Harvesting energy from old batteries?
Let me make a life a little easier with a simplification of your hard to read circuit: -



The simplification is getting rid of 50% of the changeover switches.

What can you use for these switches? I would consider using N-channel MOSFETs; two per switch element. You will need a supervisory circuit that of course needs to be very lower power but I don't see this as a difficulty a hindrance to efficiency.