SystemInfo.batteryLevel

Description

The current battery level (Read Only).

The current level of the battery, represented as a float between 0 and 1.

If the battery level is not available on your target platform, this property returns -1.

using UnityEngine;

public class Example : MonoBehaviour
{
    void Start()
    {
        // Check if the battery level is available
        if(SystemInfo.batteryLevel == -1)
        {
            Debug.Log("Battery level is not available!");
        }
        else
        {
            // Log the current battery level
            Debug.Log("Battery Level: " + SystemInfo.batteryLevel);

            // Log a message when the battery level is low
            if (SystemInfo.batteryLevel > 0 && SystemInfo.batteryLevel < 0.3)
            {
                Debug.Log("Battery is low!");
            }
        }
    }
}

Additional resources: batteryStatus.