[Nexus 7 Q&A, Help & Troubleshooting] bftb0:
Well, the 120v->5v converter certainly can be providing more current @5v because the device is active, but that's only because the motherboard is drawing current in parallel with the battery charging circuit. It doesn't mean the battery charge rate is higher.

I suppose so.

I would do a few things to determine whether that is a reasonable hypotheses, though.

1) See how fast the battery charges with the tablet turned off. Should be close to 40%/hour for a new battery. You know there is no "software" running with the tablet turned off, so if you don't see some reasonable number here (say > 20%/hr) then a bad battery or charge controller circuit in the tab are the most likely culprits. Also, if the temperature rise of the tablet while doing this seems higher than the gf's unit, that would implicate the battery, not the charging circuit.

2) There's software, and then there's software. (Preinstalled vs. User installed) Run the battery down a ways, and then observe the battery charging rate with the device on but screen off (sleeping), but on a stock 4.2 install with ZERO user apps installed. Then, install/restore all your favorite apps, reboot, maybe use a couple of your fave apps, and repeat the same charge rate trial (screen off/sleeping). Are there large differences between the two cases? If so, that would implicate one of your apps in causing either lots of additional compute operations or preventing entry into the LP0 state (perhaps because of wakelocks?)

The thing is, the N7 battery is rated at 4325 mAh; that is sort of the same thing as 4.325 amps of current for 1 hour. (Voltage range of roughly 4v to 3.5v).

So, if a "good battery" can be charged in 2.5hrs, that is sort of like stuffing 1.73 amps into the battery for that time (1.73 x 2.5 = 4.325 A-h or 4325 mA-h). That's pretty near to the max capacity of the AC charger (2A)

Now, some users have reported discharging their tabs in 4 hours under heavy continuous use; that would be about 1.08 amps for 4 hours.

Since the wall charger is rated to produce 2A, this suggests that very heavy usage simultaneous with charging would indeed cause battery charging to slow down significantly - let's suppose it drops from 1.73a to 0.65a. Now it takes the battery 6.6hrs to charge ... but that is still just over 15%/hr ... with the tab in active use.

But that's not what you were noticing - you were seeing much worse charge rates than this when the tablet was supposed to be more or less idle!

Finally I should point out that I previously mentioned that the % charge number is a prediction, not a measurement! If for some reason this number were screwed up, then the "charge rate" observations could be completely screwed up. (Think of this as being analogous to trying to partially fill a gas tank in a car or estimate fuel mileage with a broken gas gauge) The only way to be sure that you are not falling victim to something like this is to record battery voltages - the 100% level should be up around 4v, and the 10% values down around 3.5v.

You can observe this value at /sys/devices/platform/tegra-i2c.4/i2c-4/4-0055/power_supply/battery/voltage_now

(note value is reported in uV)

Whew - long post. It doesn't directly answer your question about "why was 4.1 so different?" - but gives you an idea about why I was skeptical when you saw charging rates as low as you did.

I dunno, maybe the % charge prediction value numbers are screwy on your tab for some strange reason in 4.2, perhaps because of a minor hardware difference. I can't rule it out - I once saw a bug expression in a hardware/software combination that required three independent conditions (from three separate vendors!) to have precise configurations before the bug would show itself.

I hope this post gives you some ideas to try; it certainly doesn't give a solution.

Good luck - if you feel like spending more time investigating, go for it; just don't let the clock run out on the warranty period if you have one left.