Re: iPad won't charge completely any more.
Hi and welcome to iMore. Which generation iPad is it?
I had an iPad 2 that did the same. It stopped at 78% and wouldn't go any higher. Now it's dead completely. You may be able to get the battery swapped out at an Apple store, if it's a newer iPad under warranty. If it's an older one, you'll have to get it replaced with a third party.
Before any of that is considered, try a reset by holding down the power and home buttons and only releasing them when the Apple log appears, not before.
Then plug it back into the power and see if it charges higher. Sometimes the battery percentage might be misreported and a power cycle can rectify that.