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Google Nexus 7 Charging

When I first got my Nexus 7, I was curious how it was able to charge at different rates on different chargers. There are quite a few threads concerning people having problems with their devices charging at a reasonable rate, but there was a dearth of actual data. Using a bench power supply, and a cut up USB extension cable, I took the measurements myself.

The Google Nexus 7 2013 uses the Qualcomm PM8921 Quick Charge IC, which allows it to charge at a range of currents depending on the voltage sensed at the device. For this reason, both the voltage drop of the charger and the voltage drop of the cable under load will factor into the maximum current drawn. When the device is first plugged in, it ramps the current up in stages until the voltage falls below 4.9 volts. If at any point the voltage falls, the charger will revert down to the next level. However, under no circumstance will the charger increase to a higher current level after the initialization process. This prevents the device from cycling between two charge levels as the voltage drop rises and falls. For this reason, measuring the voltage across a charger while in use is not a good indication of the current level -- by the time a measurement can be taken, the Nexus will have reverted to a lower current level, and the voltage will have come back up.

Nexus 7 2013 USB Charging Current
Voltage Current (A)
>4.90 1.15
4.75 0.83
4.65 0.45
4.50 0.25
<4.50 <0.25

The data pins of the USB cable were shorted together in these tests, and bench power supply was used to supply and measure the current. The internal battery voltage was around 4000 mV during these tests.

Nexus 7 charging current chart

It is also worth mentioning that below ~4.5 volts the current tappers down to zero at a voltage dependent on the internal battery voltage. It appears that the bucking converter in the charging circuit of the Nexus 7 has a minimum voltage drop of ~0.2 volts. This means that the Nexus 7 will stop charging if the input voltage is less than the internal battery voltage plus 0.2 volts.

Image from http://thinkertry.com/2013/08/25/nexus-7-2013-review/

As long a cable of decent quality is used, the stock charger (rated at 1.35 A) should be able to charge the Nexus at the highest rate. While any charger rated 1.2 amps or higher should also function at this rate, the lower the sticker rating, the more likely the module will exhibit excessive voltage drop. I imagine this is why many people recommend a charger rated to at least 2 amps.
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