The battle of the smartwatches has brought everyone’s attention to wearables again. In almost every review and technical comparison, the one thing that came up the most was battery runtime. No matter how many fancy features a smart watch has, they are useless if the battery is dead.
The battery runtime is affected by various factors including battery capacity, power consumption of the PCB components, user habits, etc. Among all of these factors, battery capacity is absolutely the most decisive one. Normally, battery capacity is in proportion to the physical size of the battery pack, which is already limited by the size of the smart watch. Of the several major smart watches on the market today, battery capacities range from 130mAh to 410mAh, and runtime is from less than one day to a couple of days. For other wearable devices such as wrist bands, Bluetooth headsets, glasses and jewelry, the battery capacity must be smaller, making each milliamp hour (mAH) critical for the battery run time.
Two parameters affect battery capacity and runtime, especially for small batteries:
Battery leakage current and charging termination current.
To demonstrate how critical battery leakage is, let’s assume that a wristband is using a 50mAh battery, which can support 30 days of operation if the IC itself consumes zero current from the battery – which is the ideal case. By adding different battery leakage currents to this model, the battery runtime will be reduced by different amounts. As is shown in Figure 1, with 75nA of leakage, there’s essentially no difference; the battery can still support 30 days of operation. With 5µA of leakage, however, battery runtime is two days shorter. With 10µA leakage, runtime is four days shorter; with 20µA leakage, the IC consumes 25% of the battery capacity. Obviously, the smaller the battery capacity, the more critical the leakage is.
Figure 1: The impact of battery leakage current on battery runtime
So why is termination current so important? We took two charge-cycle data from a 41mAh battery with the same 40mA fast charging current and two different termination currents. In Figure 2, the green line represents a normal 10% termination ratio, with charging terminated at 4mA and a charge time of 97 minutes. The red line in Figure 2 represents a 1mA termination current and a total charge time of 146 minutes. So the extra 50 minutes of charging provided an extra 2mAh, which is 5% of the total battery capacity. Is it worth it? Well, this 5% can provide as much as two more hours of operation for a smart watch.
Again, the smaller the battery, the more critical the termination control. For a 20mAh battery, if you cannot control the termination current below 5mA, you are losing more than 10% of battery capacity even before you start using it.
Figure 2: Charge cycle for a 41mAh battery with 4mA and 1mA termination currents
There are several charger solutions from Texas Instruments that are widely used in the low power applications today, such as bq24040 and bq24232. To accommodate the special requirements of wearable applications, the bq2510x charger family was introduced. The battery leakage current is less than 75nA and the termination current can be accurately controlled at 1mA. The bq2510x’s extremely small 0.9mm by 1.6mm package size is also ideal for these space-limited low-power applications.