Let it charge overnight. Ideally, you want to use the larger iPad brick, not the small square one they ship with phones as they don't deliver enough current to charge quickly. Computer ports are even worse.
If you have access to a [https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=USB+Ammeter|USB Ammeter], you could use that to determine if the phone is really drawing current when it says it's charging.
If it charges properly overnight, I would turn on the display at full brightness with no auto shut-off and let the battery almost completely deplete (stop at ~10%)...then charge it up again. If it handles this, then seal up the device!