This indicator indicates the battery voltage in three stages: low battery, good battery and overcharging.
It is suitable for any vehicle (car, motorbike, ...) with 12V network and after a slight modification, it can be used for 24V system.
In low battery condition only yellow LED is lit. If the battery is sufficiently charged, the green LED is also lit.
If overcharging the battery, third LED, the red one, is lit.
Description: Schematic of the car voltage indicator is in Figure 1. The hearth of indicator is a dual operational amplifier (OpAmp) with open collector outputs LM393. The OpAmp measures voltage via potentiometers P1 and P2 and lights up the amount of LEDs corresponding to the actual voltage. The LED are connected in series and therefore do not take more current when more than one LED lit at same time. If you want to use a classic OpAmp, for example LM358, LM1458 or MC4558, add series resistance to LED2 and LED3, see Figure 2. In this case, only one LED is always lit. The thresholds are set using potentiometers P1 and P2. P1 sets the voltage at which the 2nd LED lights up. It should be about 9.5 - 10V, because at lower voltage lead-acid battery is considered completely discharged. P2 sets the voltage at which the third LED is lit. This should be about 14 - 15V, because the high voltage means overcharging. The reference voltage is provided by zener diode ZD. Setting the indicator should be done with a controllable power supply.