Lithium Polymer Batteries pack a lot of power in a small package. But they can be tricky to charge safely. The Adafruit LiPo Chargers all provide a charging cycle designed to safely charge 3.7v Lithium Polymer cells. But what if your project needs more than 3.7v?
Simply pumping more voltage into a multi-cell pack is risky. If the cells become unbalanced, some will end up with more of a charge than others. When put into service, the cells with the higher charge will end up working harder than the other cells. This will have an adverse effect on the capacity of the pack. Over time, it can result in damage and/or failure of one or more cells. And LiPo failures have been known to be quite spectacular.