Why Build a Battery?
I have recently been building a solar-powered water pump for use in the field (so completely portable and no resources other than water). The main reason was to assist a friend in getting water out of a stream beside his allotment garden to water the plants, but I also fancied it as an interesting hobby project!
As part of that project I have learnt about switching power supplies and solar panels, pcb design and testing, and more. This is the prototype PCB: