I used to charge my phone during the day at work with a USB cable (plugged into my workstation). Obviously USB doesn't provide the same amount of juice as an outlet so it took all day to charge but after the charge I noticed that the battery life was very low, maybe 2-3 hours.

When charging using an outlet, obviously it only takes about an hour to charge, but afterwards it seems that my battery life is much longer about 5-6 hours.

Both tests were performed playing MP3s all day, nothing that would cause a serious difference between the two tests.

Is it just me?

Edit: My question is about charging methods vs battery run time, not the charging methods. "Faster" in this case doesnt mean modified charging, it means using OEM charger with a power outlet.

  • 2
    \$\begingroup\$ Are you basing this 5-6 Hour life off of actually running the battery down or the software power meter? \$\endgroup\$
    – Kellenjb
    Mar 4, 2011 at 16:17
  • \$\begingroup\$ actuall usage. How long I can play music until I need to charge (which I let the battery bar get to 25% before plugging in) \$\endgroup\$ Mar 4, 2011 at 16:20
  • \$\begingroup\$ I can say from personal experience that (for whatever unknown reason) USB charged devices charge about 2x as fast when plugged into the SunRay on my desk as the PC... A real headscratcher as it should be identical... \$\endgroup\$ Mar 4, 2011 at 17:56

2 Answers 2


When charging over a USB port which is also used for communication, you're limited to 2.5W/500mA. When using a port dedicated for charging (which can be a female USB-A plug), you can use almost 10W/2A. From these numbers, the dedicated charger is clearly better.

However, batteries have thermal and chemical properties which often limit the charge rate to something less than these values. This is especially true for small batteries in confined spaces like your cell phone battery, and the lithium-ion chemistry which is likely used has the added potential of explosive thermal failure, so the charge rate is limited by some circuitry.

It would be reasonable to assume that the maximum charge rate for your cell phone battery is a little less than 2A and more than 500mA. Cell phone batteries are usually charged at about 1C, and the maximum cell voltage of a Li-ion is about 4.2V, so assuming a 1000mAh battery and a 90% efficient charger (which is generous), you'd need 4.6W of power to charge your battery as fast as possible.

Charging more slowly than this (i.e. at 2.5W) shouldn't diminish the charge level of the battery in any noticeable way. A software power meter could be erroneously reporting a higher battery life due to heating of the battery and the consequent higher voltage, or any number of other errors. If an accurate test (identical use patterns, signal strength, previous charge levels, etc) indicates that your phone has markedly less battery life after charging by USB, then either your battery is defective or your phone circuitry is poorly designed.


I have a few guesses as to what is actually happening.

1) It could be an artifact of how the phone is determining how much battery life there is. The more advanced methods of battery life indication will watch the current draw over some sort of moving average. I could see a potential for a programming error that would cause you to have invalid readings after unplugging the power cable. There might actually be more power in the battery, but it would be hard to tell unless you know more about your device and how it is monitoring the voltage. You could always look at the voltage of the battery at 20% after being charged from USB vs wall and seem if the voltages are about the same.

2) The lower current limit is able to get the battery up almost to full charge, but can't quite get it there all of the way. This could be checked in a similar manor as I mentioned before where you check the voltage of the battery after USB charge vs wall charge.

3) Some devices do a method of charging and discharging after getting full even though your device is still plugged in. So it will get to 100% and then it will discharged to 90% and then start charging again. This is supposed to help with keeping some types of batteries healthy. I could see the situation where your device has discharged, but since it is on a lower current limit supply it takes it longer to charge.


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