The craze for smart watches has once again ignited concerns about wearable devices. In almost every product evaluation or technical comparison, battery life is the first to bear the brunt. No matter how cool the features or functions of a smart watch, if you don't have long battery life, you will eventually lose its color.
The battery life of a smart watch can be affected by many factors, such as the capacity of the battery, the power consumption of the PCB components, and the user's usage habits. Of all these factors, the capacity of the battery undoubtedly plays a decisive role. Normally, the battery capacity is proportional to the physical size of the battery pack, and the small size of the smart watch is limited by the size of its internal battery. At present, the battery capacity of several mainstream smart watches on the market is between 130mAh and 410mAh, and the running time ranges from less than one day to several days. For other wearable devices such as smart bracelets, Bluetooth headsets, smart glasses and smart jewellery, the battery capacity is even smaller, which makes each milliamp (mAH) of electricity very important during battery operation. .
Battery leakage current and charge termination current are usually two major parameters that affect battery capacity and run time, and this effect is more pronounced for small batteries.
To illustrate the importance of battery leakage, we assume that a smart bracelet has a battery capacity of 50 mAh. Ideally, the battery IC does not consume any current, and it can maintain the bracelet for 30 days. However, if different levels of battery leakage current are added to this model, the battery's endurance will be affected by different amplitudes. As shown in Figure 1, when the leakage current is 75nA, the battery's endurance is essentially unchanged, and it can still run for 30 days. However, when the leakage current is increased to 5 μA, the battery life is reduced by 2 days. By analogy, 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 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 leakage current on the battery's endurance.
Figure 1: Effect of battery leakage current on battery life
So, how does the termination of current affect the battery life? The set of data in Figure 2 shows two charge cycles for a battery with a capacity of 41 mAh. During these two charging cycles, the charging current is 40mA fast charging current, but the termination current is different. The green line in the figure represents a charging cycle with a termination charging current of 4 mA, a charge termination ratio of 10%, and a charging time of 97 minutes. The red line represents the case when the termination current is 1 mA, and the total charging time reaches 146 minutes. In the second case, the charging time is 50 minutes longer, and the battery power is increased by 2 mAh, which is about 5% of the total battery power. Is it reasonable to get 5% of the electricity in 50 minutes? Be aware that increasing the power by 5% can make the smart watch work for 2 more hours.
Therefore, the smaller the battery, the more critical it is to terminate the control. For a battery with a capacity of only 20 mAh, if the termination current cannot be controlled below 5 mA, then 10% of the battery power has been lost before starting to use the battery.
Figure 2: Charging cycle of a 41mAh battery when the termination current is 4mA and 1mA
Currently, several charger solutions such as Texas Instruments bq24040 and bq24232 are widely used in a variety of low power applications. In addition, in order to meet the special needs of wearable applications, TI has also introduced the bq2510x charger series, which has a battery leakage current of not less than 75nA, and can accurately control the termination current to within 1mA. With a package size of only 0.9mm x 1.6mm, the Bq2510x series is ideal for low volume, low power applications.
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