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I'm a bit OCD about efficiency and I have noticed that at least one of my two Android devices tells me to unplug it after charging is complete to save energy. My questions are:

  1. Is it true that a chargeable USB device may or will continue to consume similar amounts of power if it remains plugged in after fully charging?
  2. Does this vary based on hardware and/or software (Is it possible my LG Optimus T phone continues to consume similar power after it is charged, but my Nexus 7 doesn't)?
  3. If a device continues to "consume" power after it is charged, where does it go (laws of thermodynamics and all)?

In searching for my answer, I have found the following questions relevant but not entirely applicable to my question:

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A number of interesting points to consider, more electronics orientated in nature, than Android.. –  Simon Feb 28 '13 at 14:59
@Simon yes, it was hard to pick the most suitable forum for this question, but since it was partly specific to a warning message specific to my android device, I decided here would be appropriate. And I'm sure (seeing the rating on this question) many Android users have a similar question. –  BlueMonkMN Mar 6 '13 at 19:25
Perfectly fine by me –  Simon Mar 6 '13 at 19:57

1 Answer 1

up vote 7 down vote accepted

Like any other device, regardless if the battery is full or need charging it consumes energy all the time. When your device is fully charged and still plugged in then the charger replenishes the energy used by the phone. It uses less energy than when charging but still. This is called Trickle charging It is advised to unplug device because keeping you battery fully charged all the time will reduce its capacity and make your battery die faster. Battery needs to be partially discharged and then be fully charged on a semi-regular basis in order to prolong its life.

The following sentence is from Article on prolonging battery life

Let’s look at real-life situations and examine what stresses lithium-ion batteries encounter. Most packs last three to five years. Environmental conditions, and not cycling alone, are a key ingredient to longevity, and the worst situation is keeping a fully charged battery at elevated temperatures. This is the case when running a laptop off the power grid. Under these conditions, a battery will typically last for about two years, whether cycled or not. The pack does not die suddenly but will give lower runtimes with aging.

You can also read about partial discharging and charging here:

Techrepublic - Five tips for extending lithium-ion battery life

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Reading the headlines of the linked article I don't see anything about it being bad to keep the battery fully charged. In fact (for Lithium ion batteries) it seems to be preferred not to discharge the battery much. Can you reconcile that with your comment? Is your comment about avoiding trickle charging specific to non Li batteries? –  BlueMonkMN Feb 28 '13 at 18:48
Lithium ion batteries should not be discharged below the safe voltage rate because it may damage the cell. If you read the full article you will see that: "A continuous trickle charge would cause plating of metallic lithium, and this could compromise safety. To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible." And lots more about parasitic load and stress for the battery. –  Sephiroth Feb 28 '13 at 20:58
Lithium batteries can NOT be trickle charged. What actually happens is the phone will cycle between charging and discharging fairly quickly, draining the battery maybe 5% per cycle. –  Fake Name Feb 28 '13 at 22:03
During the times when the battery is draining in that quick cycle, is the power consumption less, or is power being used to drain the battery? (My original question: is it wasteful or not?) –  BlueMonkMN Mar 1 '13 at 2:17
Hi, I have tested that for you at home. Normal charging 4,5 Watts of energy drain from the socket, then when fully charged my meter shows 0 for 2-4 minutes, then it starts charging and it takes 2.9 Watt from the socket. Charging lasts for 2-3 minutes and then it stops again to wait until the battery goes down a bit. Depending on condition of you phone (screen turned on, on call etc.) the charing process is more frequent but every time it uses slightly less energy then normal charging. –  Sephiroth Mar 11 '13 at 9:39

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