Rechargeable batteries have less potential than normal batteries because they have different chemistry. Nickel and Cadmium or Nickel and Hydrogen batteries can have a potential of 1.2V. Alkaline and Zinc Carbon can store both up to 1.5 (maybe 1.6v if fresh). The fact that batteries with different chemistry have the same potential is just a coincident. Lithium batteries can have a potential of 3V, while Lithium-Ion rechargeable batteries may store up to 3.6V, that's 0.6 volts higher than the normal Lithium batteries.