Single Transistor Lead Acid Battery Monitor
We can make a very simple battery monitor using a dual-colour LED and a few additional components. When the red and green LEDs are illuminated, the LED produces orange. The following battery monitor circuit turns on the red LED below 10.5v. The green LED illuminates above 11.6v. The orange LED illuminates between 10.5v and 11.6v.
This simple battery monitor circuit is suitable for your 12V lead acid car battery: simple indication of the battery health. [Source: talkingelectronics.com].