Okay, I've had this same issue, as many of have had, and the only way I could boot would be the disconnect AC remove battery hold button,,,, all the other things I've found, CMOS Battery, ETC...., Then I decided to take the perfectly conditioned keyboard apart, and broke the on/off button, the contact still function as before, but I need a new KB a this point, or keep using a toothpick. (Also the W520 worked with a dock, just couldn't use on/off on the W520) anyway back to the fix..... Today I got my new KB, Snapped it in place, and everything works again. So the capacitor drain theory, looks like it has to do with the Keyboard,,,,, I don't know how or why, and as I mention, my KB was in perfect condition, until I broke the on/off button. The replacement KB only cost, like $17.00 new plus Free shipping from ebay. You really need to make sure you get a New one, I wouldn't trust a used one. Just a wild guess,,,, but I do think this is a fix for other models, and other issues, and who would have thought? Not Lenovo apparently. I cant wait to hear if this fixes other's with these issues. Maybe the static is built-up in the KB? One might try submerging their KB in alcohol and swishing it around real good and let it dry, and try it. I may do just that, and see what happens. Honestly, this new keyboard did fix my W520.
I just tried the submerging my old KB in alcohol and swishing it around real good and letting it dry. No Go. But if you look on the back side of these, you'll see debris , dirt and crap, that's stuck to they clear plastic film that adhered and the debris is under the pads. You may be able to remove this film, then clean the KB. I put the new KB back in, and she fired up. I do know from working on touchscreen, that any debris, dust, etc, will cause a capacitance issue.
After peeling the plastic film off and cleaning, it was another no go, so I got to looking with a magnifying app, and noticed wear marks "through" the ribbon cable, from the metal chassis that it lays on, and takes its toll So when putting in a new KB, you would want to use some tape on that part of the chassis, as it does have a couple pretty bad edge's there.