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
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
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
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
initiated (Hour 9)
Policy 1 (Hour 9. 5)
Policy 1 (Hour 18)
completely for Policy 2
(Hour 19. 2)
Hour of the day
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.
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