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.
The natural choice of wearable devices Wearable devices are not only very limited in size, but because they require long-term wear, comfort is 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 batteries: wearable devices for comfort and convenience · Small batteries, long battery life, high energy density · Higher operating voltage means fewer cells and smaller systems · Slower self-discharge: less charging Number of times, ready to use charging challenges To avoid these safety issues, lithium-ion batteries require constant current (CC), constant voltage (CV) charging processes. 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.
Ensuring safety Due to 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 is first The phase remains constant and 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 regulates the Japanese Electronic Information Technology Industry Association (JETIA) to develop 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.
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: The maximum current is 50% of the battery capacity
· Standard temperature zone: The maximum current is 70% of the battery capacity
· Low temperature zone: The maximum current is 60% of the battery capacity
Figure 2 shows these safe currents and the corresponding safe voltage zones.
Figure 2: Lithium-ion battery safe charging current and voltage as specified by the JEITA specification
Using the DA1468x product line to ensure rechargeable safety The size and weight of wearable devices and many other IoT devices are very limited. To help ensure battery safety while meeting size and weight requirements, Dialog's SmartBond® DA1468x family of SoCs (system-on-a-chip) integrates power management features, plus some external components and connectivity to meet JEITA specifications. .
At the heart of this feature is a flexible CC/CV charging algorithm. The algorithm supports 200 A to 400 mA charging current and can charge batteries with different capacities. Figure 3 shows how to connect the DA1468x SoC to the battery for charging.
Figure 3: Example of a Li-Ion battery charging circuit using DA14680 or DA14681
The algorithm has four different charging phases: precharge, constant current, constant voltage, and voltage monitoring.
The charge cycle charging process begins immediately after the input voltage is detected. If the battery is severely depleted (eg, the voltage is already below 3V), the charging algorithm triggers the precharge phase and “precharges†the battery at a low current (about 10% of the battery capacity) until the battery can accept full charge current. This prevents overheating.
After the voltage reaches a suitable level, the charging algorithm switches to the constant current phase, charging the battery at a higher current (which can reach the capacity of the battery) until the voltage reaches 4.2 V (standard value). At this time, the charger enters the constant voltage stage to avoid the danger of overcharging. At this stage, the voltage is held at 4.2 V and the current is reduced to about 10% of the battery capacity. The transition from the constant current phase to the constant voltage phase is gradually and smoothly completed to avoid damage to the battery.
At this point the battery is full. If the battery is still connected after being fully charged, the charger enters the voltage monitoring phase, providing periodic refilling to compensate for the power consumption due to self-discharge of the battery. Recharge is usually done when the battery open circuit voltage drops below 4.0 V.
Figure 4: Charging cycle of Dialog DA14680 and DA14681
In this basic cycle, the system design engineer can adjust a series of parameters to customize the charging process. These parameters include:
· Precharge current (should be set to 10%[1] – ie 1mA - 15mA)
· Precharge voltage Vpcv (should be set to 3.05V)
· Precharge timer (default = 15 minutes)
· Constant charging current Icc (should be set to 70%1)
· Charging voltage Vfloat (range 4.2 – 4.6V)
· Constant current (CC) timer (default = 180 minutes)
· Constant voltage (CV) timer (default = 360 minutes)
Built-in protection features To help ensure safety, the DA14680 and DA14681 offer a range of built-in protection features to prevent problems caused by non-standard charging conditions, including:
· Undervoltage discharge · Overvoltage charging · Overcurrent charging · Timeout in precharge, constant current and constant voltage phases · Low and high battery temperature (with external NTC and connection)
We strongly recommend that the battery provide built-in protection for overvoltage, undervoltage, and overcurrent (discharge). In addition, the battery must have a built-in negative temperature coefficient (NTC) sensor that should be connected to the DA14680 or DA14681. NTC pin. This pin should also be connected to an analog-to-digital converter (ADC) input (such as ADC5).
The safe, convenient and reliable wearable device market for wearable devices 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. Dialog's DA1468x family offers integrated battery management features, including JEITA-compliant charging algorithms and built-in protection. Providing all of these features on a single chip helps further reduce system size while maximizing design flexibility and simplifying the design process. Design engineers are therefore able to design wearable devices that are consuming, comfortable, and beautifully looking faster.
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