The battery life of a smart watch can be affected by a number of factors such as battery capacity, power consumption of the PCB components, and user habits. Of all these factors, the battery's capacity undoubtedly plays a decisive role. Under normal circumstances, the battery capacity is directly proportional to the physical size of the battery pack, and the smart and exquisite pursuit of the smart watch limits the size of its internal battery. Currently, several mainstream smart watches on the market have battery capacities ranging from 130mAh to 410mAh, and run times ranging from less than one day to several days. For other wearable devices such as smartbands, bluetooth headsets, smart glasses, and smart jewellery, the battery capacity is even smaller, which also makes the power of each milliamp (mAH) appear to be critical during battery operation. .
Battery leakage current and charge termination current are usually the two main parameters that affect battery capacity and operating time, and this effect is more pronounced for small batteries.
In order to explain the importance of battery leakage, we assume that the battery capacity of a smart bracelet is 50mAh. Under ideal circumstances, the battery IC does not consume any current. At this time, it can maintain the bracelet for 30 days. However, if different levels of battery leakage current are added to this model, battery life will be affected by different levels. As shown in Figure 1, when the leakage current is 75nA, the battery's battery life is essentially unchanged, and it can still run for 30 days. However, when the leakage current increased to 5μA, the battery's battery life was reduced by 2 days. By analogy, when the leakage current is 10μA, the battery's 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, which will reduce the battery's battery life by a full week. Obviously, the smaller the battery capacity, the greater the impact of the leakage current on the battery's battery life.
Figure 1: Effect of battery leakage current on battery life
So, how does stopping the current affect battery life? The set of data in Figure 2 shows the two charge cycles of a 41mAh battery. In both charge cycles, the charge current is a 40mA fast charge current and the termination current is different. The green line in the figure represents the termination of the charge cycle with a 4mA charge current, a charge termination ratio of 10%, and a charge time of 97 minutes. The red line represents the case when the termination current is 1 mA, and the total charge time reaches 146 minutes. In the second case, the charging time was more than 50 minutes, and the battery power increased by 2mAh, which is about 5% of the total battery power. Is it reasonable to take 5% of the electricity in 50 minutes? You know, adding 5% of power can make smartwatch work for 2 hours.
Therefore, the smaller the battery, the more critical it is to terminate control. For a battery with a capacity of only 20mAh, if the termination current cannot be controlled below 5mA, 10% of the battery power is lost before the battery is used.
Figure 2: 41mAh battery charge cycle at 4mA and 1mA termination
Currently, several charger solutions such as the Texas Instruments bq24040 and bq24232 are widely used in various low-power applications. In addition, in order to meet the special needs of wearable applications, TI has also introduced the bq2510x charger series. Its battery leakage current is not only less than 75nA, and it can also accurately control the termination current within 1mA. With a package size of only 0.9mm x 1.6mm, the Bq2510x family is ideal for volume-constrained low-power applications.
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