I have a DIFFERENT approach folks can try:
My phone discharged completely and went dead, would not charge the battery, just sat there rebooting and showing a gray battery. I theorized the battery was too weak to accept a charge, at least that's what the phone was probably thinking. Even car batteries can get in such a state that a "smart" car battery charger won't charge it. Some of you might be familiar with this type of situation, where a battery is drained "too low to charge".
What happens is the "smart" charger sends a brief pulse of electricity to the battery and then samples the voltage of the battery to see if it is in a good enough condition to accept a charge. If your battery is too low on power, you're up the creek right? WRONG! You can often get a weak battery going again by "tricking" it into charging up just a little bit; enough to get the automatic charger to accept it again. Here's what I did on my Samsung Galaxy S4 Active that was only a month old:
I inserted the charger for a few seconds, removed it for a few seconds, and did this over and over again, perhaps 10 or 15 times. Eventually the green bar showed up and I gently placed the phone down and walked away from it for awhile. Then I held the power button and it booted up fully and was once again running and charging!
The moral of my story is sometimes the battery isn't bad, the phone isn't bad; it's just that the battery got too "dead" and these so-called "smart" phones can be awfully stupid sometimes!
Have you ever gotten your (modern) car stuck in an inch of snow, at the end of your driveway? Because that damn "traction control" refused to allow even the slightest bit of tire slippage? If so, you probably know all about that traction control button in your car and how you can turn it off so the darn car will actually MOVE for you !!!! Got to love technology!!!
-AtariJeff001