Designing smarter: single-battery applications
The market for battery-powered applications is vast, lending itself to a multitude of products in the consumer, medical, personal-care and entertainment market segments. Successful battery-powered products in all of these markets require intelligent control, maximized battery life, as well as minimal size and weight that support device portability. Rechargeable batteries can be a good option in devices that are used frequently and operate at higher drain rates, but for many applications, primary batteries are the best solution because they support simpler and lower-cost implementation options that enable a truly portable device. Whether the device is a blood-glucose meter, computer accessory, camera or wireless headphone, battery-powered products continue to become physically smaller as each iteration of their design enters the market.
There are a number of popular battery technologies, such as alkaline and lithium batteries, that are available to developers who are designing primary batteries into their projects. Each battery option targets different types of devices and use cases (Figure 1). Lithium coin batteries (typically Li/MnO2) feature packages in 13 different sizes that fit within small and lightweight devices, such as timers and watches, which use relatively low amounts of energy and need a long shelf life on the order of seven to 10 years.
Figure 1: Several popular primary battery technologies target different types of applications that have different current draws and usage scenarios.
Two primary battery technologies that are available in a cylindrical form factor are alkaline and lithium (LiFeS2). Alkaline cylindrical batteries are suitable for a wide variety of portable devices that exhibit a low to moderate drain rate. They are widely available and inexpensive, compared to the other primary battery technologies, and they are available in three compact form factors: AA, AAA, and the smaller AAAA package (as well as larger cylindrical and 9V forms that are less relevant for this article).
Lithium cylindrical batteries are available in AA and AAA form factors, and are best suited for applications that exhibit a medium to high drain rate or need to be used in cold temperatures. They provide high-reliability operation, are 33% lighter than alkaline batteries, and have a long shelf life of up to 15 years (2 to 3 times longer than alkaline batteries).
The single-battery approach
While portable, battery-powered products keep shrinking in physical size, their overall size is becoming more and more limited by the size and shape of the battery cavity, which may house two or more batteries. Therefore, the ability to operate a microcontroller on a single 1.5V battery is becoming increasingly valuable in a growing number of applications, in order to reach even smaller sizes.
Using a battery boost converter in a design makes it possible to operate the system on a single battery, where earlier designs relied on two or more batteries. A battery boost converter, such as the Microchip MCP1640, is a power supply that steps an input voltage up (boosts it) to a higher, regulated voltage, such as 2.0 to 5.0V. By incorporating a battery boost converter, a device can startup with an input voltage that is significantly lower than the operating voltage.
For example, many microcontrollers cannot operate with a voltage that is lower than 2V, and the battery boost converter can provide the voltage that the microcontroller requires, with a startup voltage as low as 0.65V (Figure 2). This enables an application using a boosted single alkaline battery to start up at any point in its discharge curve, provided that it has enough remaining capacity to operate the device. While a battery boost converter may be able to deliver a consistent output voltage from very low input voltages (for example 0.35V), battery suppliers, such as Energizer, do not recommend discharging alkaline or lithium batteries below 0.8V, as it can damage the battery.
Figure 2: A typical startup waveform, where the low-voltage startup begins to charge the output voltage up to the input voltage. Once the output voltage is charged, the N-Channel begins to switch, pumping the output voltage up, after which the internal bias switches from the input to the output.
Using a single-battery implementation offers different advantages, depending on what alternate battery configuration you are comparing it to. The most straightforward comparison is a single alkaline battery compared to two alkaline batteries. The most obvious difference is the volume savings by eliminating one battery. Another obvious difference is the weight saved by not including the second battery or its extra housing. There is a small amount of area that is taken up by the converter and supporting components, which is discussed in the trade-offs later in this article. Using a battery boost converter also maximizes the system power efficiency by providing regulated power over the battery’s entire operating range. This regulated voltage can make the microcontroller operate more efficiently, by enabling it to run at a lower and flatter voltage. For example, reducing the microcontroller operating voltage from 3.3V to 2.2V provides 1.8 times lower power consumption on the microcontroller side (voltage difference squared). Additionally, the boost converter provides short-circuit protection through current limiting.
The OEM cost for implementing the boost converter with a single battery is a little higher than the cost of two alkaline batteries, but the consumer operating cost can be similar between the two approaches if the boost converter is operating at high efficiency. Adopting a single alkaline battery approach enables developers to shrink their product size without switching between battery chemistries from a legacy multi-battery alkaline design. Working with a single battery also simplifies the mechanical considerations of the battery housing and reduces the risk that batteries will be installed incorrectly by the user.
Designers comparing the pros and cons of implementing a single alkaline battery versus a rechargeable lithium ion polymer battery will find similar but different reasons for selecting the single-battery approach. Because the boost converter provides a regulated power output, it allows the tuning of the voltage to maximize system efficiency. Similar to the alkaline example, the boost converter provides short-circuit protection through current limiting. The footprint for a single AAAA alkaline battery (352 mm2) is smaller than a single lithium ion polymer battery (650 mm2: 31.0 x 21.0 x 3.5 mm), even though they have the same volume (2.3 cc), assuming a two-sided board design.
Using an alkaline implementation versus the lithium ion polymer simplifies logistics because it avoids the need for charging circuitry and conforming to lithium shipping regulations. For those applications where using a replaceable battery is more convenient, implementing a boost converter with a single AAAA alkaline battery can provide a 50-70% cost reduction for the battery portion of the system compared to a single lithium ion polymer battery plus a charge controller and charger.
Comparing a single alkaline battery implementation versus a lithium coin battery demonstrates yet another set of conditions where the single battery approach is favorable. For this example, the comparison is between an AAAA alkaline battery (with a 600 mAh rated capacity) and a CR2032 lithium coin battery (with a 225 mAh rated capacity). Similar to the lithium ion polymer example, using a single boosted alkaline battery avoids the need to conform to lithium shipping regulations. Cylindrical batteries, like the AAAA, are more familiar to consumers than the coin batteries, and this makes ensuring the battery is installed correctly easier. The footprint of using either power source is similar, with the single alkaline AAAA battery taking approximately 352 mm2 versus 314 mm2 for the CR2032 lithium coin battery, assuming a two-sided board.
The single alkaline battery continues to benefit from the tunable voltage to maximize efficiency and provide short-circuit protection through current limiting. However, the alkaline battery is able to deliver a higher continuous current, whereas the CR2032 cannot. Specifically, the boosted alkaline battery can source 150 mA continuously, while the CR2032 can only do that for very short pulses. Extended continuous current beyond 10 mA from the CR2032 battery dramatically decreases its usable capacity and, even at 20 mA, it can only deliver 1/5th the energy that boosted AAAA batteries can (Figure 3).
Figure 3: A single AAAA battery with boost is able to deliver higher continuous current draw than a lithium coin, when the current draw is higher than a few mA.
While using a battery boost converter provides many advantages, such as enabling a microcontroller-based design to operate with a single battery, it is important to consider the system-level tradeoffs between a boosted single-battery implementation and a multiple-battery design. The efficiency of the battery boost converter is highly dependent on the current draw, as well as the input and output voltages. The dominant loss for a boost converter is resistance, so the efficiency of lower input/output voltages is lower than the efficiency of higher input/output-voltage applications (Figure 4). Other factors that can impact the boost converter’s efficiency are the resistive losses in the inductor and capacitors. Inductors with lower DC-series resistances, and capacitors with low ESR (Equivalent Series Resistance), allow higher efficiency with the potential trade-offs being size and cost.
Figure 4: Efficiency of the boost converter is highly dependent on the current draw, as well as the input and output voltages.
To support a lower system current draw, the boost converter may include a low quiescent operation mode that typically draws 20 μA, and a shutdown mode that draws less than 1 μA. When coupled with a microcontroller, a booster can provide a voltage to power the application in a sleep mode, via a “coast down” method. The microcontroller turns on the boost circuit when the system voltage falls below a pre-determined level, then shuts it off to eliminate the operating current of the booster while continuing to power from the booster’s output capacitor. Using this method, the total power consumption of the MCU + boost regulator can drop by as much as 87%.
Implementing a battery boost converter does enable a design to reduce the overall system area by the size of the second battery, with the trade-off of including the boost converter, two resistors, two capacitors and an inductor (Figure 5). The weight of the boost converter and its accompanying components is negligible for most designs. The area of a typical boost-converter circuit is approximately 60 mm2, which is considerably smaller than the 450 mm2 area of a second AAA battery or the 352 mm2 area of a second AAAA battery, and still yields a 290 to 390 mm2 area reduction in the overall system, assuming a 2-sided board design. Lastly, the OEM cost to implement the boost converter and its accompanying components is about $0.20, which may or may not be offset by the alternative battery technology you are comparing.
Figure 5: A typical battery boost-converter configuration consists of a boost converter, two resistors, two capacitors, and an inductor.
Many semiconductor companies offer application notes and/or reference designs for their boost converters, such as Microchip’s MCP1640 single AAAA battery boost converter reference design (Figure 6). The purpose of these types of reference designs is to help developers reduce their product design cycle by providing a pre-engineered circuit that they can modify to meet their project’s specific needs.
Figure 6: Reference designs can provide a low-risk platform for developers to more quickly understand how to use a boosted single battery in their designs.
A boosted single battery implementation not only enables differentiation through a smaller form factor for many applications, but it can provide a cost advantage over using coin or rechargeable batteries. Being able to minimize the width and weight of an electric toothbrush’s handle enables the handle to feel more natural in a user’s hand. Using a single battery also simplifies the mechanical design effort, to ensure that the user has installed the battery correctly.
Another benefit of using a boosted battery is that the power supplied to the device is regulated, so that it receives a flat voltage profile throughout the life of the battery. This enables the toothbrush to provide a consistent vibration throughout the life of the battery, without a noticeable drop-off in vibration strength, and thus performance, as the battery discharges. Additionally, the microcontroller can modulate the vibration signal, such as generating pulsating vibration to signal to the user that the battery is approaching an end-of-life condition.
Flashlights, especially LED ones, can also benefit from a regulated supply voltage by using the boost converter to power a microcontroller, which in turn controls a more complex supply for the LEDs. The regulated supply can avoid a dimming of the flashlight as the battery discharges – hence providing consistent lighting through to the battery’s end of life.
Using a boosted, single alkaline battery can be appropriate in value-priced devices, such as a portable, wireless mouse. A value-priced mouse focuses on longer battery life by operating with low-frequency positioning updates appropriate for Web browsing and text editing, as opposed to higher-frequency updates for gaming. Wireless mice are able to deliver a compact form factor with the single replaceable battery, and save on design and materials cost versus implementations that rely on a rechargeable battery. In addition to the cost difference between the battery technologies, using a replaceable battery means the mechanical design avoids including an expensive connector for a recharging source.
Most consumers are familiar with AA, AAA, and AAAA cylindrical form factors, and they know where to acquire them. Designs using these form factors can provide a simpler access hatch to replace the battery, so that the user is confident the battery is installed correctly. While other form factors, such as a coin battery, are not unusual, most consumers are not as familiar with the notations for the different coin form factors – nor are they as familiar with ensuring that the battery is installed correctly. The clear orientation for installing a cylindrical battery and ease of acquiring replacement batteries provides an ease-of-use differentiation for a design.
An additional differentiation opportunity for boosted equipment is to incorporate RF connectivity in the volume that was freed up from using only a single battery. Applications that can benefit from this are portable medical equipment, such as blood-pressure modules and blood-glucose meters, as well as beacons used in industrial environments for tracking transportation containers.
The opportunities to deliver efficient energy via a single battery span a wide range of application spaces. The aforementioned example applications demonstrate different ways that a device using a boosted single battery can enable a smaller form factor, lower weight, a lower cost of materials, and a simpler mechanical design than a multiple-battery approach. Additionally, because consumers are more familiar with how to use cylindrical batteries, a single-battery system is more valuable to the end user. All of these benefits make it easier to consider migrating to a boosted single-battery implementation for the next iteration of your design.