why I measured 12.6 V?

[Electronman]

A full charging 12V battery seems to be more than 12V (12.6V in my battery)
what's the reason? is that due to environmental parameters or is a machination from the origin company for some reasons?

 

[Sceadwian]

12 volt batteries are 'nominally' 12 volts. They can go as high as 13.8 volts and much lower if they're heavily discharged. You should research more on basic battery technology, if you plan on working with them it's important to know how they work, charge/discharge methods and general rules of thumb for the different types of batteries out there, and there are many different types.

 

[crutschow]

A "12V" lead acid battery is normally 12.6V when fully charged at room temperature.