Ipad Air not turning on but has battery symbol and recognizes charger.

So, I got an iPad Air that was not turning on. I got it to come on for a minute somehow and it had even charged to 100% battery. I used it for a few minutes (playing a game) and it just randomly died as if the battery had drained. Somehow I got it to come on one more time and the battery was down a few percent. The customer asked me to change out the battery, so I decided that would be my next move. He claimed it wouldn't come on unless he kept it on the charger. I replaced the battery only to have the same thing happen, and I can't get it to turn on. So all it does is when I press the power button it has the battery symbol and signal to plug it in. I plug it in and the symbol does go away and it acts as if it were charging but I have left it on the charger for hours and it still doesn't come on. Only the red battery symbol displays. I do believe it is charging the battery, but may be a board level issue. Where would I start? What parts of the board would be causing this? I have SOME micro soldering training, and some experience, and now a microscope as well. I am still learning though. Any help is greatly appreciated, thank you! :)

Turns out, it was a bad battery all along, embarrassing as that is. Even tho I changed the battery out, that was not the end of it. I decided to order ANOTHER battery through a reputable source (https://www.ifixit.com/Store, thank you so much for your quality batteries! <3). Plugged it all in and it all worked great :) Another great way to check this is to get ibackupbot and check the information on the battery there. Thank you so much for your very helpful suggestions!