USB Fast Chargers

I've been a bit confused lately at what constitutes fast charging versus normal charging, and why newer Android devices complain repeatedly if you use the wrong cable, or the wrong charger, or the wrong cable AND charger. How does it know?

Then I found an article on LifeHacker that partially explains it, but this comment thread clarified in an excellent manner: http://lifehacker.com/theres-a-bunch-of-misunderstanding-around-charging-via-1532885435

According to this article and the super helpful comment thread, the multiple things that affect an Android's ability to fast charge are:

1. Battery current rating - I have a 3000mAh / 11.4Wh battery capacity (it is able to provide 3 amps at a nominal voltage for one hour or 11.4 Watts for one hour). P/I=V = 3.8V which is what this battery indicates it's able to deliver. This battery also has a minimum rating of 2940mAh / 11.2Wh (also 3.8V).
2. USB Spec - Normally USB is supposed to output 5V +/- 0.25V. To convert from the 3.8V battery, we simply draw less current at the USB and convert it accordingly: (3.8V x 3A) = 11.4W. (11.4W / 5V) = 2.28A. So ideally my battery wants to present a current draw of 2.28A on the charger when acting as a load. With the 0.25V allowed voltage margin, this could be anywhere from 2.17A to 2.4A.
3. A standard USB 2 port on a computer is rated only for 500mA (0.5A). If too many devices hang off of a USB port, you could start current limiting and the voltage to each device will drop, usually below the allowed 4.75V. This is why powered hubs are recommended. Even with a single device trying to draw more current than the port is capable of, this is when you get those errors that "your phone is charging slowly. Please use the charger and charging cable that came with your phone." A good charger should be able to provide all 2A needed for a standard charging rate the phone requires.
4. Diameter of charging cable - USB 2.0 has five lines that it uses:

• Rx
• Tx
• V+
• V-
• GND (braided shield)

Normally the four main lines are all 28AWG. According to http://www.powerstream.com/Wire_Size.htm, this means that the charging lines will provide 0.23A before they start heating up and creating an appreciable resistance (higher than 213 Ohms per km) which will in essence place a second, noticeable load in series with the battery. The power lines (V+/V-) in the wire can be increased to 24AWG to allow 0.58A at 84.2 Ohms per km and thus decreasing this second parasitic load. Additionally, the load of the charging cable can also be reduced by shortening the cable itself.

Gauge	Test Current	Impedance (per km)	Wire length (km) (~6 feet)	Cable Impedance
28AWG	0.23	213	0.002	0.426
24AWG	0.58	84.2	0.002	0.1684





	Volt drop (test)	Voltage Left for Phone
28AWG	0.09798	4.90202
24AWG	0.097672	4.902328





	Volt drop (0.5A)	Voltage Left for Phone
28AWG	0.213	4.787
24AWG	0.0842	4.9158





	Volt drop (2A)	Voltage Left for Phone
28AWG	0.852	4.148
24AWG	0.3368	4.6632


With a standard 0.5A charger, even a 28AWG 6-foot cable will be able to charge the phone at a standard voltage (4.79V being higher than the minimum allowed of 4.75V). However, if we wanted a full 2 Amps for fast charging, we would need a bigger cable (24AWG), which would even then present enough of a load to drop the voltage on the battery to 4.66V which, while better than 4.15V on the 28AWG wire, is still out of spec.

5. Androids have the ability to detect a "fast charger." Fast chargers indicate their ability to provide 2A by a simple short between the unused Rx/Tx lines. If the android sees this "loop-back" connection on its data lines, it is programmed to assume that whatever its connected to is able to provide 2A. Otherwise it will only draw 0.5A.
6. Additionally, some fast chargers which are paired with a standard length and gauge USB cable are able to do some rudimentary current sensing and output a voltage higher than 5.0V to compensate for the drop on the cable itself.

So in short, for a proper fast charger, two things should be used:

1. A cable with higher diameter power lines
2. A charger that is able to output the current required by the battery being charged

And an additional feature that could be desireable:

• A charger that is able to sense the current, calculate the voltage drop across a known cable and boost its voltage output to compensate.

More on Skype and overhead setups

Well, last night at men's 6:33 we had our Skype call from Argentina. And it worked...kind of. However, the connection they had down there was extremely poor and our own LAN wasn't doing so hot for external connections either. However, we did like we had done for when they went to India. We bypassed audio transmission and sent only video through Skype. Then we plugged someone's Blackberry into the sound board via headphone jack and were able to get audio that way. So Skype for video and Blackberry for audio. It worked fairly well and we were able to get a decent show up on the screen.

However, WE of course, were stressing out the entire time and we had leadership up in the sound booth bugging us as to why we weren't getting this feature to work or why they couldn't hear us or why why why...

And of course, we didn't know because we hadn't anticipated the Blackberry/cell-phone idea one bit. They just handed us the cell phone and said, "This is how we did it this morning!" But neither of us were there "this morning," so we had to figure out what they were talking about. Anyway, the same leadership person that was pretty much instigating all the stress also said afterward that we should just come up with a standard. He said, "Skype team," but I assume that to mean that we just need to write up a whitepaper or standard on how to make Skype work for missions trips that we want to bring in during a service. And I've come up with a few ideas.

The first idea is to codify the setup we had last night. Have a headphone plugin going to a cell phone from the board for audio and Skype the video in and combine them successfully. However, Skype, I noticed, is pretty horrible with video, especially when the camera is low resolution. Besides that, we only have a framerate of maybe 2 frames/sec if the connection is good and less if the connection is bad or flaky. In fact, there were times where we had 1 frame/2 minutes last night.

So maybe we should scrap the Skype idea and go for something a bit more reasonable like UStream. UStream is like an online video recorder that will also stream your recording live as you record it. Of course, I've no idea what the quality of the transmission would be if they were on a slow connection, so this might require a bit of thought.

As for audio, working with everybody else's cell phone was just horrible. The first one we got was a flip-phone that required security unlocks due to the international connection and it kept disconnecting on us. The second was a Blackberry and we got audio okay, but had a bit of trouble sending audio back. Besides that, the guy was up in the sound booth yelling at his phone the whole time. Thus I've come up with a solution. Perhaps the best idea would be to create/research a box which would replace the telephone handset. We DO have a landline in the sound booth which we can probably more easily call and get a reasonable connection (cell phones either way increase chances of dropped calls). My idea is to somehow vandalize or 'decompile' the telephone handset which is on a PBX network and replace it with a plain old RCA I/O. This would require a bit of impedance matching and possibly some audio amplifiers to interface the phone earpiece/mic with a regular audio input/output. This is probably the best solution that I can think of. Of course, the store-bought one costs somewhere around $400, so that's out. I can't imagine a church paying that much money for a once-a-year occurrence. Besides, I think I can make my own, with my dad's help.

So those are my ideas and reflections. I didn't think last night was at all efficient, and standardizing it will make things a whole bunch better.