There are a number of such measuring devices for USB charging current. I chose this product branded "PortaPow" because it itself consumes little power for its own operation, as it has only a power-saving LCD display (no LED) backlight off or similar frills. Characterized the measurement is not distorted, and no separate power supply (battery) for the measuring device requires, as is the case with some other products.
The handling is very simple: The meter is simple addition between the charger (or PC) and the device to be charged plugged. With a small switch can be between the display of the voltage (should always be 5 volts +/- 5%) select and the charging current; I'm interested mostly the latter. The current value is displayed in two decimal places, ie with a resolution of 10 mA, which is sufficient for this purpose in practice.
With the help of this device, I could now examine exactly which combinations of chargers, cables and devices work best. And I have gained some surprising findings. Firstly, the cable can have a significant impact on the charging current and thus the duration of charging. That meant that I was able to identify two particularly bad cable, I promptly discarded and replaced by better ones.
Another interesting discovery: In a certain combination I could find that a tablet initially attracted only 800 mA charge current, then turn up after a few minutes to 1.3A. The reason for this behavior is unknown to me. It shows, however, that one should not make for a test of a few seconds his judgment, but that it may be worthwhile to bring your patience.
By the way: The largest charging current is always drawn with empty battery. If the battery is gradual, the charging current decreases slowly until it then at 100% battery level to almost zero (or to the operating value if you want to load the device is not turned off). This is the normal load curve for LiIon and LiPo batteries, as the charging circuit switches from a certain cell voltage from current to voltage monitoring in order to avoid an overload of sensitive cells. So if you want to measure and compare load currents, the device to be charged should be nearly empty, so that the charging circuit draws with maximum amperage.
Conclusion: The charge devices via USB is a complex issue with many variables. This little instrument has been a great help to some mysteries to get to the ropes. I highly recommend it.
