Generally, battery capacity is proportional to the physical size of the battery pack, and the compact and exquisite design pursued by smartwatches further limits the size of their internal batteries. Currently, the battery capacities of several mainstream smartwatches on the market range from 130mAh to 410mAh, with runtimes varying 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, making every milliampere (mAh) of power crucial during battery operation.
Battery leakage current and charge termination current are usually two major parameters that affect battery capacity and runtime, and this effect is more pronounced for small batteries.
To illustrate the importance of battery leakage, let's assume a smart bracelet has a 50mAh battery. Ideally, the battery IC consumes no current, allowing the bracelet to operate for 30 days. However, increasing the leakage current in this model affects the battery life to varying degrees. As shown in Figure 1, when the leakage current is 75nA, the battery life remains essentially unchanged, still lasting 30 days. However, when the leakage current increases to 5μA, the battery life decreases by 2 days. Similarly, at 10μA, the battery life decreases by 4 days. And when the leakage current reaches 20μA, the battery IC will consume the equivalent of 25% of the battery capacity, reducing the battery life by a full week. Clearly, the smaller the battery capacity, the greater the impact of leakage current on battery life.
So, how does the termination current affect battery life? Figure 2 shows two charging cycles of a 41mAh battery. In both cycles, the charging current is a 40mA fast charging current, but the termination current differs. The green line represents the charging cycle with a termination current of 4mA, a charging termination rate of 10%, and a charging time of 97 minutes. The red line represents the scenario with a termination current of 1mA, where the total charging time reaches 146 minutes. In the second case, the charging time is 50 minutes longer, while the battery capacity increases by 2mAh, which is approximately 5% of the total battery capacity. Is it reasonable to gain 5% battery capacity in 50 minutes? It's worth noting that a 5% increase in battery capacity could extend the working hours of a smartwatch by 2 hours.
Therefore, the smaller the battery, the more critical the termination control becomes. For a battery with a capacity of only 20mAh, if the termination current cannot be controlled below 5mA, then 10% of the battery capacity will have already been lost before the battery is even used.
Currently, several charger solutions from Texas Instruments (TI), such as the bq24040 and bq24232, are widely used in various low-power applications. Furthermore, to meet the specific needs of wearable applications, TI previously launched the bq2510x charger series, whose battery leakage current is less than 75nA, and whose termination current can be precisely controlled to within 1mA.
With a package size of only 0.9mm x 1.6mm, the Bq2510x series is ideal for size-constrained, low-power applications.
