Here’s a cool car battery charger diagram.
This very simple circuit uses a transformer ,two diodes , a capacitor and an ammeter. To charge a battery just connect the + and – terminals of the circuit to the corresponding terminals of the battery. When the battery is not charged, the ammeter reading shows 1-3 amps. When the battery is fully charged the ammeter reads Zero or nearly zero, after which the battery should be removed from the charger.