The All-in-One Nexus 7 Charger Issues thread

Update 8/9/12:
janedoesmith did a bunch of testing with different charger and cable combinations. http://xdaforums.com/showthread.php?p=29983624#post29983624

Sounds like the most common problem is just cables with micro USB connectors that are too short.

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There are a handful of threads that are a mish mash of good and bad/non-information. The N7 charger situation is a mess and finding a charger that works well is
almost impossible. It seems the cable you use is important, too.

"Confirmed working" chargers are chargers that work at > 500mA

Confirmed working 3rd party charger/cable combos (Updated 7/30/12):
- OEM Asus charger / $25 / ~1.8A / 3' cable
- HP Touchpad charger / ~$15 / ~1.6A / 6' cable
- Blackberry 1.8A Playbook charger / 1.8A rated / $15 / 6' cable
- Asus Transformer charger model AD827M / $? / 2A / length?
- Nokia AC-10U / 1.2A / $? / 6ft integrated cable
- Blackberry 700mA charger / 700mA rated / $5 / 6' cable

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airphoto has noticed the actual micro usb connector is a little longer on the OEM cable than a typical cable. This is the only confirmed physical difference we know about at the moment.

http://xdaforums.com/showpost.php?p=29361415&postcount=19






Here is my experience:

- Chargers seem to work in 3 different ways: 1) they'll charge at full capacity, 2) they'll charge but slower than they are rated, or 3) they won't charge at all (and the N7 will NOT show a charging icon) (Update: Chargers that don't show a charging icon will actually trickle charge, roughly 5% an hour)

- Shorting the data pins on a charger that doesn't charge at all (#3 above) results in NO CHANGE

- The USB cable you use is important, in what way I'm not sure. Using a cheapo cable I have with the OEM 2A charger results in my N7 charging slow, probably 350ma.

And here are the chargers I've tried with results:

- OEM charger with OEM cable: Charges from 0-100% in about 3 hours

- OEM charger with cheap cable: Charges from 0-100% in 18 hours or worse if the screen is off, drains FASTER than it charges with screen on

- Blackberry 700ma charger (integrated cable): Charges from 0-100% in about 8 hours, which is in line with what you would expect with it's output

- Cellet 1000ma charger with any cable: will not charge at all

- Cellet 1000ma charger with data pins shorted: will not charge at all

- Motorola dual charger (Razr Maxx): Same slow charge as above, about 18 hours to charge fully

In addition, I've used a handful of different brands and types of generic cables. Something is different about the OEM charger but no one (AFAIK) has been able to identify it yet. There is clearly more at play here than the rated charge rate, so if that is your only comment, please keep it out of this thread.

Really, try a charging-only cable. I've used mine with 3 separate wall chargers and 4 separate car chargers and every one shows up as charging from AC. I haven't actually measured the charge rates, but if I try it with a normal cable it doesn't even show up as charging at all. Just make sure you don't short the red and black wires. This is what you need to do... please forgive my crude ascii art...

The charger issue to me seems to be the cables, the micro usb for the N7 is longer than a normal micro usb.

Sent from my Nexus S 4G using Tapatalk 2

This may be a software bug ..

Gentlemen,
I have been testing different chargers with the two different USB cables, and what I am finding is the tablet is in fact charging even when the battery icon does not show that I am plugged in.

Using the Nexus 7 OEM cable here are my results:
1. OEM Nexus 7 charger @2A = shows proper icon and charges tablet within a few hours.
2. Belkin charger @ 1.8A = no icon but charges tablet in just over three hours.
3. HTC charger @ 1A = no icon but does indeed charge (was too impatient to let charge complete, but gained 25% charge in two hours).
4. Samsung charger @ 0.8A = shows proper icon and charges (one hour gave 11% charge).
5. No name car charger @ 2A = shows proper icon and charges.

Using Samsung OEM usb cable (came with my SGS2 i777):
1. OEM Nexus 7 charger @2A = no icon but does show charge by battery percentage (went from 75% to 100% in about 45min).
2. Belkin charger @1.8A = shows icon and charges
3. HTC charger @1A = no icon but charges by battery percentage


In all cases, I show the battery as charging when reading battery percentage, although the battery icon in the notifications bar is not consistent. Yes there are likely background checks being made by the hardware for resistance, voltage, and current. The issue is that the icon should always indicate when there is a charge source applied; even if the source can only trickle-charge.

-thank you for letting me share some of my findings
-happy gadgetry to all


***Update (8/1/2012)*** when I turn the device off, all cords and charge sources register as charging.

Here's what I've learned from a hardware perspective...

We're working on a hardware + firmware + software product that uses an Android tablet, which right now is specifically the Nexus 7. We have found that the standard charge rate of the battery is insufficient to support tablet operation with a few sensors enabled and high screen brightness. Therefore, even if plugged in to a power source, the Nexus 7 discharges during normal use. The charging system cannot keep up with normal power requirements. That is a TERRIBLE hardware design decision.

Rather than guess at what is happening, or jump to conclusions based on various apps that claim to report current, we connected an actual ammeter (current meter) in line with the USB cable plugged in to a 2012-era Nexus 7 and ran a lot of experiments to characterize its behavior.

A few basic things to keep in mind:

* The current rating on a power supply/charger is the max current that device can provide. The device being charged controls how much current is actually drawn from the supply. A higher-current charger cannot, simply by virtue of its higher capacity, force more current into the device.

* USB hardware specs very clearly define the max current that a Portable Device (PD, in this case an N7) can draw from various power sources. Earlier posts in this thread properly referred to the names of the various types of ports including the one that we need, a Charging Downstream Port (CDP), which supports both data and higher charging current. A CDP uses certain voltage levels on its D+ and D- pins to signal the availability of greater current; a "dumb charger" can just short the two data pins together to signal the same thing, but obviously this won't work if you also want to pass data and not just charge the battery. (The "shorted data pins" trick is a documented way to let cheap chargers inform the PD of higher current capacity without having to add intelligence to the charger.)

* USB software specs also define how the PD can negotiate with the upstream port (in our case, a CDP), essentially letting the N7 specify how much current it wants to draw and letting the upstream port respond with approval or disapproval. In this case, the hardware does its thing, and then the software on both ends talks back and forth to agree on a (potentially higher) current rate.

I'll cut to the chase: The N7 never draws more than 440mA. Ever. With any charger, with any cable, with any combination we've tried. This includes the Asus-labeled 5V 2100mA OEM charger and the Asus-labeled OEM cable that came with the tablet. We really, really want it to, but we have not been able to figure out how to convince it to use more current (and thus stop draining the battery while plugged in). Yes, I've read the other comments in this thread that report higher currents and I don't know how to explain what they're seeing. But in a laboratory environment, with real test equipment run by Engineers, 440mA is the number.

On the hardware side, our product has a dedicated 5A 2000mA+ power supply for the N7 connection. This is really clean power - it's a little switching power supply with great filtering that powers nothing but the USB connector. An oscilloscope shows an absolute flat line, no ripple, no noise, nothing, even under load. We have tested its current output capacity and it goes well beyond 2000mA with no degradation of the voltage level nor quality. This power is as clean as its gets. The power is there if the N7 wants it.

On the software side, the N7 does in fact do the "USB software negotiation" for current and we tell the N7 that 2000mA is available. Nevertheless, the N7 always requests 500mA (we've captured and analyzed the USB data), and in reality never draws more than the 440mA mentioned above.

We cannot short D+ and D- since we need to communicate with the N7. But there are other tricks supported by the USB spec, including specific voltage levels on the data lines to indicate that the port to which the N7 is connected is a CDP. Briefly, a PD which takes advantage of a CDP first performs "primary detection" by looking at D- for 0.4-0.8VDC. If a voltage in that range is found, "secondary detection" causes the PD to apply voltage to the D+ pin looking for a pulldown resistance of (nominally) 19.5K.

So we added circuitry to provide this environment, thus indicating we are a CDP. Result: No change at all. The N7 draws 440mA with or without the CDP circuitry.

We kept at it for a long time, because we thought it strange that Asus would ship a 2100mA charger when a 500mA charger would suffice. We figured that was proof the N7 could charge at a higher rate. But after hours and hours of analysis, testing, experiments, etc. we were forced to conclude that the N7 simply never draws more than 440mA. Presumably its onboard charging circuitry cannot handle higher current rates. This is a real shame, because it means the Nexus 7 cannot be used in kiosk mode - it cannot run indefinitely when plugged into external power. It will always require some "down time" to recharge its battery. That's fatal to a lot of applications for this tablet, including ours.

We have not yet tested a 2013 N7, but we're hopeful it has a better charging circuit.

Hope this helps someone!