1 page.title=Measuring Device Power 2 @jd:body 3 4 <!-- 5 Copyright 2015 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18 --> 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 27 such as a Summit SMB347 or Maxim MAX17050 (available on many Nexus devices). Use the in-system 28 gauge when external measurement equipment is not available or is inconvenient to 29 connect to a device (such as in mobile usage).</p> 30 31 <p>Measurements can include instantaneous current, remaining charge, battery capacity at test start 32 and end, and more depending on the supported properties of the device (see below). For best 33 results, perform device power measurements during long-running A/B tests that use the same device 34 type with the same fuel gauge and same current sense resistor. Ensure the starting battery charge 35 is the same for each device to avoid differing fuel gauge behavior at different points in the 36 battery discharge curve.</p> 37 38 <p>Even with identical test environments, measurements are not guaranteed to be of high absolute 39 accuracy. However, most inaccuracies specific to the fuel gauge and sense resistor are consistent 40 between test runs, making comparisons between identical devices useful. We recommend running 41 multiple tests in different configurations to identify significant differences and relative power 42 consumption 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> 49 import android.os.BatteryManager; 50 import android.content.Context; 51 BatteryManager mBatteryManager = 52 (BatteryManager)Context.getSystemService(Context.BATTERY_SERVICE); 53 Long energy = 54 mBatteryManager.getLongProperty(BatteryManager.BATTERY_PROPERTY_ENERGY_COUNTER); 55 Slog.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> 63 BATTERY_PROPERTY_CHARGE_COUNTER Remaining battery capacity in microampere-hours 64 BATTERY_PROPERTY_CURRENT_NOW Instantaneous battery current in microamperes 65 BATTERY_PROPERTY_CURRENT_AVERAGE Average battery current in microamperes 66 BATTERY_PROPERTY_CAPACITY Remaining battery capacity as an integer percentage 67 BATTERY_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. 71 However, the exact properties, resolution of property values, and update frequency 72 available 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 78 intervals 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 87 short period of time (such as power consumption metering tests), the fuel gauge does not make 88 corrections, 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, 92 meaning any measurement is +/-1.5mA (PCBoard layout can also affect this variation). For example, 93 when 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 102 percentages give a total error of 7% or to a small current (15mA) where +/-1.5mA gives a total 103 error of 10%.</p> 104 105 <p>For best results, we recommend measuring greater than 20mA. Gain measurement errors are 106 systematic and repeatable, enabling you to test a device in multiple modes and get clean relative 107 measurements (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 113 sample-to-sample variation due to noise, with each sample delivered at 175.8ms. You can expect a 114 rough +/-100uA for a 1 minute test window and a clean 3-sigma noise less than 100uA (or 1-sigma 115 noise 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 117 with 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 141 resolution 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 158 current sense resistor.</td> 159 </tr> 160 <tr> 161 <th>Properties</th> 162 <td>BATTERY_PROPERTY_CAPACITY<br> 163 BATTERY_PROPERTY_CURRENT_NOW<br> 164 BATTERY_PROPERTY_CURRENT_AVERAGE<br> 165 BATTERY_PROPERTY_CHARGE_COUNTER<br> 166 BATTERY_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> 171 CURRENT_AVERAGE resolution 156.25uA, update period configurable 0.7s - 6.4h, default 11.25 secs.<br> 172 CHARGE_COUNTER (accumulated current, non-extended precision) resolution is 500uAh (raw coulomb 173 counter read, not adjusted by fuel gauge for coulomb counter offset, plus inputs from the ModelGauge 174 m3 algorithm including empty compensation).<br> 175 CHARGE_COUNTER_EXT (extended precision in kernel) resolution 8nAh.<br> 176 ENERGY_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 193 current sense resistor.</td> 194 </tr> 195 <tr> 196 <th>Properties</th> 197 <td>BATTERY_PROPERTY_CAPACITY<br> 198 BATTERY_PROPERTY_CURRENT_NOW<br> 199 BATTERY_PROPERTY_CURRENT_AVERAGE<br> 200 BATTERY_PROPERTY_CHARGE_COUNTER<br> 201 BATTERY_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> 206 CURRENT_AVERAGE resolution 156.25uA, update period configurable 0.7s - 6.4h, default 11.25 secs.<br> 207 CHARGE_COUNTER (accumulated current, non-extended precision) resolution is 500uAh.<br> 208 CHARGE_COUNTER_EXT (extended precision in kernel) resolution 8nAh.<br> 209 ENERGY_COUNTER is CHARGE_COUNTER_EXT at nominal voltage of 3.7V.<br> 210 Accumulated current update period 175.8ms.<br> 211 ADC 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 228 resistor.</td> 229 </tr> 230 <tr> 231 <th>Properties</th> 232 <td>BATTERY_PROPERTY_CAPACITY<br> 233 BATTERY_PROPERTY_CURRENT_NOW<br> 234 BATTERY_PROPERTY_CURRENT_AVERAGE<br> 235 BATTERY_PROPERTY_CHARGE_COUNTER<br> 236 BATTERY_PROPERTY_ENERGY_COUNTER</td> 237 </tr> 238 <tr> 239 <th>Measurements</th> 240 <td>Current measurement (instantaneous and average) resolution is 156.3uA.<br> 241 CURRENT_NOW instantaneous current update period is 3.5 seconds.<br> 242 CURRENT_AVERAGE update period is 28 seconds (not configurable).<br> 243 CHARGE_COUNTER (accumulated current, non-extended precision) resolution is 625uAh.<br> 244 CHARGE_COUNTER_EXT (extended precision in kernel) resolution is 144nAh.<br> 245 ENERGY_COUNTER is CHARGE_COUNTER_EXT at nominal voltage of 3.7V.<br> 246 Update period for all is 3.5 seconds.</td> 247 </tr> 248 </tbody> 249 </table>
