The USB Charger Doctor, that is.I went on a small eBay shopping spree, though it was mostly restocking some components like resistors, capacitors, and some RGB LEDs. The only interesting item in the mix was this USB Charger Doctor which reads out the voltage and current through a USB connection. I bought it because I’ve always been curious to see exactly how much current my devices were drawing. I tested the stock charger for my Nexus 7 (2012), the stock charger for my Samsung Galaxy S2 Skyrocket, and that battery pack I got from China. The results are all over the place, and I like that.
The voltage regulation of the two chargers and battery are not bad. It seems as though all three test subjects can sway +/-200mV with or without a load.
A bonus test was with my Powerocks 2600mAh battery. It’s of much better quality than the China battery pack in build quality, and as gives a very nice regulated 5V compared to the 5.08V (no load) output of the China battery pack.
The current tests were what I was really interested in. I got some pretty fascinating notes out of it.
The picture above is my Skyrocket (phone) being charged. When I was doing this test, it would actually pull that 910mA only when the screen was on. It would drop down to around 400mA when I turned the screen off. I suspect it’s a current limit for the battery when it’s already charged to >90% which is what it was during that test. As I test it now with my phone around 70%, it draws a consistent 960mA regardless of whether the screen is on or not. The Samsung charger for my phone is rated at 1A so the charger is perfect.
As for the Nexus 7 charge test, it was pulling 910mA. I find that curious since the included charger is rated for 2A. Perhaps it’s a software limitation, or a change in the Nexus 7 hardware after the charger was already designed?
The current test for my China battery pack was the one I was most interested in because I wanted to know how much current it draws so I could use an appropriate charger to charge it. There’s no label for the input current, unlike the Powerocks battery, but it’s not like I’d put much trust in it if it was labeled anyway. Using the Doctor, I see it pulls a constant 700mA so any one of my 1A chargers would suffice.It was pretty neat to see what was going in with the batteries and chargers I’ve been using for years. I’ll still have to play around with it more as my tests have been pretty quick and not very practical. I would totally recommend one of these USB Charger Doctors if you’re interested in seeing what your devices are drawing. Thanks for reading!