There is no battery memory issues in the Lithium Polymer batteries like there was in the Nickle based batteries. However, there has been a lot of research indicating that Lithium batteries last longer if you don't let them discharge more than half way before recharging. If you consistently wait until 15% charge is left, you can damage the battery. So best practice is to charge it before it reaches less than the half charged level. It doesn't matter how often you charge it and you certainly don't need to wait until it reaches 50%.