It is important to be aware that certain applications will have battery packs that may be exposed to corrosive elements such as acidic, salt, and on occasion conductive substances and fluids. Over time this type of exposure can slowly corrode the PCBA, components, and solder joints. This type of corrosion can cause premature failure, short circuits, and or dangerous conditions that could lead to fire or even an explosion.
When building a lithium-ion battery pack, there will always be some sort of protection circuitry necessary that will safely separate the cells from the external connections. The protection may be as simple as a pair of charge and discharge Field Effect Transistors (FETs) with voltage and current detectors, or as complicated as adding firmware controlled fuel gauging and secondary protection.