Android smart phone with longer battery life

Smart phones have been developed a lot with more cpu cores, more computation capabilities, with larger and brighter screens, with higher resolution cameras and etc. But battery is one of the areas have no breakthrough. Talking to the battery life, because of the faster cpu/gpu, bigger screen, it even gets worse. Do you still remember the last time you got so mad to find a power supply outlet?
Smart phones use Li-ion batteries. Before Li-ion batteries improve the capacity with the technology breakthrough, how to save the battery life becomes very important as we dont have too much to waste. To save the battery life keeping closely monitoring the battery life is the first step.
Start from Android KK (Kitkat) Android adds the system/core/healthd, which is built into part of Android system image as /sbin/healthd, to monitor the battery state. In this article let's take a close look into healthd to understand how can we monitor the battery state.

What does healthd monitor and tell?

First let's take a look at the healthd logs:

<14>[21867.192464] healthd: battery l=78 v=4024 t=18.8 h=2 st=4 c=-239 chg=a
<14>[21867.421852] healthd: battery l=78 v=4067 t=18.8 h=2 st=2 c=5 chg=a
<14>[21867.448320] healthd: battery l=78 v=4067 t=18.8 h=2 st=2 c=5 chg=a

The healthd monitors the battery state of charge and reports 7 outputs. They are battery level (l=78), voltage (v=4024), battery temperature (t=18.8), health state (h=2), battery status (st=4), charging/discharging current (c=-239), and charging status/charger type (chg=a).

• Battery level: values in 0 ~ 100, means no battery to full battery.
• Battery voltage: voltage in mV, lower battery level, less voltage.
• Battery temperature: temperature in Celsius (°C).
• Health state: values in 1 ~ 7. It is a enumeration with following health state:
• 1 -> Unknown
• 2 -> Good
• 3 -> Overheat
• 4 -> Dead
• 5 -> Over voltage
• 6 -> Unspecified failure
• 7 -> Cold
• Battery status: values in 1 ~ 5. It is a enumeration with following battery status:
• 1 -> Unknown
• 2 -> Charging
• 3 -> Discharging
• 4 -> Not charging
• 5 -> Full
• Charging/discharging current: positive value means discharging current in mV, negative value means charging current in mV.
• Charging status: empty if not on charging. a/u/w otherwise:
• a: charger AC online
• u: charger USB online
• w: charger wireless online

How does healthd read the battery health data?

The battery driver exports the virtual sysfs under /sys/class/power_supply for upper layer to get the battery health data. Let's review the code https://github.com/android/platform_system_core/blob/master/healthd/BatteryMonitor.cpp to understand how healthd gets the battery health data.

BatteryMonitor::init(...) function: open /sys/class/power_supply directory, read and traverse

void BatteryMonitor::init(struct healthd_config *hc) {
    String8 path;
    char pval[PROPERTY_VALUE_MAX];

    mHealthdConfig = hc;
    DIR* dir = opendir(POWER_SUPPLY_SYSFS_PATH); /* open folder /sys/class/power_supply */
    if (dir == NULL) {
        KLOG_ERROR(LOG_TAG, "Could not open %s\n", POWER_SUPPLY_SYSFS_PATH);
    } else {
        struct dirent* entry;

        while ((entry = readdir(dir))) { /* loop and read files in /sys/class/power_supply */

In sysfs folder /sys/class/power_supply, there is a list of power_supply supported by the given smart phone. In each power_supply folder, there are some attributes. One of the attributes every power_supply shall have is the type. Following code snippets check the type of each power_supply to determine the next things to check:

394            // Look for "type" file in each subdirectory
395            path.clear();
396            path.appendFormat("%s/%s/type", POWER_SUPPLY_SYSFS_PATH, name);
397            switch(readPowerSupplyType(path)) {
398            case ANDROID_POWER_SUPPLY_TYPE_AC:
399            case ANDROID_POWER_SUPPLY_TYPE_USB:
400            case ANDROID_POWER_SUPPLY_TYPE_WIRELESS:
... ...
407            case ANDROID_POWER_SUPPLY_TYPE_BATTERY:
... ...
506            case ANDROID_POWER_SUPPLY_TYPE_UNKNOWN:
... ...

If power_supply type is other than Battery or Unknown, then power_supply type should be one of the following:

• ANDROID_POWER_SUPPLY_TYPE_AC
• ANDROID_POWER_SUPPLY_TYPE_USB
• ANDROID_POWER_SUPPLY_TYPE_WIRELESS

and /sys/class/power_supply/%power_supply_type%/online will be further checked to see if such power_supply is plugged to charge the given smart phone.

401                path.clear();
402                path.appendFormat("%s/%s/online", POWER_SUPPLY_SYSFS_PATH, name);
403                if (access(path.string(), R_OK) == 0)
404                    mChargerNames.add(String8(name));
405                break;
... ...
215    for (i = 0; i < mChargerNames.size(); i++) {
216        String8 path;
217        path.appendFormat("%s/%s/online", POWER_SUPPLY_SYSFS_PATH,
218                          mChargerNames[i].string());
219
220        if (readFromFile(path, buf, SIZE) > 0) { /* online */
221            if (buf[0] != '0') {
222                path.clear();
223                path.appendFormat("%s/%s/type", POWER_SUPPLY_SYSFS_PATH,
224                                  mChargerNames[i].string());
225                switch(readPowerSupplyType(path)) {
226                case ANDROID_POWER_SUPPLY_TYPE_AC:
227                    props.chargerAcOnline = true;
228                    break;
229                case ANDROID_POWER_SUPPLY_TYPE_USB:
230                    props.chargerUsbOnline = true;
231                    break;
232                case ANDROID_POWER_SUPPLY_TYPE_WIRELESS:
233                    props.chargerWirelessOnline = true;
234                    break;
235                default:
236                    KLOG_WARNING(LOG_TAG, "%s: Unknown power supply type\n",
237                                 mChargerNames[i].string());
238                }
239            }
240        }
241    }

If power_supply is Battery, it checks more attributes under sysfs folder /sys/class/power_supply/battery, such as status, health, present, capacity, voltage_now/batt_vol, current_now, current_avg, charge_counter, temp/batt_temp, technology.
Read battery level in percentages and battery voltage in mV:

191    props.batteryLevel = mBatteryFixedCapacity ?
192        mBatteryFixedCapacity :
193        getIntField(mHealthdConfig->batteryCapacityPath);
194    props.batteryVoltage = getIntField(mHealthdConfig->batteryVoltagePath) / 1000;

Read battery temperature in Celsius (°C):

196    props.batteryTemperature = mBatteryFixedTemperature ?
197        mBatteryFixedTemperature :
198        getIntField(mHealthdConfig->batteryTemperaturePath);

Read battery status:

204    if (readFromFile(mHealthdConfig->batteryStatusPath, buf, SIZE) > 0)
205        props.batteryStatus = getBatteryStatus(buf);
57int BatteryMonitor::getBatteryStatus(const char* status) {
58    int ret;
59    struct sysfsStringEnumMap batteryStatusMap[] = {
60        { "Unknown", BATTERY_STATUS_UNKNOWN },
61        { "Charging", BATTERY_STATUS_CHARGING },
62        { "Discharging", BATTERY_STATUS_DISCHARGING },
63        { "Not charging", BATTERY_STATUS_NOT_CHARGING },
64        { "Full", BATTERY_STATUS_FULL },
65        { NULL, 0 },
66    };
67
68    ret = mapSysfsString(status, batteryStatusMap);

Read battery health state:

207    if (readFromFile(mHealthdConfig->batteryHealthPath, buf, SIZE) > 0)
208        props.batteryHealth = getBatteryHealth(buf);
77int BatteryMonitor::getBatteryHealth(const char* status) {
78    int ret;
79    struct sysfsStringEnumMap batteryHealthMap[] = {
80        { "Unknown", BATTERY_HEALTH_UNKNOWN },
81        { "Good", BATTERY_HEALTH_GOOD },
82        { "Overheat", BATTERY_HEALTH_OVERHEAT },
83        { "Dead", BATTERY_HEALTH_DEAD },
84        { "Over voltage", BATTERY_HEALTH_OVER_VOLTAGE },
85        { "Unspecified failure", BATTERY_HEALTH_UNSPECIFIED_FAILURE },
86        { "Cold", BATTERY_HEALTH_COLD },
87        { NULL, 0 },
88    };
89
90    ret = mapSysfsString(status, batteryHealthMap);

After BatteryMonitor.cpp traverse the sysfs folder /sys/class/power_supply and update the attributes, the following code snippets is to notify the applications who concern the battery health state.

275    healthd_mode_ops->battery_update(&props);

It is a callback function registered at the beginning of the health monitoring service. Have to cut here. Will continue with a new article.