My 1-month old tablet, the Asus Transformer T100 tablet started draining slowly while it was plugged in.
Today I drained the battery down to around 30%, plugged it in, and continued with web-browing and streaming music. An hour later I noticed that the battery percentage wasn't climbing, and it had actually discharged another 5%. Windows claimed it was "plugged in, charging" and the LED on the power button was lit up like normal, but the battery percentage was still slowing going down. Once the battery was below 20% I decided to shut it down and let it charge with the system off.
I waited about an hour to power it back up, when I found it only charged 2%. I also felt the AC adapter, and it didn't feel warm at all. I tried another cable, using a different 5V 2A charger that I knew to be good, but nothing changed. I wiggled both cables around to see if there were any bad connections, but the light never turned off, so I assumed both were fine.
I searched Google a bit and found a few forum posts where people reported similar issues. A few said that this was fixed in firmware version 220, but I was already running 220. 223 is available on their site, so I flashed it, but it didn't fix my charging issue.
I was thinking about cutting up a USB cable so I could meter the power used while charging, but I found some software that promised to tell me the battery's charge rate, BatteryBar. Lifehacker checked it out in 2009, along with a few other reptable sites, so It's nothing new. It looks just like the battery meter that comes on some on Lenovo laptops.
I decided to give it a try. The software runs perfectly on Windows 8.1. I found that both cables I tried earlier were only charging at ~2500mW, which was right around or below the discharge rate. I then tried a new 5' long cable that I picked up from Monoprice. With this cable BatteryBar claims the charge rate is 4500-5000mW on both of the AC adapters I tried, and now it's actually charging!
5000mW at 5v is only 1A, but I don't know how the accurate the charge rate is. Also the PC might be drawing more power since it's on AC, thus possibly using the full 2 Amps available. I'll check the total current draw through a meter another day.
Before I throw away the two 'bad' cables, I want to find out what is wrong with them. When I plug the stock Asus cable in to my phone and to my PC, the device is recognized by Windows, I'm able to copy files over, but it doesn't even try to charge. The second cable I tried is apparently a charge-only cable. It will charge my phone, but my PC doesn't recognize a device has been plugged in.
TL;DR: T100 charged slow because of bad USB cables.