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[SOLVED] Nexus 7 charger/cable issues

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janedoesmith
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(Last edited by janedoesmith; 16th August 2012 at 06:03 AM.)
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Default [SOLVED] Nexus 7 charger/cable issues

I have been closely watching these threads and found some mixed results.
http://forum.xda-developers.com/show....php?t=1791717
http://forum.xda-developers.com/show....php?t=1780211
http://forum.xda-developers.com/show....php?t=1793059
http://forum.xda-developers.com/show....php?t=1781680
http://forum.xda-developers.com/show....php?t=1784322
http://forum.xda-developers.com/show....php?t=1785976

So, I conducted some experiments and the results are below. For those of you that don't care for the data, you can scroll down to the CONCLUSION.

Some of the information presented here are derived directly from the above threads and from the Internet. All values are approximate and measured using a digital multimeter. Some of you may already know but for the sake of other readers, I will be using the following terminology and acronyms.

AWG = American Wire Gauge. The thickness or the diameter of an electrical conductor(s) within a single wire. Most flexible wires contain multi-stranded conductors instead of a single solid conductor. The important thing is, as the AWG number DECREASES, the thickness of the conductor INCREASE. Thicker wires can carry more current
OEM = Original Equipment Manufacturer
Connector Pigtail = A connector with its individual wires exposed and part of the wire's insulation removed as a means to perform electrical measurements.
(XX / YY) = "XX" represents the AWG of the data wires (D+ / D-). "YY" represents the AWG of the power / ground wires (+5v / GRN). Some USB cables will be imprinted with its rating on the cable itself such as "28AWG/1P 26AWG/2C". I will be referring to this cable as (28/26) meaning the data cables are 28AWG and the power / ground wires are 26AWG
USB data wires = The White wire (D-) and the Green wire (D+) contained within the USB cable.

Testing Jig:
1. 12 inch USB A male to USB A female extension cable (28/28) cut in half to produce 1-USB A Male pigtail and 1-USB A Female pigtail.
2. 12 inch MicroUSB B male to MicroUSB B female extension cable (28/26) cut in half to produce 1-MicroUSB B Male pigtail and 1-MicroUSB B Female pigtail. http://www.ebay.com/itm/Micro-USB-B-...item3f1497b3e1
3. 18 inch test leads (18 AWG). http://www.harborfreight.com/18-inch...ads-66717.html

I used 3 different MicroUSB cables and 1 USB A extension cable:
OEM = 3 feet, unknown AWG (not imprinted on the cable), impedance of power/ground wires = 0.2 ohms, length of MicroUSB male end = 6 mm, resistance / continuity check reveals nothing special. It is a standard MicroUSB cable.
Monoprice = 6 feet, (28/24), impedance of power/ground wires = 0.2 ohms, length of MicroUSB male end = 5.5 mm, resistance/continuity check reveals nothing special. I purchased 2 of this cable. The MicroUSB male end on the other cable measured 6 mm. http://www.monoprice.com/products/pr...seq=1&format=2
Amazon = 6 feet, unknown AWG (not imprinted on the cable), impedance of power/ground wires = 0.5 ohms, length of MicroUSB male end = 5.5 mm on both, resistance/continuity check reveals nothing special. http://www.amazon.com/Case-Star-Blac...microusb+cable
Generic extension cable = USB A male to USB A female, 6 feet, (28/24), impedance of power/ground wires = 0.2 ohms.

I used 4 different chargers:
OEM = Rated at 2 A, resistance between data pins = 0.5 ohms (shorted).
iPhone 4 OEM charger = rated at 1 A, resistance between data pins = 53100 ohms.
Belkin = 2 port, each port rated at 500 mA, resistance between data pins = 58000 ohms. http://www.amazon.com/Belkin-Mini-Su.../dp/B0015DYMVO
AT&T Car charger = An iPhone charger with an auxiliary USB charging port, unknown rating, resistance between data pins = not connected (infinity).

Amperage readings measured at the cable using connector pigtails with test leads and with a completely drained Nexus 7 (Surprising Results!!!):
OEM charger + OEM cable = Charges at 821mA AND displays AC charging.
OEM charger + OEM cable with the data wires disconnected (open)= Charges at 821mA BUT displays Discharging.
OEM charger + OEM cable + Generic extension cable = Charges at 631 mA AND displays AC charging.
OEM charger + OEM cable with the data wires disconnected (open) + Generic extension cable = Charges at 631 mA BUT displays Discharging.
OEM charger + Monoprice cable = charges at 823 mA BUT displays Discharging.
OEM charger + Monoprice cable with data wires shorted = charges at 823 mA AND displays AC charging.
OEM charger + Monoprice cable + Generic extension cable = charges at 635 mA BUT displays Discharging.
OEM charger + Monoprice cable with data wires shorted + Generic extension cable = charges at 635 mA AND displays AC charging.
OEM charger + Amazon cable = charges at 451 mA BUT displays Discharging.
OEM charger + Amazon cable with data wires shorted = charges at 451 mA AND displays AC charging.
OEM charger + Amazon cable + Generic extension cable= charges at 258 mA BUT displays Discharging.
OEM charger + Amazon cable with data wires shorted + Generic extension cable= charges at 258 mA AND displays AC charging.

iPhone 4 OEM charger + all above cable combination = charges at a maximum rate of 635 mA BUT displays Discharging. Increasing the length of the cable decreased the charging rate.
iPhone 4 OEM charger + all above cable combination + data wires shorted = charges at a maximum rate of 635 mA AND displays AC charging. Increasing the length of the cable decreased the charging rate.

Belkin charger + all above cable combination = charges at 259 mA BUT displays discharging. Increasing the length of the cable did not change charging rate.
Belkin charger + all above cable combination + data wires shorted = charges at 259 mA AND displays AC charging. Increasing the length of the cable did not change charging rate.

AT&T car charger + all above cable combination = maximum charging rate of 830 mA BUT displays discharging. Increasing the length of the cable decreased the charging rate.
AT&T car charger + all above cable combination + data wires shorted = maximum charging rate of 830 mA AND displays AC charging. Increasing the length of the cable decreased the charging rate.

NOTE: With the Nexus 7 powered off, all combinations of above chargers and cables, regardless of whether the data wires were shorted or not displayed the charging indicator.

CONCLUSION:
1. Nexus 7 OEM charger is nothing special, just the data pins are shorted.
2. Nexus 7 OEM MicroUSB cable is nothing special. It is a standard MicroUSB cable.
3. Any USB car/wall charger should charge the Nexus 7.
4. Nexus 7 will charge regardless of whether it displays "AC Charging" or not.
5. The charging rate is dependent on the capacity of the charger and the total charging circuit impedance (lower the impedance the better).
6. Increasing the length of the cable increases the charging circuit impedance and decreases the charging rate.

SOLUTION:
1. Use a high quality, high amperage USB charger.
2. Use a cable with an AWG of (28/24) or lower.
3. Cables longer than 6 feet is not suggested regardless of lower AWG as the connectors itself (especially at the MicroUSB end) will be the bulk of the impedance for that cable.
4. If you desire to have "AC Charging" be displayed on your Nexus 7, short the data wires/pins. Remember, this has no effect on the charging rate.
5. If you plan to modify the charger, take a resistance measurement of the data pins first (unplugged), any readings other than 0 ohms or infinity (open or not connected), you will need to isolate the data pins from the charging circuit prior to shorting. If you do not, you risk damaging the charger and/or your precious Nexus 7.
6. If you plan to modify the cable, short the data wires at the MicroUSB end and leave the data wires open or not connected at the charger end. Use heat shrink to insulate the exposed conductor of the data wires.
7. Make sure the MicroUSB connector end is about 6.0 mm in length.
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stolencar
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Thanks for this great post.

I have been switching back and forth all morning between the oem cable/charger and a blackberry charger with removable usb cable. This post solves the riddle of why the oem charger with the longer blackberry usb cable was charging demonstrably slower than the oem combo (at least according to my battery widget).
 
unclebeuford
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Thank you for the detailed information. This was driving me nuts.
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gogol
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I am surprised with the charging rate of 821mA ...

Quote:
OEM charger + OEM cable = Charges at 821mA AND displays AC charging.
So, basically, we don't really need 2A charger?
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generationgav
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Will charging my phone (HTC Desire S) with the Nexus 7 charger have any damage on my phone?
 
redmonke255
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Quote:
Originally Posted by generationgav View Post
Will charging my phone (HTC Desire S) with the Nexus 7 charger have any damage on my phone?
No, there's current limiters that keep it at a safe level.

Sent from my LG-P999 using xda app-developers app
LG G2x P999 CM7.2
Nexus 7 16GB
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qoncept
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(Last edited by qoncept; 9th August 2012 at 04:05 PM.)
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Thanks for all the info, that was what we really needed.

Now.. what's our best source for a quality cable with long enough connector for a reasonable price?

Edit: I think I'll short some data pins and trim some hoods back with a knife tonight.
 
player911
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Awesome. Perhaps I don't need to replace my car setup. I've shorted out the DATA pins IN the charger. My Galaxy Nexus reads it as AC charging now but the N7 doesn't even detect it. I'll throw it on the charger on my way home from work and see if it does in fact charge it.

This is exactly the way my Samsung Galaxy Tab behaves. It was dead when I got it (used) and none of my chargers would work (display charging). I found that even if it didn't say charging, it was, in fact, charging.

So technically the unit is sucking down the juice but telling us it isn't. Perhaps some devs can figure out how to force it to display charging any time it grabs juice. Just seems like a software bug. There is no reason my Galaxy Nexus see's my car charger as AC (1A) but nothing on the N7.

Glad you did this test. It proves very helpful.
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janedoesmith
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Quote:
Originally Posted by gogol View Post
I am surprised with the charging rate of 821mA ...



So, basically, we don't really need 2A charger?
You do need a 2A charger. All testing was done when the N7 was completely dead. I suspect when the battery is at like 50%, the charging rate will peak to 1300-1500mA and start decreasing when the battery nears its capacity.

I didn't make measurements at the various charge stages as this experiment was just about whether it charges with the various chargers and cable combinations.
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AZImmortal
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janedoesmith: When you said that the Nexus 7 was completely drained, does that mean that it was turned off? If it was off, then would you mind performing the stock charger test with it turned on (and maybe something like 50% charge)? I'm just curious to see if it draws more current when turned on. Would you also mind testing charging from your computer's USB port? Since your testing has shown that the Nexus 7 tries to draw greater than 500mA even when the data pins are open, then it means that it doesn't follow USB charging specs (with open data pins, a device should never try to draw more than 500mA).

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