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Scenario: A smartphone (Nexus S) battery is charged via USB connection, connected with a laptop. A measuring device (USB Tester) is running between laptop and smartphone.

Observation: The measured current and voltage while charging battery is - more or less -constant. After the battery is charged, the current drops to zero. This is the expected behavoir, but here it comes: After using the charged phone a few seconds, although the phone shows a "battery charged" status, the measured current rises again. It is not constant, but depends on the screen status, cpu load, apps running etc.

Question: Is this state - variable current, depending on actions taken on the smartphone - a direct measurement of the smartphone power consumption? Or do you have another idea what is causing this phenomenon?

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    \$\begingroup\$ Sounds like you're observing trickle charging. \$\endgroup\$ – Alexios Dec 19 '13 at 18:15
  • \$\begingroup\$ How much milliamps do you observe when doing something constant-drain on the phone, e.g. playing a HD movie? \$\endgroup\$ – anrieff Dec 22 '13 at 7:39
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In normal conditions i.e your phone is in standby mode and your screen is locked, then as you said the once the phone is charged, the current consumed by the charger is 0.

Now when you unlock your device(but still the charger is connected) and start to do some operations(e.g. playing games or watching video), then the phone will start to consume power from its battery. At this instant your phone has drained its battery to a small percentage. But the phone won't display since its a small drop. In parallel your phone will try to charge the phone since the battery has drained.

This causes the current to be drawn from the charger. As per the operation you do on the phone, the battery will get drained soon and the charger will try to charge the battery as soon as possible by providing more current. This causes a variable current change on a charged phone.

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  • \$\begingroup\$ You are partially right, because some power management IC may allow the current from the USB port to power directly the system load instead of the battery. The system load is said to be decoupled from the battery. This allows the end product to operate immediately when plugged in, regardless of the battery’s state of charge: linear.com/products/… \$\endgroup\$ – RawBean Jan 20 '14 at 12:57

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