How much voltage does a lead acid battery produce in an airplane?

A lead acid cell has an off charge/no load voltage of 2.2 volts, so that 12 cells in series will have a voltage of 26.4 volts. When you float this across a charging supply, standard practice is to increase the voltage to 2.3 volts per cell, so that the total voltage becomes 27.6 volts.

Airplanes normally have a 27.6 volt DC system, and larger aircraft also have a 115 volt 400 Hz system. Also, rare these days, possibly a 115 volt wild frequency system.

Note that the so called 12 volt system in a car is actually 13.8 volts.