How is a lead acid battery and a Ni-Cd battery charged on an aircraft, such as an A320?
Is it constant current or voltage?
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.
Constant-voltage, but there's more going on.
Batteries prefer a mildly complex charging curve, because both lead-acid and NiCd charge efficiently in their mid-cycle (20-80%) -- but inefficiently at the top of their range (80-100%). In a perfect world, a charger pushes them heavily (constant-current) in their midrange, but tapers off for the top-up charge. The state-of-the-art in lead-acid is 3-stage charging.
If power is pushed into a battery when it should not, the battery will produce a lot of heat, "boil off" water excessively, partly splitting water into hydrogen and oxygen, carry battery acid (or alkaline in the case of NiCd) into the battery box, and shorten the life of the battery pack, especially with lead-acid which already has a short life.
It would be neat if battery voltage and impedance behaved such that constant-voltage charging caused the correct charging curve passively. No such luck.
Constant-voltage charging is a compromise - the same one cars make - since the charging system is both charging the battery and powering the airplane. The voltage is chosen for least damage/heat/boil-off/hydrogen, at the trade-off of slower charging than a dedicated smart charger.
If the battery is lead-acid and you have depleted it, recharge it ASAP, as it shortens the life of lead-acid batteries to be stored partially discharged. That won't faze a nickel-cadmium set, they are highly resilient and long-lived.