112 COMMUNICATIONS OF THE ACM | DECEMBER 2016 | VOL. 59 | NO. 12
conserving charge on the battery that is more capable of
handling such a workload in an efficient manner.
The SDB software component that resides in the OS implements a set of policies, and uses simple application programming interfaces (APIs) to communicate with the SDB hardware.
The algorithms implemented by this software use various
metrics to decide the ratios in which to discharge and charge
each battery, such that the charge–discharge duration of
the device is increased, and degradation of the batteries is
reduced. We present the details of the APIs and policies in
Section 3. 3.
The SDB design is cross-layer and involves new chemistries, additional hardware, and new OS components. This
approach opens up new battery parameters, previously
unavailable to OS designers, for resource optimization. In
existing mobile devices, the battery is usually treated as a
black box, and is simply assumed as a reservoir of charge. As
we show in Section 5, OS techniques yield substantial gains
in battery usage. This design also allows a system designer
to select any combination of batteries for an optimal design,
including new chemistries as they are developed using just
software updates.
Even with existing batteries, SDB enables several new
scenarios, such as: (i) fast-charging devices that can gain a
significant percentage of their charge in just a few minutes
without causing unexpected battery degradation, (ii) long-lived wearables created by combining flexible bendable
batteries with traditional batteries, and (iii) efficient 2-in- 1
laptop-tablet convertible devices with battery usage tailored
to the user’s behavior.
2. BATTERY BACKGROUND
A Li-ion battery contains a negative electrode (the anode),
which is usually made of graphite and a positive electrode (the
cathode), which is typically a metal oxide. A separator ensures
physical separation between the anode and the cathode to
prevent shorting, and the battery is filled with an electrolyte
composed of a lithium-based salt whose ions can easily pass
through the separator. Current is discharged when the electrodes are connected externally over a resistive load while
positive lithium ions flow from the anode to the cathode
through the electrode and the separator. During charging,
Li-ion batteries store energy by trapping positive lithium ions
in the anode when an external potential is applied.
Li-ion battery capabilities, such as longevity, energy density, and internal resistance, are largely determined by the
materials used for the electrodes and the separator. The
battery’s gravimetric and volumetric energy densities are
affected by the strength of the separator. The resistance of
the battery, and hence its inefficiencies, depend on the resistance of the separator, which typically increases with the
age of the battery. The power density of the battery is also
affected by aging. The structural integrity of the electrodes
determines how much energy they can store—some lithium
ions get permanently trapped in the anode. The anodes can
develop cracks as they age, which can ultimately reduce both
energy and power densities.
Figure 1a demonstrates the capabilities of four different Li-ion batteries, which differ in the chemistry of materials used for the cathode and the separator. Batteries of
Type 1 are typically used in powered tools that need to
charge quickly and provide high power for a short duration of time. Such batteries are a poor choice for mobile
devices because of their poor energy density—a Type 1 battery is usually double the volume of a Type 2 battery with
the same energy capacity. Type 2 batteries are commonly
used in most mobile devices today. We measure the loss
in capacity with respect to number of charge–discharge
cycles for a sample Type 2 battery, and observe that the
battery degrades much faster when discharged at higher
current (Figure 1b).
Type 3 batteries are an emerging variation over Type 2 that
have a slightly higher power density at the expense of some
energy density. This is achieved by making the separator less
dense allowing more lithium ions to pass through per unit
time. This usually leads to decreased energy density as separators cannot store energy—only the electrodes can. Finally,
Type 4 is another emerging battery that is flexible and bendable
because of the physical properties of the rubber-like (
ceramic-based) separator used—while the electrodes are implemented
by coating material along the cell’s walls. Unfortunately, such
separators increase the resistance to passage of ions and
thereby result in higher inefficiency, as shown in Figure 1c.
2. 1. Typical power management
Figure 2 shows a block diagram of the typical power management hardware. It consists of a (i) battery, (ii) fuel gauge,
(iii) battery charger, and (iv) voltage regulator.
Figure 1. Li-ion battery properties. (a) Li-ion batteries compared. (b) Charging rate affects longevity. (c) Discharging rate versus lost energy.
(a) (b)
1.0 A 0.5 A 0.7 A
0 100 200 300 400 500 600
C
ap
a
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y
af
te
r
N
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c
le
s
(%
) 105
100
75
80
85
90
95
Cycle count
(c)
10
15
20
25
30
35
Power used to drain battery (C rate)
I
nt
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h
e
at
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(
%
)
0.00 0.25 0.50 0.75 1.00 1. 25 1. 50 1. 75 2.00
Type 2 Type 4 Type 3
5
0
Form-factor
flexibility
Efficiency
LongevityAffordability
Power
density
Energy
density
Type 1: LiFePO4 Cathode, high-density liquid polymer separator
Type 2: CoO2 Cathode, high-density liquid polymer separator
Type 3: CoO2 Cathode, low-density liquid polymer separator
Type 4: CoO2 Cathode, rubber-like solid ceramic separator
