what determine the voltage of the battery?

what determine the voltage of the battery?
why the voltage of the battery is fixed?
e.g. the AA battery is 1.5V. dose the length effect the voltage?
why the battery can not be current source?

in one word, the battery I can not understand.
Any nice guy give in deep explanation?

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The potential at the terminals of a battery can be calculated from the Nernst equation ( Nernst Equation and Faraday's Law ). Wikipedia also has a section on it. The constants for the Nernst equation can be found in any reference text on electrochemistry. The CRC Handbook of Chemistry is one such reference and is available in most libraries.

Here is a simplified discussion of the lead-acid battery: Electron activity in chemical reactions : BATTERIES AND POWER SYSTEMS

John

Voltage for a electrolytic cell:
EMF definition: 1 Volt = 1 Joule/Coulomb.
Joule= Required amount of energy to force the electrons to move between two electrodes (oxidation at one electrode and reduction at the other). This force is depending on concentration of available ions.
Concentration of available Ions is depending on number of moveable electrons/mole for the chemical structure.
When there is a difference in concentration of ions between two electrodes can we measure this as a voltage.
Actual cell voltage is depending on what chemical components that are involved. When you know these and their number of moveable electrons/mole can you calculate unloaded cell voltage.
Some examples

Scaling cell size is a energy parameter and not a voltage related parameter.

Battery is a current source. If there is no stream of electrons (current) available when battery is loaded by a resistive load=>Useless battery.

This is a very simplified description as voltage for an electrolytic chemical cell also is very depending temperature and other physical circumstances.

For a deeper explanation, try in a chemical forum.