Here is a simple but useful battery level indicator to monitor the battery terminal voltage. Efficiency of the battery reflects in its terminal voltage. A good Lead Acid battery will show 13.8 volts in the fully charged state and if it is below 10 volts, the battery needs immediate charging. If the battery voltage goes above 14 volts, it is in the overcharged condition. This circuit uses a two lead bicolour LED to indicate three battery levels. If the voltage is below 10 volts, the Green part of the LED lights and if the voltage is above 14 volts, the Red part lights. If the battery voltage is between 10 to 14 volts LED remains off.
Fig. 1: Image of Bi- Colour LED