1page.title=Measuring Device Power
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19<div id="qv-wrapper">
20  <div id="qv">
21    <h2>In this document</h2>
22    <ol id="auto-toc"></ol>
23  </div>
24</div>
25
26<p>You can determine device power consumption for Android devices that include a battery fuel gauge
27such as a Summit SMB347 or Maxim MAX17050 (available on many Nexus devices). Use the in-system
28gauge when external measurement equipment is not available or is inconvenient to
29connect to a device (such as in mobile usage).</p>
30
31<p>Measurements can include instantaneous current, remaining charge, battery capacity at test start
32and end, and more depending on the supported properties of the device (see below). For best
33results, perform device power measurements during long-running A/B tests that use the same device
34type with the same fuel gauge and same current sense resistor. Ensure the starting battery charge
35is the same for each device to avoid differing fuel gauge behavior at different points in the
36battery discharge curve.</p>
37
38<p>Even with identical test environments, measurements are not guaranteed to be of high absolute
39accuracy. However, most inaccuracies specific to the fuel gauge and sense resistor are consistent
40between test runs, making comparisons between identical devices useful. We recommend running
41multiple tests in different configurations to identify significant differences and relative power
42consumption between configurations.</p>
43
44<h2 id="power-consumption">Reading power consumption</h2>
45
46<p>To read power consumption data, insert calls to the API in your testing code.</p>
47
48<pre>
49import android.os.BatteryManager;
50import android.content.Context;
51BatteryManager mBatteryManager =
52(BatteryManager)Context.getSystemService(Context.BATTERY_SERVICE);
53Long energy =
54mBatteryManager.getLongProperty(BatteryManager.BATTERY_PROPERTY_ENERGY_COUNTER);
55Slog.i(TAG, "Remaining energy = " + energy + "nWh");
56</pre>
57
58<h2 id="avail-props">Available properties</h2>
59
60<p>Android supports the following battery fuel gauge properties:</p>
61
62<pre>
63BATTERY_PROPERTY_CHARGE_COUNTER   Remaining battery capacity in microampere-hours
64BATTERY_PROPERTY_CURRENT_NOW      Instantaneous battery current in microamperes
65BATTERY_PROPERTY_CURRENT_AVERAGE  Average battery current in microamperes
66BATTERY_PROPERTY_CAPACITY         Remaining battery capacity as an integer percentage
67BATTERY_PROPERTY_ENERGY_COUNTER   Remaining energy in nanowatt-hours
68</pre>
69
70<p>Most properties are read from kernel power_supply subsystem attributes of similar names.
71However, the exact properties, resolution of property values, and update frequency
72available for a specific device depend on:</p>
73
74<ul>
75<li>Fuel gauge hardware, such as a Summit SMB347 or Maxim MAX17050.</li>
76<li>Fuel gauge-to-system connection, such as the value of external current sense resistors.</li>
77<li>Fuel gauge chip software configuration, such as values chosen for average current computation
78intervals in the kernel driver.</li>
79</ul>
80
81<p>For details, see the properties available for <a href="#nexus-devices">Nexus devices</a>.</p>
82
83<h2 id="maxim-fuel">Maxim fuel gauge</h2>
84
85<p>When determining battery state-of-charge over a long period of time, the Maxim fuel gauge
86(MAX17050, BC15) corrects for coulomb-counter offset measurements. For measurements made over a
87short period of time (such as power consumption metering tests), the fuel gauge does not make
88corrections, making the offset the primary source of error when current measurements are too small
89(although no amount of time can eliminate the offset error completely).</p>
90
91<p>For a typical 10mOhm sense resistor design, the offset current should be better than 1.5mA,
92meaning any measurement is +/-1.5mA (PCBoard layout can also affect this variation). For example,
93when measuring a large current (200mA) you can expect the following:</p>
94
95<ul>
96<li>2mA (1% gain error of 200mA due to fuel gauge gain error)</li>
97<li>+2mA (1% gain error of 200mA due to sense resistor error)</li>
98<li>+1.5mA  (current sense offset error from fuel gauge)</li>
99</ul>
100
101<p>The total error is 5.5mA (2.75%). Compare this to a medium current (50mA) where the same error
102percentages give a total error of 7% or to a small current (15mA) where +/-1.5mA gives a total
103error of 10%.</p>
104
105<p>For best results, we recommend measuring greater than 20mA. Gain measurement errors are
106systematic and repeatable, enabling you to test a device in multiple modes and get clean relative
107measurements (with exceptions for the 1.5mA offset).</p>
108
109<p>For +/-100uA relative measurements, required measurement time depends on:</p>
110
111<ul>
112<li><b>ADC sampling noise</b>. The MAX17050 with its normal factory configuration produces +/-1.5mA
113sample-to-sample variation due to noise, with each sample delivered at 175.8ms. You can expect a
114rough +/-100uA for a 1 minute test window and a clean  3-sigma noise less than 100uA (or 1-sigma
115noise at 33uA) for a 6 minute test window.</li>
116<li><b>Sample Aliasing because of load variation</b>. Variation exaggerates errors, so for samples
117with variation inherent in the loading, consider using a longer test window.</li>
118</ul>
119
120<h2 id="nexus-devices">Supported Nexus devices</h2>
121
122<h5 id="nexus-5">Nexus 5</h5>
123
124<table>
125<tbody>
126<tr>
127<th>Model</th>
128<td>Nexus 5</td>
129</tr>
130<tr>
131<th>Fuel Gauge</th>
132<td>Maxim MAX17048 fuel gauge (ModelGauge™, no coulomb counter)</td>
133</tr>
134<tr>
135<th>Properties</th>
136<td>BATTERY_PROPERTY_CAPACITY</td>
137</tr>
138<tr>
139<th>Measurements</th>
140<td>The fuel gauge does not support any measurements other than battery State Of Charge to a
141resolution of %/256 (1/256th of a percent of full battery capacity).</td>
142</tr>
143</tbody>
144</table>
145
146
147<h5 id="nexus-6">Nexus 6</h5>
148
149<table>
150<tbody>
151<tr>
152<th>Model</th>
153<td>Nexus 6</td>
154</tr>
155<tr>
156<th>Fuel Gauge</th>
157<td>Maxim MAX17050 fuel gauge (a coulomb counter with Maxim ModelGauge™ adjustments), and a 10mohm
158current sense resistor.</td>
159</tr>
160<tr>
161<th>Properties</th>
162<td>BATTERY_PROPERTY_CAPACITY<br>
163BATTERY_PROPERTY_CURRENT_NOW<br>
164BATTERY_PROPERTY_CURRENT_AVERAGE<br>
165BATTERY_PROPERTY_CHARGE_COUNTER<br>
166BATTERY_PROPERTY_ENERGY_COUNTER</td>
167</tr>
168<tr>
169<th>Measurements</th>
170<td>CURRENT_NOW resolution 156.25uA, update period is 175.8ms.<br>
171CURRENT_AVERAGE resolution 156.25uA, update period configurable 0.7s - 6.4h, default 11.25 secs.<br>
172CHARGE_COUNTER (accumulated current, non-extended precision) resolution is 500uAh (raw coulomb
173counter read, not adjusted by fuel gauge for coulomb counter offset, plus inputs from the ModelGauge
174m3 algorithm including empty compensation).<br>
175CHARGE_COUNTER_EXT (extended precision in kernel) resolution 8nAh.<br>
176ENERGY_COUNTER is CHARGE_COUNTER_EXT at nominal voltage of 3.7V.</td>
177</tr>
178</tbody>
179</table>
180
181
182<h5 id="nexus-9">Nexus 9</h5>
183
184<table>
185<tbody>
186<tr>
187<th>Model</th>
188<td>Nexus 9</td>
189</tr>
190<tr>
191<th>Fuel Gauge</th>
192<td>Maxim MAX17050 fuel gauge (a coulomb counter with Maxim ModelGauge™ adjustments), and a 10mohm
193current sense resistor.</td>
194</tr>
195<tr>
196<th>Properties</th>
197<td>BATTERY_PROPERTY_CAPACITY<br>
198BATTERY_PROPERTY_CURRENT_NOW<br>
199BATTERY_PROPERTY_CURRENT_AVERAGE<br>
200BATTERY_PROPERTY_CHARGE_COUNTER<br>
201BATTERY_PROPERTY_ENERGY_COUNTER</td>
202</tr>
203<tr>
204<th>Measurements</th>
205<td>CURRENT_NOW resolution 156.25uA, update period is 175.8ms.<br>
206CURRENT_AVERAGE resolution 156.25uA, update period configurable 0.7s - 6.4h, default 11.25 secs.<br>
207CHARGE_COUNTER (accumulated current, non-extended precision) resolution is 500uAh.<br>
208CHARGE_COUNTER_EXT (extended precision in kernel) resolution 8nAh.<br>
209ENERGY_COUNTER is CHARGE_COUNTER_EXT at nominal voltage of 3.7V.<br>
210Accumulated current update period 175.8ms.<br>
211ADC sampled at 175ms quantization with a 4ms sample period. Can adjust duty cycle.</td>
212</tr>
213</tbody>
214</table>
215
216
217<h5 id="nexus-10">Nexus 10</h5>
218
219<table>
220<tbody>
221<tr>
222<th>Model</th>
223<td>Nexus 10</td>
224</tr>
225<tr>
226<th>Fuel Gauge</th>
227<td>Dallas Semiconductor DS2784 fuel gauge (a coulomb counter), with a 10mohm current sense
228resistor.</td>
229</tr>
230<tr>
231<th>Properties</th>
232<td>BATTERY_PROPERTY_CAPACITY<br>
233BATTERY_PROPERTY_CURRENT_NOW<br>
234BATTERY_PROPERTY_CURRENT_AVERAGE<br>
235BATTERY_PROPERTY_CHARGE_COUNTER<br>
236BATTERY_PROPERTY_ENERGY_COUNTER</td>
237</tr>
238<tr>
239<th>Measurements</th>
240<td>Current measurement (instantaneous and average) resolution is 156.3uA.<br>
241CURRENT_NOW instantaneous current update period is 3.5 seconds.<br>
242CURRENT_AVERAGE update period is 28 seconds (not configurable).<br>
243CHARGE_COUNTER (accumulated current, non-extended precision) resolution is 625uAh.<br>
244CHARGE_COUNTER_EXT (extended precision in kernel) resolution is 144nAh.<br>
245ENERGY_COUNTER is CHARGE_COUNTER_EXT at nominal voltage of 3.7V.<br>
246Update period for all is 3.5 seconds.</td>
247</tr>
248</tbody>
249</table>