It is common with lead-acid battery charging to use a constant-voltage source and series resistor for current limiting. As the battery voltage rises to its fully-charged value so the current reduces to a trickle. The battery voltage measured at its terminals depends on the current, because of the finite internal resistance of the battery. A '12V' car battery is typically regarded as fully charged when its terminal voltage reaches 13.8V off-charge, but it's on-charge voltage may be 14V or more. An on-charge voltage above 14.3V is excessive/dangerous and likely to damage the battery.