Communications of the ACM

The OS sets a low parameter value for times when the external battery is expected to be plugged in for longer duration while high parameter values are for times when the external battery is plugged during battery crises.

Figure 10 shows the comparison of two extreme parameters for various application workloads on a development 2-in- 1 device with two equal sized traditional Li-ion batteries. Results show that the parameter causing simultaneous power draw from both batteries provides 22% more battery life than the parameter that causes one battery to charge another. However, this gain is not realizable for a user who only keeps the base with the secondary battery plugged in for short periods of time. The OS must, therefore, learn, predict and adapt to user behavior to set appropriate parameters.

6. CONCLUSION AND FUTURE WORK

Device requirements are typically hard to meet with a single battery since these requirements are often in conflict with each other. We present the SDB system that allows a device to use multiple heterogeneous batteries, and get the best of all of them. The SDB hardware is designed to be low cost, and provides rich functionality to the OS. The SDB APIs allow an OS to dynamically route charge to, and from the batteries based on application workload such that the overall goals (battery life, cycle count, fast charge, etc.) are met. We show several new scenarios that can be enabled with SDB, and demonstrate its feasibility using a prototype, and detailed emulations.

Moving forward, we are taking the SDB work in two main directions. First, we are tying personal assistants like Siri, Cortana, and Google Now understand user behavior and the user’s schedule and by using this information, an OS can perform better parameter selection. For example, if the user’s profile suggests that the user plays video games in the evening, then it SDB could preserve a higher power-density battery for that workload. Second, we are working on additional devices that would benefit from this technology, such as drones, smart glasses, and electric vehicles (EVs). Each would require a different combination of battery chemistries, and the SDB logic might be different too. For example, we are building on the techniques proposed for Hybrid Energy Storage in the Grid7, 10 or hybrid power sources in Data Centers, 8 to improve the lifetime of EVs. An EV’s NAV system could provide the vehicle’s route as a hint to the

200mAh bendable battery for the setting. For a typical user who spends the entire day checking messages on his smart-watch and goes for a run in the evening, we plot the workload and the instantaneous losses in the batteries. We find that the latter method minimizes the total losses and therefore increases overall battery life by over an hour. These results provide evidence that mobile OSes that are aware of a user’s day-to-day schedule may be able to provide better battery life by setting the right parameter. On the other hand, it is interesting to note that if the user had not gone for a run then the first policy would have given better battery life suggesting that the knowledge of an impending workload can help save energy in heterogeneous battery settings.

5. 2. Battery management for 2-in-1s

2-in- 1 devices are tablets that have a detachable keyboard. Some such devices have another battery under the keyboard. In such a setting, there are two batteries exposed to the OS, often with different capacities but the same internal chemistry—traditional Li-ion. However, efficiency of the battery in the base is less as it is used solely to charge the battery in the tablet. Significant amount of energy is lost in charging the internal battery with the external one, yet the reason why device manufacturers have chosen this route is to simplify design.

SDB via the OS can improve the battery life of a combined internal and external battery by understanding user behavior and expectations. The power drawn from an external battery can either be used toward running the system, for charging the main battery or both. For a user who rarely unplugs an external battery, the better solution would be to draw power simultaneously from both batteries as the internal losses are proportional to the square of the current (resistive losses = I2R). Splitting the power draw across the two batteries, therefore, reduces the internal losses and increases the energy delivered to the system.

However, this strategy may not be ideal for a user who mostly operates in tablet-only mode. For such users, it makes more sense to draw as much power for as long as possible from the external battery to handle system load and also for charging the internal battery.

Figure 9. Fixed priority levels are bad. Priority levels have to be changed according to expected user schedules and workloads.

Total energy used by the device in each hour
Policy 2: Losses with parameter designed to preserve Li-ion battery
Policy 1: Losses with parameter designed to minimize instantaneous losses
Running workload
initiated (Hour 9)

Li-ion discharged completely for Policy 1 (Hour 9. 5)

Bendable battery discharged completely for Policy 1 (Hour 18)

Batteries discharged completely for Policy 2 (Hour 19. 2)

100,000

10,000

1000

100

10

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of the day
Jo
ule

Figure 10. Drawing power simultaneously from internal and external batteries is more energy efficient than depleting the external battery for conserving and charging the internal one.