Advanced charging features extend battery run time in wearable products-EEWORLD

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The smartwatch craze has once again sparked people's attention to wearable devices. In almost every product review or technical comparison, battery life is the first thing to consider. No matter how cool the features or functions a smartwatch has, it will eventually fade into obscurity if it does not have a long battery life.

The battery life of a smartwatch is affected by many factors, such as the battery capacity, the power consumption of the PCB components, and the user's usage habits. Among all these factors, the battery capacity undoubtedly plays a decisive role. Generally speaking, the battery capacity is proportional to the physical size of the battery pack, and the small and exquisite pursuit of smartwatches limits the size of its internal battery. The battery capacity of several mainstream smartwatches on the market currently ranges from 130mAh to 410mAh, and the operating time ranges from less than a day to several days. For other wearable devices such as smart bracelets, Bluetooth headsets, smart glasses and smart jewelry, the battery capacity is even smaller, which makes every milliampere (mAH) of power crucial in the battery operation process.

Battery leakage current and charge termination current are usually the two main parameters affecting battery capacity and run time, and this impact is more significant for small batteries.

To illustrate the importance of battery leakage, we assume that the battery capacity of a smart bracelet is 50mAh. In an ideal situation, the battery IC does not consume any current, and it can keep the bracelet running for 30 days. However, if different levels of battery leakage current are added to this model, the battery life will be affected to different extents. As shown in Figure 1, when the leakage current is 75nA, the battery life is essentially unchanged and can still run for 30 days. However, when the leakage current increases to 5µA, the battery life is reduced by 2 days. Similarly, when the leakage current is 10µA, the battery life is reduced by 4 days. When the leakage current reaches 20µA, the battery IC will consume a current equivalent to 25% of the battery capacity, reducing the battery life by a full week. Obviously, the smaller the battery capacity, the greater the impact of leakage current on the battery life.

Figure 1: Effect of battery leakage current on battery life

So how does the termination current affect battery life? The data in Figure 2 shows two charging cycles of a 41mAh battery. In both charging cycles, the charging current is 40mA fast charge current, but the termination current is different. The green line in the figure represents the charging cycle with a termination current of 4mA, a charge termination ratio of 10%, and a charging time of 97 minutes. The red line represents the situation when the termination current is 1mA, and the total charging time is 146 minutes. In the second case, the charging time is increased by 50 minutes, and the battery capacity is increased by 2mAh, which is about 5% of the total battery capacity. Is it reasonable to get 5% of the power in 50 minutes? You know, a 5% increase in power can make the smartwatch work for 2 more hours.

Therefore, the smaller the battery, the more critical the termination control. For a battery with a capacity of only 20mAh, if the termination current cannot be controlled below 5mA, 10% of the battery power will be lost before the battery can be used.

Figure 2: Charging cycle of a 41mAh battery with termination currents of 4mA and 1mA

Currently, several charger solutions such as Texas Instruments (TI) bq24040 and bq24232 are widely used in various low-power applications. In addition, to meet the special needs of wearable applications, TI has previously launched the bq2510x charger series, whose battery leakage current is not only less than 75nA, but also can accurately control the termination current within 1mA. The package size of the bq2510x series is only 0.9mm x 1.6mm, which is an ideal choice for those volume-constrained low-power applications.

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