Copyright
©
2012
2014
W3C
®
(
MIT
,
ERCIM
,
Keio
,
Beihang
),
All
Rights
Reserved.
W3C
liability
,
trademark
and
document
use
rules
apply.
This specification defines an API that provides information about the battery status of the hosting device.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
The functionality described in this specification was initially specified as part of the System Information API but has been extracted in order to be more readily available, more straightforward to implement, and in order to produce a specification that could be implemented on its own merits without interference with other, often unrelated, features.
The following changes have been made since the W3C Candidate Recommendation 08 May 2012 ( diff ):
BatteryManager
via
getBattery()
returning
a
Promise
instead
of
a
synchronous
accessor.
(
Section
5
)
BatteryManager
object
is
created
and
the
implementation
is
unable
to
report
the
battery
status
information
.
(
Section
6
)
This
document
was
published
by
the
Device
APIs
Working
Group
as
a
Candidate
Recommendation.
Last
Call
Working
Draft.
This
document
is
intended
to
become
a
W3C
Recommendation.
If
you
wish
to
make
comments
regarding
this
document,
please
send
them
to
public-device-apis@w3.org
(
subscribe
,
archives
).
W3C
publishes
a
Candidate
Recommendation
to
indicate
that
the
document
is
believed
to
be
stable
and
to
encourage
implementation
by
the
developer
community.
This
Candidate
Recommendation
is
expected
to
advance
to
Proposed
Recommendation
on
01
July
2012.
The
Last
Call
period
ends
02
October
2011.
All
feedback
is
comments
are
welcome.
Please see the Working Group's implementation report .
Publication
as
a
Candidate
Recommendation
Last
Call
Working
Draft
does
not
imply
endorsement
by
the
W3C
Membership.
This
is
a
draft
document
and
may
be
updated,
replaced
or
obsoleted
by
other
documents
at
any
time.
It
is
inappropriate
to
cite
this
document
as
other
than
work
in
progress.
This is a Last Call Working Draft and thus the Working Group has determined that this document has satisfied the relevant technical requirements and is sufficiently stable to advance through the Technical Recommendation process.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy . W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy .
This document is governed by the 14 October 2005 W3C Process Document .
This section is non-normative.
The Battery Status API specification defines a means for web developers to programmatically determine the battery status of the hosting device. Without knowing the battery status of a device, a web developer must design the web application with an assumption of sufficient battery level for the task at hand. This means the battery of a device may exhaust faster than desired because web developers are unable to make decisions based on the battery status. Given knowledge of the battery status, web developers are able to craft web content and applications which are power-efficient, thereby leading to improved user experience. Authors should be aware, however, that a naïve implementation of this API can negatively affect the battery life.
The Battery Status API can be used to defer or scale back work when the device is not charging in or is low on battery. An archetype of an advanced web application, a web-based email client, may check the server for new email every few seconds if the device is charging, but do so less frequently if the device is not charging or is low on battery. Another example is a web-based word processor which could monitor the battery level and save changes before the battery runs out to prevent data loss.
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The
key
words
must
MUST
,
must
not
MUST
NOT
,
required
REQUIRED
,
should
SHOULD
,
should
not
SHOULD
NOT
,
recommended
RECOMMENDED
,
may
MAY
,
and
optional
OPTIONAL
in
this
specification
are
to
be
interpreted
as
described
in
[
RFC2119
].
This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.
Implementations that use ECMAScript to implement the APIs defined in this specification must implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [ WEBIDL ], as this specification uses that specification and terminology.
The
Function
interface
represents
a
function
in
the
scripting
language
being
used
as
following
concepts,
terms
and
interfaces
are
defined
in
[
HTML5
].
]:
Navigator
EventHandler
Promise
objects
are
defined
in
[
HTML5
ECMASCRIPT
].
This section is non-normative.
The
API
defined
in
this
specification
is
used
to
determine
the
battery
status
of
the
hosting
device.
The
information
disclosed
has
minimal
impact
on
privacy
or
fingerprinting,
and
therefore
is
exposed
without
permission
grants.
For
example,
the
user
agent
can
obfuscate
the
exposed
value
in
a
way
that
authors
cannot
directly
know
if
there
is
a
battery
or
not
in
the
hosting
device.
device
has
no
battery,
is
charging
or
is
exposing
fake
values.
BatteryManager
The
6.1
Attributes
BatteryManager
interface
represents
the
current
battery
status
information
of
the
hosting
device.
The
charging
attribute
represents
the
charging
state
of
type
boolean
,
readonly
Represents
if
the
system's
battery
is
charging.
No
exceptions.
battery.
The
chargingTime
of
type
double
,
readonly
Represents
attribute
represents
the
time
remaining
in
seconds
until
the
system's
battery
is
fully
charged.
No
exceptions.
The
dischargingTime
of
type
double
,
readonly
Represents
attribute
represents
the
time
remaining
in
seconds
until
the
system's
battery
is
completely
discharged
and
the
system
is
about
to
be
suspended.
No
exceptions.
suspended,
and
the
level
of
type
double
,
readonly
Represents
attribute
represents
the
current
battery
level
scaled
from
0
to
1.0.
No
exceptions.
of
the
system's
battery.
interface BatteryManager : EventTarget { readonly attribute boolean charging; readonly attribute unrestricted double chargingTime; readonly attribute unrestricted double dischargingTime; readonly attribute double level; attribute EventHandler onchargingchange; attribute EventHandler onchargingtimechange; attribute EventHandler ondischargingtimechange; attribute EventHandler onlevelchange;
};
When
the
user
agent
is
to
create
a
new
BatteryManager
object
,
it
MUST
instantiate
a
new
BatteryManager
object
and
set
its
attributes'
values
to
those
that
represent
the
current
battery
status
information
,
unless
the
user
agent
is
created,
unable
to
report
the
battery
status
information
,
in
which
case
the
values
MUST
be
set
to
default
values
as
follows:
charging
must
MUST
be
set
to
true,
chargingTime
MUST
be
set
to
0,
MUST
be
set
to
level
dischargingTime
1.0
positive
Infinity,
and
MUST
be
set
to
1.0.
dischargingTime
level
The
user
agent
is
said
to
be
unable
to
report
the
value
positive
Infinity,
battery
status
information
,
if
the
implementation
it
is
unable
not
able
to
report
the
battery's
charging
state,
charging
time,
level
values
for
any
of
the
attributes,
for
example,
due
to
a
user
or
remaining
time
respectively.
system
preference,
setting,
or
limitation.
Implementations unable to report the battery status information emulate a fully charged and plugged in battery to reduce the potential for fingerprinting and prevent applications from degrading performance, if the battery status information is not made available, for example.
The
charging
attribute
must
MUST
be
set
to
false
if
the
battery
is
discharging,
and
set
to
true,
if
the
battery
is
charging,
the
implementation
is
unable
to
report
the
state,
or
there
is
no
battery
attached
to
the
system,
or
otherwise.
When
the
battery
charging
state
is
updated,
the
user
agent
must
MUST
queue
a
task
which
sets
the
charging
attribute's
value
and
fires
a
simple
event
named
chargingchange
at
the
BatteryManager
object.
The
chargingTime
attribute
must
MUST
be
set
to
0,
if
the
battery
is
full
or
there
is
no
battery
attached
to
the
system,
and
to
the
value
positive
Infinity
if
the
battery
is
discharging,
the
implementation
is
unable
to
report
the
remaining
charging
time,
or
otherwise.
When
the
battery
charging
time
is
updated,
the
user
agent
must
MUST
queue
a
task
which
sets
the
chargingTime
attribute's
value
and
fires
a
simple
event
named
chargingtimechange
at
the
BatteryManager
object.
The
dischargingTime
attribute
must
MUST
be
set
to
the
value
positive
Infinity,
if
the
battery
is
charging,
the
implementation
is
unable
to
report
the
remaining
discharging
time,
there
is
no
battery
attached
to
the
system,
or
otherwise.
When
the
battery
discharging
time
is
updated,
the
user
agent
must
MUST
queue
a
task
which
sets
the
dischargingTime
attribute's
value
and
fires
a
simple
event
named
dischargingtimechange
at
the
BatteryManager
object.
The
level
attribute
must
MUST
be
set
to
0
if
the
system's
battery
is
depleted
and
the
system
is
about
to
be
suspended,
and
to
1.0
if
the
battery
is
full,
the
implementation
is
unable
to
report
the
battery's
level,
or
there
is
no
battery
attached
to
the
system.
When
the
battery
level
is
updated,
the
user
agent
must
MUST
queue
a
task
which
sets
the
level
attribute's
value
and
fires
a
simple
event
named
levelchange
at
the
BatteryManager
object.
The
definition
of
how
often
the
chargingtimechange
,
dischargingtimechange
,
and
levelchange
events
are
fired
is
left
to
the
implementation.
If
a
hosting
device
contains
more
than
one
battery,
BatteryManager
SHOULD
expose
an
unified
view
of
the
batteries.
The
charging
attribute
MUST
be
set
to
true
if
at
least
one
battery's
charging
state
as
described
above
is
true.
Otherwise,
it
MUST
be
set
to
false.
The
chargingTime
attribute
can
be
set
to
the
maximum
charging
time
of
the
individual
batteries
if
charging
in
parallel,
and
to
the
sum
of
the
individual
charging
times
if
charging
serially.
The
dischargingTime
attribute
can
be
set
to
the
maximum
discharging
time
of
the
individual
batteries
if
discharging
in
parallel,
and
to
the
sum
of
individual
discharging
times
if
discharging
serially.
The
level
attribute
can
be
set
to
the
average
of
the
levels
of
batteries
of
same
capacity,
or
the
weighted
average
of
the
battery
level
attributes
for
batteries
of
different
capacities.
The
following
are
the
event
handlers
(and
their
corresponding
event
handler
event
types
)
that
must
MUST
be
supported
as
attributes
by
the
BatteryManager
object:
event handler | event handler event type |
---|---|
onchargingchange
|
chargingchange
|
onchargingtimechange
|
chargingtimechange
|
ondischargingtimechange
|
dischargingtimechange
|
onlevelchange
|
levelchange
|
This section is non-normative.
This trivial example writes the battery level to the console each time the level changes:
// We get the initial value when the promise resolves ... navigator.getBattery().then(function(battery) { console.log(battery.level); // ... and any subsequent updates. battery.onlevelchange = function() { console.log(this.level); };});
Alternatively,
the
same
using
the
addEventListener()
method:
navigator.getBattery().then(function(battery) { console.log(battery.level); battery.addEventListener('levelchange', function() { console.log(this.level); });});
The following example updates the indicators to show the charging state, level and time remaining in minutes:
<!DOCTYPE html> <html> <head> <title>Battery Status API Example</title> <script> window.onload = function () { function updateBatteryStatus(battery) { document.querySelector('#charging').textContent = battery.charging ? 'charging' : 'not charging'; document.querySelector('#level').textContent = battery.level; document.querySelector('#dischargingTime').textContent = battery.dischargingTime / 60; } navigator.getBattery().then(function(battery) { // Update the battery status initially when the promise resolves ... updateBatteryStatus(battery); // .. and for any subsequent updates. battery.onchargingchange = function () { updateBatteryStatus(battery); }; battery.onlevelchange = function () { updateBatteryStatus(battery); }; battery.ondischargingtimechange = function () { updateBatteryStatus(battery); }; }); }; </script></head><body> <div id="charging">(charging state unknown)</div> <div id="level">(battery level unknown)</div> <div id="dischargingTime">(discharging time unknown)</div></body></html>
The group is deeply indebted to Mounir Lamouri, Jonas Sicking, and the Mozilla WebAPI team in general for their invaluable feedback based on prototype implementations. Many thanks to the people behind the System Information API and Device Orientation Event specification for the initial inspiration. Also thanks to the nice folks bringing us the Page Visibility specification, which motivated the editor of this specification to write the introduction chapter discussing some real-world high value use cases that apply equally to this specification. Special thanks to all the participants of the Device APIs Working Group and others who have sent in substantial feedback and comments, and made the Web a better place for everyone by doing so.