I recently read about RadioShack losing $200 million last quarter. I went to one RS store and found they wanted over $20 for a 5 foot USB male A to micro USB cable. I told the salesdroid that I could get a similar cable for a fraction of that price at a dollar store. He tried to dazzle me by throwing tech talk at me, telling me about impedances, when I interrupted him in mid sentence by telling him that I had already measured the difference and it was only about 15 to 20 percent. (I should have told him that the impedance had nothing to do with the charging rate, it was the wire size, and hence the wire DC resistance that limited the charging current – but I figured, why embarrass him further).
I walked across the parking lot and bought a USB to micro USB cable for a dollar at Dollar Tree store. The average consumer would not realize that the cheaper cable with thinner wire was reducing the charging current, but he would much rather buy the cheaper cable than pay 20 times more for the RadioShack cable.
Then I went to Fry’s Electronics And bought a dozen 3 foot long USB to micro USB cables for $1.99 each. The cable seems a lot thicker than the cheap Dollar store cable, but I haven’t cut one up to see if the wires really were heavier or if the jacket is just thicker. There is no doubt that they will charge at a higher current because the cable is half the length of the cheap dollar cable.
I wanted to increase the charging current, so I shortened one cable. But the thin wire requires a lot of soldering patience and small diameter heat shrink tubing. It’s just better to buy them already made short. I have a few 2 inch or 50mm very short A male to micro USB cables, so I used one between my Samsung Galaxy S4 and a charger rated for up to 2.1 amps. This charged at about 1 amp – it jumps up and down as the phone does different activities. I changed to a 6 foot or 1.8M cable and the charge current went down to 1/2 amp. This shows that the cable length has a significant effect on the charging current and hence the length of time it takes to charge.
Wire Size
I looked at the AWG (American Wire Gauge) wire tables to see what the copper wire resistance was for the various sizes. I’ll give a few examples.
I’ll start with the smallest, 32 AWG. This size has about 1/6 ohm per foot. Thus a 6 foot cable has about 1 ohm resistance in the positive wire, and 1 ohm in the negative wire, for a total of 2 ohms. A 1/2 amp current would drop 1 volt in the cable, so what starts out as 5 volts at the charger end is only 4 volts at the phone end. That is probably too low to charge, so the current would drop to a lower value to reduce the loss in voltage. The charging rate goes down and the time to fully charge increases.
AWG 30 is the next common wire size. This wire size seems to be a common size for wires used in many cables in inexpensive consumer electronics. It has about 1/10 ohm per foot, or 1.2 ohms for a 6 foot cable. A 1/2 amp current would drop about 0.6 volts across the cable. We now have 5 volts at the charging end and 4.4 volts at the phone end. Not as bad as 32 AWG, but still enough to cause a significant reduction in charging current.
AWG 28 is the next common wire size up the chart. I’ve found this size wire used in USB cables with full size A and B connectors, for the data line. For the power wires, typically a larger size is used. AWG 28 has .065 ohms per foot so 12 feet of wire has .78 ohm resistance and drops .39 volt at 1/2 amp. The phone would get 4.61 volts, still too much loss. The current would have to be reduced somewhat.
AWG 26 looks like it’s getting to the point where the voltage drop is not so unreasonable. It has .041 ohms per foot resistance for a total of about 1/2 ohm for 12 feet of wire. For 1/2 amp current, the voltage drop would be about 1/4 volt, just on the edge of being tolerable. The charging current will have to be reduced very little to compensate for the voltage drop. I would consider this to be a minimum size for a 6 foot or longer cable.
AWG 24 wire is a common size used for telephone house wiring, datacomm wiring, etc. It has .026 ohms per foot, or about .3 ohms total for 12 feet of wire. A half amp would have .15 volts drop. That’s not bad, and would be acceptable in my opinion. But consumer electronics makers find that the high price of copper makes good quality copper cable expensive so they cut the copper – use thinner wire – to save $$$.
So if the cable has thinner wire, then making it shorter will reduce the negative effect thin wire has on the cable’s performance.
You may see a ‘high current’ charger rated at 2 amps for the tablets such as the iPad being sold. This charger may put out 2 amps, but if you use the cable that came with it, you may find that the cable is restricting the current to much less than its full 2 amp capacity. Surprised? It’s the same cost cutting that I mentioned earlier. 🙁
And spending more money for a USB charging cable does not mean you will get a cable with heavier copper wire. Hopefully it might be thicker than the inexpensive cable, but it’s unlikely that the wires are heavy enough to not negatively affect the charging current. The best way to charge is with the shortest USB cable you can get, and add an extension cord to the power outlet to make the overall length long enough for your needs.