Every electronics device needs a power supply. In many cases we just go with AA batteries, 9V + regulator, wall mounted adapter and LiPo cells. First ones are obvious enough – easy to obtain and pretty safe. Lithium technology batteries falls in to different category. These cells carry much more capacity than casual batteries, but also are more dangerous and sensitive to too much discharge and overcharge. This is why in almost all cases LiPo batteries come with small PCB attached to them. This circuit at least takes care of disconnecting battery in case of discharge bellow threshold (2.7V). In battery packs this situation is more complex. Proper circuit should monitor individual cells and disconnect whole pack if at least one cell reaches the threshold. But in reality cheap battery packs lack individual protection. So in case of failure of single cell, whole pack is condemned.
Karman suggests not to throw such battery packs away, but instead take them apart and check if there are any of individual cells alive. They can be reused in other projects. For instance he took Macbook Pro battery pack and found two of three cells working. Then he took individual cell protection circuit from dead picture frame and paired with one cell. This way he got decent power supply for same picture frame or any microcontroller project. Lithium cells are still pretty costly, so look around maybe there are few sources to scavenge them and reuse.
We recommend EasyEDA for circuit design and PCB prototype