South Bank

Alright everyone - just a small problem I have that I was hoping you could help me with. Cell One is powering a component Cell Two is powering a component Cell Three is charging Cell Four is also charging I thought I would ask any of you if you know of an automatic way to reverse this setup after a set time (The time it takes for Cells 1 and 2 to have nearly run out of power). Obviously once the setup is reversed Cells 3 and 4 are powering the components whilst Cells 1 and Two are charging. I have figured out a mechanical way to do this but I feel an electronic method would be more apt and reliable so if any of you know of a way, your help will be greatly appreciated.