Why are rechargeable AA batteries 1.2V instead of 1.5V like alkaline batteries?

The chemistry of the battery. Everything chemical has a certain electric potential. The battery voltage is the difference between these two potentials. The classical chemicals for batteries was carbon and zinc, which produce 1.5 V. Rechargables, which are based on nickel and cadmium or nickel and hydrogen, produce 1.2 V. The lead acid cells in your car battery, on the other hand, produce 2 V, and some lithium chemistries produce 3 V.