[SOLVED] Nexus 7 charger/cable issues

I have been closely watching these threads and found some mixed results.
http://xdaforums.com/showthread.php?t=1791717
http://xdaforums.com/showthread.php?t=1780211
http://xdaforums.com/showthread.php?t=1793059
http://xdaforums.com/showthread.php?t=1781680
http://xdaforums.com/showthread.php?t=1784322
http://xdaforums.com/showthread.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..._USB_Cables_Hubs_Adapters&hash=item3f1497b3e1
3. 18 inch test leads (18 AWG). http://www.harborfreight.com/18-inch-low-voltage-multi-colored-test-leads-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/p...=10303&cs_id=1030307&p_id=5458&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-Bla...id=1344416273&sr=8-70&keywords=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-Surge-Protector-Charger/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.

gogol said:

I am surprised with the charging rate of 821mA ...



So, basically, we don't really need 2A charger?

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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.

AZImmortal said:

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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The tests were performed with the N7 powered on and powered off. The current draw was the same for both conditions.

My N7 is currently fully charged. I will make one additional amperage measurement when it discharges to 50%.

Basically, a charging circuit cannot draw no more than what the charger can supply. When my N7 discharges to 50%. I'll make some measurements from the computer's USB port.

AZImmortal said:

I understand about not being able to provide more than the charger can supply. I brought up the USB charging specs issue because some computers will shut down if the connected device tries to draw more than 500mA, which is why there's supposed to be a difference between having the data pins bridged or not. Open is supposed to make the device think that it's connected to a computer's USB port, meaning it shouldn't try to draw over 500mA. Bridged is supposed to signal that it's ok to draw up to 1A because it's supposedly connected to a dedicated charging port.

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I'm no expert when it comes USB specifications but this is what I suspect:

When a device connects to a host via USB, the data lines are no longer open and it becomes and active data circuit. Once a handshake takes place, the device and the host knows what its roles are and the N7 displays " USB charging". Due to this active data circuit, I will not be disconnecting nor shorting the data lines when I perform the additional tests when my N7 discharges to 50%. I will be just measuring amperage with various cables and lengths.

Mustang7302 said:

I picked up one of these: http://www.monoprice.com/products/p...=10303&cs_id=1030307&p_id=5458&seq=1&format=2

Has worked without a concern of charging or not and seems to feed sufficient current to charge the device in a timely manner. I know this isn't scientific, but wanted to provide the cable that I am using without issue.

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Thank you. That cable was used in the experiment. Any 6 foot cable with a AWG of (28/24) will work flawlessly.

Entropy512 said:

janedoesmith said:

Thank you. That cable was used in the experiment. Any 6 foot cable with a AWG of (28/24) will work flawlessly.

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Not necessarily - it seems that a lot of MicroUSB cables have male connectors that are excessively short. These interfere with the N7's case.

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The length of the MicroUSB connector was covered in the first post.

Salty Wagyu said:

Interesting results, what did you use to measure the mA rate? Is it possible from a battery app? I have a kill-a-watt plug lying around though, will see tomorrow if it can display mA but I doubt it.

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I used a digital multimeter and measured it at the cable.