test result of battery current against display brightness and GPS power mode

Qingyou Meng meng.qingyou at gmail.com
Thu Mar 26 22:21:59 CET 2009

Because display and GPS chip may be powered on for a long while, I
choose to test them here.

Phone is GTA02v5. Distribution is latest SHR unstable, with almost 0% CPU load.
No devices(WIFI, GSM, etc) opened before this test. Battery capacity
is about 88%.

My test method is, for example: set display background light to 100%,
then get battery current every 5 seconds for a while...

To power on GPS chip: write 1 to file
To set brightness: write (brightness_percent / 100 * 255) to file
To get "current" battery power: read file
    - It seems kernel update this file every 30 seconds
    - the output unit is uA.
To enable/activate FixNow sleep mode: write UBX binary messages
CFG-FXN and CFG-RXM to /dev/ttySAC1.
    - when activate, write dummy packet and RXM-POSREQ message:
      { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,     0xB5,
0x62, 0x02, 0x40, 0x00, 0x00, 0x42, 0xC8 }

Here is the results:

1. when GPS chip is powered off, test brightness vs. battery current:

* brightness = 100%: battery current ~= 203 mA
* brightness =  75%: battery current ~= 153 mA
* brightness =  50%: battery current ~= 116 mA
* brightness =  25%: battery current ~= 101 mA
* brightness =   0%: battery current ~=  95 mA

2. when brightness is 0%, test GPS power state vs. battery current:

* GPS normal power mode: battery current ~= about 144 mA
* GPS FixNow sleep mode:  battery current ~= about  97 mA when it goes to sleep
* GPS FixNow sleep mode:  battery current ~= about 146 mA when it is waken up

>From test #1, (assume battery voltage is constant) we can see:

1) the naked OS with almost zero CPU load and 0% brightness consumes ~95 mA.
2) brightness greatly affect battery life, 100% brightness consumes ~108 mA
3) comparing to 100% brightness,
   25% brightness saves 95% power,
   50% brightness saves 80% power,
   75% brightness saves 55% power

>From test #2, we can see:

1) GPS chip consumes about 50 mA when run in normal mode
2) GPS chip consumes near 2 mA when it goes to FixNow sleep

I've been testing FixNow for a while. Now I doubt whether it is useful
for phone users, because:

1) to save power, we can't frequently wake up FixNow from sleep,
because on each wakeup it tries getting fixes for a while then goes to
sleep (off).
2) the position data is not accurate just after wakeup, so we have to
poll for a while, if we're lucky enough we get good fix.

I think, FixNow can only be possibly used in this kind of scenario on
-- For a long trip, we need log position data (say every 3~5 minutes,
I think it's bad to set the frequency less than 1 minute).
   A logger sets GPS chip to FixNow mode, frequently wakeup it to get fix.
   To save power, we set display brightness to 0% by locking screen,
but OS still consumes 95 mA, leaving at most ~10 hours battery life!

How to utilize FixNow feature?
Can we make the power consumption of "naked" OS down to tens of mA?

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