is designed to be disposable, fully sealed (no battery replacement), waterproof, and have Bluetooth® communication capabilities to transmit ECG data wirelessly. The patch will also include a MAX30208 temperature sensor (to record patient temperature) and an ADXL367 accelerometer (to monitor patient activity information). It can be used in hospital environments, outpatient clinics, and patients’ homes. In this application, we use the MAX30001 as the ECG analog front end (AFE) and the MAX32655 as the microcontroller unit (MCU). The power management scheme will be selected based on the battery.