Maximizing available battery capacity requires a system-level approach to power efficiency plus components that tightly control battery operation while consuming minimal power.
“Doing more with less” isn’t just a line from your CEO’s latest speech about corporate cost-cutting – it’s practically a mantra for designers of wearable electronic devices. Every new generation of smartphone, fitness band, and smartwatch offers an ever-increasing array of features. The Apple Watch Series 3 (Fig. 1), for example, includes an accelerometer, gyroscope, heart-rate sensor, microphone, speaker, barometric altimeter, ambient light sensor, Wi-Fi, Bluetooth, NFC, GPS, and optional LTE phone. Oh, and it tells the time, too.
|Figure 1.||Like most wearables, the Apple Watch Series 3 leaves little room for
its 279-mAh battery. Nonetheless, the watch can run for up to 18 hours
with occasional use. Just be careful about talking on the phone – that
can reduce the run-time to as little as one hour. (Source: ifixit.com)
This increased functionality results in greater power usage. Add in the continuing trend toward smaller, thinner packages, and something’s gotta give.
In this case, it’s often the space allotted for the battery. No longer do current-generation smartphones allow the user to swap out the battery when its performance starts to degrade; the user-replaceable battery has become a victim of the drive toward ever-thinner devices.
There’s even less space available for a battery in a wearable. A smartwatch or fitness band often only has room for a single-cell Li-Ion battery with a voltage of 3.8 V and a capacity of 130 to 410 mAh.
Not surprisingly, a smaller battery has a smaller capacity – less energy for those power-hungry new features. As a result, one of the most common complaints from buyers of wearables is short battery life.
The combination of increased power consumption and a smaller battery forces the designer to look for power savings at every opportunity. An effective system-level approach is to divide up the functional blocks, group them into power domains based on their common needs, and then formulate the best approach for each domain to maximize efficient use of power (Fig. 2).
|Figure 2.||A typical wearable architecture can be segmented into several blocks
based on their shared power profiles. The most efficient design optimizes
the power strategy for each block. (Source: TI Training video: “Nuances in
ultra-low power designs for wearable products”)
For instance, some domains can be completely turned off when not in use. Examples include radios and highly duty-cycled functions that aren’t time-dependent.
Other domains need low power in standby mode, but also need to be able to respond quickly. Examples include microcontrollers and sensors that spend most of the time asleep, but wake up to perform scheduled functions or respond to user inputs.
Another domain might put the emphasis on high efficiency under full load. The ability to fully disconnect the battery from the rest of the system is always an advantage, to ensure that the battery retains charge while the product is being shipped or stored in the warehouse.
Turning to the battery-management system (BMS) itself, Li-ion is the most popular chemistry for small rechargeable batteries. It has many advantages when compared to its competitors, but Li-ion batteries suffer from a relatively small safe operation area (SOA). Conditions such as overcharging, over-discharging, or overtemperature can have potentially catastrophic results. Thus, the BMS must carefully monitor the current, voltage, and temperature during charging and operation.
Three primary goals of a BMS design are to minimize the BMS current consumption during operation to maximize the available battery energy; minimize the time needed for recharging; and recharge the battery to the greatest degree possible.
The battery run-time is affected by various factors, including battery capacity, power consumption, and usage patterns, among others. For small batteries, two BMS parameters have the greatest effect on battery capacity and runtime: the leakage current and charging termination current.
To demonstrate the effect of the BMS leakage current, consider a wristband with a 50-mAh battery that can support 30 days of operation.
If we assume that the BMS is ideal and consumes zero current, the baseline current consumption of the rest of the wearable design is therefore 64 µA. By adding different BMS leakage currents, the battery run-time will be reduced by different amounts. With 75 nA of leakage, there’s a negligible difference; the battery can still support 30 days of operation. A leakage of 5 µA, however, reduces the battery run-time by two days, and a leakage of 20 µA reduces run-time by seven days, or 23%, so a low leakage current is a requirement for a wearable battery-management system.
Why is the termination current important? It’s the charging current at which the charger ceases operation because it considers the battery fully charged. If the charger sets the level too high, it will stop charging the battery while usable capacity remains, needlessly shortening system run time.
Figure 3 illustrates that the charging current declines over time. The graph also shows the variation in capacity for a 41-mAh battery after two different charging cycles, both of which begin with a charging current of 40 mA. The green line represents a charging cycle with a normal 10% termination ratio; i.e., a termination current of 4 mA. The red line represents a charging cycle with a 1-mA termination current; this provides an extra 2 mAh of capacity – a 5% improvement.
|Figure 3.||Shown are the charge cycles for a 41-mAh battery with 4- and 1-mA
termination currents. (Source: TIE2E Community: “Advanced
charging features extend battery run time for wearables”)
There’s a tradeoff in capacity versus time: The 2 mAh took an additional 50 minutes for this battery (146 minutes charge time versus 97 minutes.) Is it worth it? That’s a decision for the designer, of course, but an extra 5% can mean two more hours of operation for a smartwatch.
The smaller the battery, the more critical the termination control. A charger with a termination current of 5 mA will waste more than 10% of the capacity of a 20-mAh battery.
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