Re: extremely slow charging, any fixes?
FYI, I'm an electrical engineer and I design, build and test LiIon charging circuits (among other things) so I know a bit about this topic.
I see a number of half-truths and myths about electricity and charging in this thread. The Nexus-7 specific information was true, that it should take 3.6 hrs, not 12 hrs, and any significant applications running can extend the charging time. Certain chargers and cables may extend charging time, more on this below.
The Myths:
1. No amount of extension cords or power strips or surge protectors on the A/C side of things can possibly affect charging. USA power is maintained between 110 and 125VAC. You might get a few fractions of a volt or a few volts drop over extension cords IF you have something like a space heater running at the far end of the cords where you plug in your charger. But the charger is made to operate down to 100VAC for Japanese use (and even below that voltage; they're always overrated) so there will never be a plug in the USA that won't operate it, as long as there is power going to the plug at all, and we're not under severe brownout conditions.
2. There aren't USB "Charging Cables" and "Data Cables" as such. ALL USB cables can both charge and carry data. (Some permanently attached cords on chargers have a USB plug on the end but don't have data wires in the cables, but these aren't USB cables.) That being said, different USB cables can have different gauges of wire on the power conductors. I did a little research on this. If the wire gauge is 24ga, you can have about few tenths of a volt drop along a 3' cable at 1A charging current. A few tenths of a volt is tolerable by the charging circuit. If the wire gauge is 26, this changes to a full volt or more of drop along the cable and that can interfere with charging, causing it to charge at a lower current, so the voltage doesn't drop so much and remains at a level that the charging chip inside the Nexus 7 requires.
Note that a thick USB cable doesn't necessarily mean that it has thicker power conductors inside, but a thin one usually means that the wires are thinner. I would go with thicker overall cables and try several. Shorter cables, it's true, have lower voltage drop and would limit the charging less, but this doesn't mean that a shorter cable always charges faster. But if your longer cable is limiting charging current, a shorter cable will reduce this limitation.
Now, all that being said, the Nexus 7 has a 4325mAh battery, so for a 3.6 hr rated charging time it should draw an average current of 1.2A. LiIon batteries draw much higher current up front (at the beginning of their charging) and then trail off toward the end. So I would expect initial charging currents of 1.5-2A on a completely discharged battery at the beginning of the charge cycle, trailing off to under 500mA at the end of charge. LiIon charging of a sizeable cell usually shuts off at around 50-100mA charge current at the constant terminal voltage of 4.1-4.2V.
What this means is that for full speed charging you should use a 2A charger like it came with. A 1.5A charger would probably work, but it may heat up a lot and may limit charging at the very beginning a little bit (on a fully discharged cell) so it may take, say, 4 hrs instead of 3.6 hrs.
NOW... All that being said...
My research and experience leads me to believe that there are a certain subset of Nexus 7's that have a defect or assembly error (wrong current-set resistor or something) that causes them to limit their charging current internally such that it takes around 12hrs to charge.
My Nexus 7 has never charged quickly. I recently got a USB Ammeter that displays the charging current. I have my original Nexus 7 (ASUS) transformer as well as a number of other high-current USB transformers. What I have observed is that the Nexus 7 will never draw more than 430mA, and always exactly 430mA (with a fully or partially discharged battery) no matter if the unit is so dead that it won't power on, or when it initially powers on, or when the OS is fully booted, with lots of applications running or with none, with the display on or off. Always 430mA. Doesn't matter which charger I use (all are 1.5A-2A capable), doesn't matter which cable I use (I have 7 different cables of different lengths and thicknesses, even some very short 1-foot cables), and doesn't matter what wall plug I use (for you electrically superstitious types).
Notably, a 4325mAh cell would take about 10 hours to charge at 430mA, and since the charging current trails off toward the end, 12hrs is right on the nose, in keeping with the current that I'm measuring on my Nexus 7.
For those of you who don't trust my charging setups, my Nexus 10 charges at somewhat varying rates with different charges and cables (with a partially discharged battery) but all over 1A up to 1.5A on the same chargers and cables that my Nexus 7 limits to 430mA.
I know this thread isn't really active any more, but I'm posting this for posterity in case others find the thread as I did. If any of you have subscribed to it and have questions or comments about what I've said, please post and I'll subscribe and answer.
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