Car Battery Charge Indicator Replaced with LED then Dimly Lit When Off
You should not replace the battery warning light with an LED.

The battery warning light is connected between the ignition supply and a terminal on the alternator, and it serves two functions. First, it provides a current to the alternator to drive the field coils and allow it to generate enough output to power the car's electrical system and charge the battery. Second, it warns you when the ignition circuits are turned on but the battery is not being charged (most commonly when the ignition is on but the engine is not running, but also in cases such as the drive belt breaking).

Replacing the light with an LED will not supply enough current to the alternator to allow it to function correctly. You must use the correct filament lamp as originally specified for the make and model.