According to JuicePlotter, my HTC Evo's 3G radio is always on when the screen is on -- even when doing absolutely nothing. I tested this by turning the brightness down to 1 and opening the settings menu and just leaving it there. The screen was set to never shut off. I got pretty significant battery drain.

I ask because I want to improve my battery life when reading downloaded books with the Kindle app. It seems to me that the radio should be in sleep mode when not in use. I'd rather not switch to airplane mode because I'd still like to receive calls/texts.

To clarify:

  • I have a strong signal at all times
  • When the screen is off, JuicePlotter says the radio is off (and the battery lasts practically forever)
  • When in airplane mode with the screen on, JuicePlotter says the radio is off (duh) and the battery life is much better.
  • Android reports that my display only consumes between 5 and 10% of power
  • Android reports that Cell Standby consumes 30% (and that's with great signal)

I think these tests demonstrate that the display itself is not the problem, the radio is. So again the question is: why does the radio use so much power when the screen is on but it's not being used?

  • Are you referring to the FM radio or 3G radio or Wi-Fi?
    – cofiem
    Sep 23, 2010 at 12:44
  • I clarified (3G radio)
    – colithium
    Sep 24, 2010 at 4:07
  • Good question. I didn't realize how nuanced this behavior was. Thanks for asking. Hope this can be solved.
    – Matt
    Sep 24, 2010 at 8:49

2 Answers 2


With cdma phones the data and voice use the same radio. Its going to run all the time anyway because its constantly syncing data, towers, location... If you want to stop it without turning off voice you should turn off the apn. Doing this will not put your radio to sleep but it will reduce your power consumption. Check into apndroid or quick settings, and tasker to set triggers and automation.

  • But it is off when the screen is off (yet of course it still receives voice calls, SMS, and syncs). When the screen is off, the radio only turns on once every half an hour or so for approximately 1 minute. I have carefully set up syncing so that it only syncs at most once an hour.
    – colithium
    Sep 24, 2010 at 7:00
  • 1
    Are you using juice defender? If so have you run the tests w/ out it enabled? Juice defender by default sets the apn to turn on with the screen. I'm not sure what the default behavior of the evo is w/o jd enabled. Either way, managing the apn will take care of the issue. You may need to disable jd if you can't tweak the settings to stop the screen trigger. I couldn't figure out how to stop that trigger w/o causing other problems. I recomend using tasker to set triggers, for ex. Have it turn off the apn when you open kindle. You can actually get more specific than that if you need.
    – Matt
    Sep 24, 2010 at 8:54
  • +1 I will definitely try this. I don't use JD though, just the plotter. And I think I have my terminology wrong. It seems I should have been saying APN instead of 3G radio.
    – colithium
    Sep 24, 2010 at 10:16
  • At the very least if you use the quick settings app you can manually toggle the APN with just a few clicks.
    – Matt
    Sep 24, 2010 at 10:58
  • @colithium You misunderstand what is going on. The radio is active for brief period of time keeping updated with the towers any time that it is not in airplane mode. What you are thinking of is waking up the application processor periodically to make checks which the radio processor cannot accomplish by itself. May 31, 2011 at 3:31

The radio is not "on" all the time. Just as in any other cellphone, it makes very brief contacts to the towers to update its registration with the network; this consumes a small amount of power, but the power management design of the radio and the coprocessor which runs it are pretty well done, so it can power up the circuity it needs just long enough and then power it back down again. Without that we wouldn't be getting multi-day battery life even from simple candy bar phones.

The screen in a smartphone itself consumes some power, but any any time the screen is on, the application processor - which runs linux and android - is also going to be on, and that consumes a lot of power compared to keeping the radio processor registered on the network but not actively exchanging data. With the screen off and nothing holding wakelocks, the application processor can try to go to sleep, and only wake up every few minutes to make quick checks. That's where your real power savings comes from, and yes, scheduling more or less frequent email polling can change this.

  • This is what it's probably supposed to do. However, like I said, there is a huge difference between having airplane mode on vs. off. That rules out the processor and screen battery drain.
    – colithium
    May 31, 2011 at 21:06
  • @colithium - by enabling airplane mode, you discourage anything that wants to use the network from periodically running. But yes, as in any cell phone, being registered with the network is going to consume some average power over time. That does not mean the radio is "on" - it just means that it turns on for a tiny timeslice fairly frequently to stay in touch - a far more efficient process than the application CPU trying to wake up and do anything. May 31, 2011 at 21:14

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