How is a lead acid battery and a Ni-Cd battery charged on an aircraft?
Airliners like the Airbus A320 use Nickel Cadmium batteries; in case of A320, a pair of main batteries with 23 Ah capacity is used. 

While on the aircraft, the batteries are charged using a constant voltage method. As this article notes:


  ... are "constant-voltage chargers" that apply a fixed voltage to the battery as it charges, similar to what the aircraft electrical system does in flight. With a constant-voltage charger, the charging current starts out relatively high and gradually tapers down toward zero as the battery becomes fully charged.


FAA Aircraft Electrical System Handbook also notes:


  The battery charging system in an airplane is of the constant 
  voltage type.