why do AA & AAA rechargeable batteries run at 1.2V instead of 1.5V?
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It is a completely different chemical process which happens in them. And this produces very different voltages. And while alkaline batteries provide much lower voltage when empty, down to about .9V when completely out the reachargable Nickel-metal hydride batteries that can replace them maintain a voltage of about 1.2V throughout its discharge cycle.
They use different chemicals.
Batteries work by allowing electrons to move from one chemical to another, but only when the ends of the battery are connected. This is called an electrochemical reaction.
The electro potential depends on the chemicals involved.
For the 1.5V the battery uses Zinc and Maganese-Dioxide. The electric potential (voltage) between these two chemicals is 1.5V, but these aren’t suitable for rechargeable batteries due to the reverse process not getting the full charge back, and potential for them to burn/explode if you try.
To make higher voltage batteries they link up cells. A 9V battery is basically six 1.5V batteries in a wrapper.
For rechargeable lithium ion batteries the electric potential of the chemicals is 1.2V, but for this trade off of voltage you get reliable recharging.
haven’t they figured out how to get them up to 1.5 yet? We have Li-Ion and NI-MH but they still work the same as the old Ni-Cads?
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