What are the reasons that affect battery life?

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 jewelries, the battery capacity is even smaller, which makes each milliamp (MA/H) of electricity seem vital during battery operation. .

Battery leakage current and charge termination current are often the two main parameters that affect battery capacity and run time, and this effect is even more pronounced for small batteries.

To illustrate the importance of battery leakage, we assume that a smart bracelet has a battery capacity of 50 MA/H. 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 one week. Obviously, the smaller battery capacity, the greater impact of leakage current is on the battery's endurance.

So, how does the termination current affect the battery life? The set of data in Figure 2 shows the two charge cycles of a 41 MA/H battery. During these two charging cycles, the charging current is 40mA fast charging current, and 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 MA/H, which is about 5% of the total battery power. Is it reasonable to get 5% of the power in 50 minutes? You know, increasing the power by 5% can make the smart watch work for 2 hours.

Therefore, the smaller the battery, the more critical it is to terminate the control. For a battery with a capacity of only 20 MA/H, 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.

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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