how to make a flashlight battery system

Unity Engine
2

Please read the code tags post .

You can do this by using a simple timer which will count up while the flashlight is on. Example:

public class Flashlight : MonoBehaviour
{
    bool m_isOn;
    public GameObject light;
    float m_batteryTimer;
    public float batteryDuration;

    public void Flip()
    {
        if (m_isOn) TurnOff();
        else TurnOn();
    }

    public void TurnOn()
    {
        if (m_batteryTimer < batteryDuration)
        {
            m_isOn = true;
            light.SetActive(true);
        }
    }

    public void TurnOff()
    {
        m_isOn = false;
        light.SetActive(false);
    }

    void Start()
    {
        m_isOn = light.activeInHierarchy;
    }

    void Update()
    {
        if (m_isOn && m_batteryTimer < batteryDuration)
            m_batteryTimer += Time.deltaTime;

        if (Input.GetMouseButtonDown(0))
            Flip();
    }
}

Links you might find useful:
Time.deltaTime
GameObject.activeInHierarchy
Input.GetMouseButtonDown()