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[SOLVED] Nexus 7 charger/cable issues

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janedoesmith

Senior Member
Jul 11, 2012
203
184
I have been closely watching these threads and found some mixed results.
http://forum.xda-developers.com/showthread.php?t=1791717
http://forum.xda-developers.com/showthread.php?t=1780211
http://forum.xda-developers.com/showthread.php?t=1793059
http://forum.xda-developers.com/showthread.php?t=1781680
http://forum.xda-developers.com/showthread.php?t=1784322
http://forum.xda-developers.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.
 
Last edited:

stolencar

Member
Jul 20, 2012
13
1
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).
 

qoncept

Senior Member
Feb 1, 2010
239
38
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.
 
Last edited:

player911

Inactive Recognized Developer
Sep 8, 2006
7,954
1,206
Cincinnati
www.SnapSiteAdmins.com
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.
 

janedoesmith

Senior Member
Jul 11, 2012
203
184
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.
 

AZImmortal

Senior Member
Dec 22, 2010
505
74
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).
 

janedoesmith

Senior Member
Jul 11, 2012
203
184
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).
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

Senior Member
Dec 22, 2010
505
74
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.
 

janedoesmith

Senior Member
Jul 11, 2012
203
184
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.
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.
 

janedoesmith

Senior Member
Jul 11, 2012
203
184
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Entropy512

Senior Recognized Developer
Aug 31, 2007
14,095
25,088
Owego, NY
Thank you. That cable was used in the experiment. Any 6 foot cable with a AWG of (28/24) will work flawlessly.

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

However, those Monoprice 24/28AWG "premium with ferrite" cables work flawlessly for me. I have a whole pile of them.

I have had the following chargers work properly with my N7 and cables like the Monoprice ones:
  • Samsung Tab 10.1 charger - Shorts D+ and D- and additionally floats them at 1.8 with a weak voltage divider (the latter is not necessary for N7, but it happens to be a method that is compatible with standards-compliant devices like the N7 and all of my phones)
  • Scosche iPad car charger modified so that it behaves just like the Samsung Tab charger (Shorted D+ and D-, removed one of the pullup resistors in the voltage divider)
  • Galaxy Note charger (shorts D+ and D-)
  • Old HTC charger (shorts D+ and D-)
Using CurrentWidget (my kernel is patched so it works - see http://review.cyanogenmod.com/#/c/20891/), I see the following:
  • Tab 10.1 charger seems to charge faster than stock. This doesn't make sense as when unloaded, the 10.1 is dead-on at 5.0 volts, and the stock charger is 5.1 volts. However the stock charger may behave differently under load. This may just be test variance - CurrentWidget is reporting current into the battery, so this will vary depending on CPU/screen power usage. I'm still looking to see if there is a separate current measurement device somewhere that will report USB input current.
  • The Galaxy Note charger reports significantly lower charging current than the stock charger or Tab 10.1 charger - not surprised, it's only rated 1.0A
 

Salty Wagyu

Senior Member
May 28, 2011
660
199
England
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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  • 113
    I have been closely watching these threads and found some mixed results.
    http://forum.xda-developers.com/showthread.php?t=1791717
    http://forum.xda-developers.com/showthread.php?t=1780211
    http://forum.xda-developers.com/showthread.php?t=1793059
    http://forum.xda-developers.com/showthread.php?t=1781680
    http://forum.xda-developers.com/showthread.php?t=1784322
    http://forum.xda-developers.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.
    3
    can I plug Nexus7 in 5V 3.0A car charger? Will it burn, melt or anything like that

    Think of it this way: the light sockets in your house might be hooked to a 10A breaker, and 10A * 120V = 1200W. If you put a 40W light bulb in there it doesn't draw the whole 1200W; it draws 40W, like it was designed to. The extra power is there, but each device draws only what it needs (UNLESS something goes horribly wrong, like when something gets shorted-out internally; but that would be a problem no matter which charger you were hooked to.)

    (There are other issues with some devices or chargers like constant-current supplies, but generally those issues do not affect commercial devices.)

    The number of amps the power supply can source does have relevance to how fast the device charges (which is why the 2A charges faster than the 500mA from a computer USB port) but they are both still limited to safe levels in commercial devices.
    2
    Bought lot of 10 usb cables
    Ebay item 390317506982 Seller goodbuy711

    Description says "Proper current to your USB device via Heavy-duty 22AWG power wire"
    But in reality this is cheap thin 28AWG cable.
    Beware this seller.

    Sorry, can't post urls. Wanted to attach photo of this s**t.
    2
    This is the other combo I'm definitely considering. Do you happen to know what the mA in (per the OP) is for this cord+charger combination?

    Do either of these look legit? I don't want to get an eBay "knock-off" variety by mistake..
    http://www.ebay.ca/itm/Brand-New-10...adapter-HP-TouchPad-FB341AA-ABA-/330772367905
    http://www.ebay.ca/itm/New-HP-Touch...er-Charger-Original-FB341AA-ABA-/200728721755

    Both of those appear legit. The "KMASHI" label on one of them is indeed the OEM -- the devices I purchased have both a small KMASHI hologram sticker and printed HP markings. The two-piece design is real but it's solid once assembled.The UPC label is a part number on a clear sleeve that peels off. Printing on the barrel indicates HP part number 157-10157-00, which seems to check out as legit.
    Listed input is 100-240V, 0.4A, output is 5.3V 2.0A

    One of the eBay sellers you quoted -- hkx-power -- was my source too, but I note they only ship to the US and Canada. The other source you quoted ships worldwide.

    BTW, I'm still happy with them. Good luck!
    2
    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.