[Nexus 7 Q&A, Help & Troubleshooting] lhakedal:
I've been using this forum as resource in debugging a battery problem for some time now. Great resource.

I have a nexus 7, 32GB, 3G.

The "Here are the similar threads we found" section I got after writing the title is neat, but they all usually lead to an answer ala "An app is eating up your battery." "Google Now is bad." "Turn off wifi when sleeping." "Leave wifi on when sleeping." Not to mention baseband_xmm_power.

My tablet eats battery at a rate of 7-8%/hour, as measured by BBS.

My case is a bit odd. At first I thought I had the baseband_xmm_power wakelock problem. So I've installed Franco's kernel. I'm currently at r58. That did in fact stop all those, but it didn't really help me with the battery.

So I've done a number of factory resets (did many of those prior to rooting and francokerneling and a few after.) I've been running the tablet with nothing installed except the apps that comes with. In safe mode. Not syncing. In airplane mode. Still eating battery.

So I bought BBS. If you have any problem with battery/power/runaway apps: Buy That App. Now. Learn what it does! I also tried Wakelock Detector. If you're serious, get BBS.

So why am I still unhappy?

BBS claims that my tablet has been sleeping for 10 hours, (Yes, really. Deep sleep.) having never been turned on once. It usually have few minutes of various wakeups, but the tablet is mostly deep sleeping. Checking the battery page in settings says "yup, sleeping. Screen off". Battery drained 80%. Sometimes it is blamed on "screen" which was on the few seconds it took me to sleep the app and the few seconds it took me get into BBS. "Tablet idle" is another favorite: "Battery used by tablet when idle" I think it says. Why not "eaten by gremlins"? :)

A few weeks ago I noticed that the tablet was hot in the upper left corner. (holding the n7 in portrait mode, with the camera on top). I repeated the 10h deep sleep maneuver only checking the heat of the tablet. Hot all through the day. Deep sleeping all day. Repeated in safe mode. Same result. After a full reset. Same result. Something is using my battery to heat up the tablet.

So whats in that corner? ifixit.com has a great pictorial of what's inside an n7. It's not quite the same as mine... Oh. Did I not mention that I voided my warranty opening my n7? :) (Being a noob, I'm not allowed to post it here. Sorry.)

The motherboard of the N7 is shaped like an L. In the corner of the L is the Azurewave AW-NH665 chip which has Everything Radio on the N7. Wifi, bluetooth, it's even got and FM receiver and transmitter and it seems as if the chip supports 802.11a, even though the n7 does not.

Inside the AW is a bcm4330 which is The Chip to use if you need low power radio. Googling "bcm4330" it seems as if every portable device out there uses it. There is also an NFC chip. (I've done a bit of testing with/without NFC enabled. Most sources claims that NFC is really, really low power. I probably concur.)

I have access to an IR thermometer. Point and shoot and it'll tell you the temperature of the object you point it at. Ice cubes: 0C (32F)

So I reopened the n7, powered it up and put it to sleep. Withing seconds the bcm4330 went from room temperature to 38-40C (about 100F?). I measured a bit back and forth to see if the NFC chip was in fact the source. I doubt it.

I've read a bit about the bcm4330. It supports rfkill, which sounded just like something I could use to find the cause. I don't have the syntax at hand but you can say something like "rfkill block wifi" which, I think, will shut down wifi. It seemed to confuse the wifi settings button. Wifi was off, the tablet was offline but the ui hadn't noticed. Neither had the bcm4330. It was still chugging down mAmps...

There are two lines in the output from dmesg right after boot that puzzles me a bit:

<4>[ 1.999795] bcm4330_rfkill_probe: can't find bcm4330_32k_clk. assuming 32k clock to chip

<4>[ 1.999927] bcm4330_rfkill_probe : can't find reset gpio.

I don't know if this is normal. I only tested it twice and it was there both times.

If I power the tablet down it does not eat battery.

I tried to start the boot process while holding the volume down button to enter the boot menu. Leaving it like that the screen never turns off and I didn't leave it like that for long so I really don't at what rate the battery is used, but the hotspot did in fact heat up so I'm guessing that it still used battery.

I don't really know where to go from here. I used to think that the bcm4330 was confused about something and was in an active loop of sorts. But wouldn't a factory reset handle that? Or many resets? I've even had the battery disconnected for half a day and that should have reset it?

I want to thank the entire community for all the hits I've gotten and the leads I've been trying to follow, but now I'm stuck. Is there anything I haven't tested or told you about?

PS: I have tried several other, presumably, dead ends. Some of them really silly. "What if it's something in the air? Like my wifi at home telling it something?" Hardly, but I still wrapped the tablet in several layers of tinfoil to see if it made a difference. It didn't.