Wawa:
Dangerous diagram.When two or more relays are active, then the batteries are shorted.
This could happen when code goes haywire, or during bootup.
If you use this diagram, then add protection resistors between cells and relays.I would ues a TPIC6x59x (e.g. TPIC6B595) instead of a darlington array.
Leo..
I build something based on this circuit, with the exception of using A5W-k relays instead of NTE-R40 relays, and 1N4148 diodes across the relay terminals instead of the resistor + LED combinations. I learned the hard way that two, or perhaps more, relays could be engaged at the same time.
You mentioned "protection resistors between cells and relays" - what size resistor would you use?