Multiple Battery entries in /sys/class/power_supply on Android

Question

I am trying to read battery information from an NDK app. The only way I know to accomplish this is to read the values out /sys/class/power_supply. My difficulty is that I find multiple entries with type "Battery" and I don't know which one to use.

On a Nexus 10 with KitKat 4.4.4 I see:

$ ls /sys/class/power_supply
ds2784-fuelgauge
manta-battery
smb347-battery
# other non-battery entries

With type=Battery for all three. For example:

$ cat /sys/class/power_supply/ds2784-fuelgauge/type
Battery

In the Android BatteryManager code they just iterate through the battery devices and take the first one that has the entry they want (http://androidxref.com/4.4.4_r1/xref/system/core/healthd/BatteryMonitor.cpp). However, on my Nexus 10 they have different values. For example, for ds2784-fuelgauge/voltage_now and smb347-battery/voltage_now:

$ cat ds2784-fuelgauge/voltage_now
4153100
$ cat smb347-battery/voltage_now
4300000

ds2784-fuelgauge/voltage_now seems to be updated regularly (e.g. it changes when I start a compute-heavy task), but smb347-battery/voltage_now seems to be fixed. I see a similar effect when I compare ds2784-fuelgauge/current_now and smb347-battery/current_now.

My questions are:

1. why are there multiple entries here with type=battery?
2. how do I distinguish them in a general way?
3. which one should I be using?

Edits

• fixed typo in voltage_now filename
• emphasize that type=Battery for all three devices

Answer 1

On android devices you may have more than one battery, like the backup for RTC or for other peripherals. Did you already cat the type file for all the 3 sysfs entries? for all the 3 you get 'Battery'?

Considering that Android expects several parameters from the battery (see below), i may guess that they are split in those 3 sysfs files you got.

For example the fuel gauge is the hardware component that reads the current flow from/to the battery and calculate the remaining capacity, thus reading the capacity file from it should give you the %, but probably the voltage is read from another file.

Battery params (i.e.: /sys/class/power_supply/my_batt/status):

• status: Charging, Discharging, Not charging, Full, Unknown
• health (string): Cold, Death, Good, Overheat, Over voltage, Unspecified failure, Unknown
• present (int): 1 when present
• capacity (int): 97 for 97%
• voltage_now (in uV) or batt_volt (in mV): 4205000 for 4205000 uV = 4.205 V
• temp or batt_temp (in tenth of °C): 305 for 30.5°C
• technology (string): Li-ion