Why do wearable devices and other small systems favor lithium battery power?

In order to collect the required data, consumers must wear wearable devices on their bodies. Therefore, they must be small and comfortable, and can work continuously for a long time. Smart home sensor nodes and other IoT applications face similar requirements. This creates the problem of how to power these devices.

The Internet of Things will improve almost every aspect of modern life. By collecting and analyzing large amounts of data, the Internet of Things can help us manage our health, reduce energy consumption in our homes and workplaces, monitor and improve our environment, and more.

The potential applications of the Internet of Things are very broad, but they also have some important features in common. Devices used to collect data need to be small, easy to use, and almost ready to use. These requirements may be most apparent on wearable devices, where millions of people around the world are already using wearable devices to track activity, monitor physical indicators, and improve health.

In order to collect the required data, consumers must wear wearable devices on their bodies. Therefore, they must be small and comfortable, and can work continuously for a long time. Smart home sensor nodes and other IoT applications face similar requirements.

This creates the problem of how to power these devices. Ideally, they can get energy directly from their environment so they can always be powered. Although we have made great progress in reducing power consumption and improving energy harvesting, there is still a gap between the ideals of achieving distance. For the foreseeable future, we also need to rely on the battery as the main source of power. In particular, in order to minimize energy waste caused by billions of devices, rechargeable batteries should be the preferred power source for some time to come.

Battery charging safety for wearables and other small systems

Wearable devices are not only very limited in size, but because they require long-term wear, comfort is also important, so they must also be very light, so the battery must be as small as possible. Not only that, IDC and GMI's repeated research shows that battery life is the number one consideration for consumers to purchase battery-powered convenience products. Therefore, high battery capacity is very important to the success of the product.

Meeting these two requirements at the same time makes the challenge of the battery even more difficult. Fortunately, many of the features of lithium batteries enable them to overcome this challenge, making them ideal for wearable device applications.

First, they provide high energy density, allowing system designers to choose smaller and lighter batteries and provide longer working hours. At the same time, lithium batteries typically operate at 3.7 V, compared to 1.2 V for NiMH or NiCd batteries. This means that lithium batteries require fewer cells, which also helps to achieve smaller and lighter systems. In addition, their self-discharge rate is much lower than that of nickel-based batteries, which is about 2% per month, while nickel-hydrogen and nickel-cadmium batteries are as high as 5% per day. This not only reduces the number of times of charging, but also allows the system to be used again at any time after the battery has been placed for a long time, making the system more convenient for customers.

Of course, all technologies have their own shortcomings. For example, lithium ion batteries are more expensive to manufacture than nickel based rechargeable batteries, so they are more expensive. But as a mass-produced product, economies of scale and continuous technological improvements are rapidly reducing their manufacturing costs.

Recent headlines also show that lithium-ion batteries have a greater potential safety risk. Due to the use of flammable electrolytes, if the charging voltage is too high or too low, it may cause a fire or explosion. However, most lithium-ion batteries have internal protection circuits that prevent overvoltage or undervoltage to some extent. However, the charging process of lithium-ion batteries is still much more complicated than nickel-based rechargeable batteries.

Lithium-ion battery: a wearable device that is comfortable and convenient

Small battery, long battery life, high energy density

Higher operating voltage means fewer cells and smaller systems

Slower self-discharge: less charge, ready to use

Charging challenge

To avoid these safety issues, lithium-ion batteries require a constant current (CC), constant voltage (CV) charging process. During this process, the battery is first charged at a fixed current until the set voltage is reached. The charging circuit then switches to a constant voltage mode to provide the necessary current to maintain the set voltage.

In order to get the best charging results, careful choices must be made between the choice of current and voltage levels. Charging at a higher voltage can increase battery capacity, but too high a voltage can cause the battery to be stressed or overcharged, resulting in permanent damage, instability, and danger. Similarly, higher charging currents can speed up charging, but at the expense of reduced battery capacity: a 30% reduction in charging current can increase the amount of charge stored in the battery by as much as 10%.

Therefore, the charging current is usually set to half of the battery capacity (the maximum current that the battery can continuously supply for one hour), and the voltage is set to 4.2 V per cell. However, it turns out that using a slightly smaller charging current and voltage can slow down the battery's aging, allowing it to pass through more charging cycles with higher storage.

Ensure safety

Because of this complexity, the charging solution must be able to closely monitor the charging current and voltage and provide a stable control loop that keeps the charging current and voltage at the right point in the charging cycle (even if the current remains in the first phase) Constant, the voltage remains constant during the second phase)

In addition, the charging solution needs to be thoroughly tested according to strict standards. These standards include test conditions that are more extensive than those required for nickel-based rechargeable batteries, as well as tests related to the battery itself.

JEITA specification

The Japan Electronic Information Technology Industry Association (JETIA) has established specifications for the use and charging of lithium-ion batteries. Although this specification is only a guideline, not a strict standard of a certification body, it is widely recognized in the industry as a guarantee to ensure the safe use of lithium-ion batteries.

As shown in Figure 1, the JEITA specification defines the lowest temperature (T1), the highest temperature (T4), and three temperature zones (low, medium, and high) between them to ensure safe charging.

Why do wearable devices and other small systems favor lithium battery power?

Figure 1: Temperature zone specified by the JEITA specification to ensure the safety of lithium-ion battery charging

The specification specifies the maximum safe current for each temperature zone.

High temperature zone: maximum current is 50% of battery capacity

Standard temperature zone: The maximum current is 70% of the battery capacity

Low temperature zone: maximum current is 60% of battery capacity

Figure 2 shows these safe currents and the corresponding safe voltage zones.

Why do wearable devices and other small systems favor lithium battery power?

Figure 2: Lithium-ion battery safe charging current and voltage as specified by the JEITA specification

Safe, convenient and reliable wearable device

The wearable device market is growing rapidly and will continue to grow in the coming years. Some progress has been made in terms of system power consumption and energy harvesting potential. But we still have a long way to go before the wearable device gets the energy from its environment for charging. As a result, wearables and other feature-rich IoT applications still require the use of rechargeable batteries.

Lithium-ion batteries are small, lightweight, and large in capacity, helping system designers meet size constraints while providing long battery life that is satisfactory to consumers. Its higher operating voltage means fewer cells are needed, which further reduces system size and design flexibility.

But these batteries require more sophisticated charging solutions to ensure safe and efficient charging.

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