[dcb]

You could use several power fets to do that (similiar to synchronous rectifying). But if you go down the 555 path, you start adding somewhat dated counter logic chips where a pic/avr (I'm a broken record, I know) would be both low power and simpler to design a circuit around. I would probably run the avr off a couple (3) nimhs just to keep it simple and avoid draining anything else, and turn the avr clock down to 1mhz (or less) on the internal clock for nice long run time. I'll draw a picture.