Why are rechargeable batteries only 1.2v?

Answer 1

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.

Answer 2

It's a consequence of the specific chemical process that's being used in the common types of rechargeables. Each type of process has it's own output voltage. LiIon has 3.6V per cell, Alkaline 1.5 V etc. If recently seen rechargeables using a new technology, those have 1.6 V per cell, see the related link for some more info.

Answer 3

Single use batteries tend to give 1.5V and rechargeable batteries 1.2V. In most cases you won't notice the difference but in some devices it can cause problems. Single use or primary batteries, such as alkaline and zinc are the most common type of battery but increasingly seen as wasteful and expensive in the long run. The exact composition of the internal chemistry will vary depending on the use the battery is designed for and the battery brand. What doesn't vary is that the chemical reaction inside the batteries cells produces 1.5V. In a rechargeable or secondary batteries, such as NiMH batteries the chemical reaction is reversible. During discharge this chemical reaction happens in two parts: Nickel oxyhydoxide is reduced to nickel hydroxide producing 0.52V. Metal hydride is oxidized to the metal allow producing 0.83V. This gives a theoretical voltage of 1.35V although the nominal voltage of 1.2V is advertised.