battery Over charge Indicator

If an LED indicator is present in battery powered Elements such as Emergency lamps, it will consume power even if the element is not using. This will reduce the battery voltage since the LED takes around 2 volts. So it is compulsory to charge the battery always to keep the battery voltage level. This circuit eliminates this and the LEDs turn on only in two states. That is in the over charged and over discharged conditions only.

The circuit is basically a voltage controlled switch using Zener diodes. Two state LED indication is provided using a Bicolour LED. Zener diode ZD1 and the PNP transistor T1 forms the over discharge indicator switch. When the battery voltage is above the breakdown point of ZD1 (around 5 volts), it conducts and keeps T1 out of transmission. So the Red half of the bicolur LED remains off. When the battery voltage reduces below 5 volts, Zener turns off allowing T1 to conduct and Red LED turns on. This indicates that the battery is going to the over discharged state.

Zener diode ZD2 and NPN transistor T2 forms the Over charge indicator switch. When the battery voltage is below 6.8 volts (most voltage level), ZD cease to conduct and T2 remains off. So that the Green half of the LED also remains off. When the battery voltage increases above 7 volts due to overcharging, ZD2 conducts followed by T2 and Green LED turns on. This is the over charged state. In short, if the battery voltage is between 5 and 7 volts, both LEDs stay off. This reduces the chance of power consumption.