...
public float jumpSpeed = 15.0f;
public float gravity = -9.8f;
public float terminalVelocity = -10.0f;
public float minFall = -1.5f;

private float _vertSpeed;
...
void Start() {
    _vertSpeed = minFall;   
    ...
}

    void Update() {
        ...
        if (_charController.isGrounded) {     
            if (Input.GetButtonDown("Jump")) {      
                _vertSpeed = jumpSpeed;
            } else {
                _vertSpeed = minFall;
            }
        } else {                                 
            _vertSpeed += gravity * 5 * Time.deltaTime;
            if (_vertSpeed < terminalVelocity) {
                _vertSpeed = terminalVelocity;
            }
        }
        movement.y = _vertSpeed;

        movement *= Time.deltaTime;      
        _charController.Move(movement);
    }
}
