This is one simple but highly useful circuit for you. Most of us use different types of Rechargeable batteries in Portable devices, Inverter, Emergency lamp etc. If the battery is deeply discharged, sometimes it will not get fully charged later. This is the most common reason for the failure of Emergency lamp. So we need an indicator to show the safe level of battery voltage. It is too simple to make it and needs only three components. Just hook up it in the battery and it will remind you when it needs charge.
Each type of rechargeable battery has a safe voltage limit. The 12 V Lead Acid battery attains full charge at 13.8 volts and Tubular battery at 14.8 V. The 9 V battery shows full charge as 9.6 V and 6 V battery as 6.8V. So how much it can discharge. Some battery can discharge to 50% but is ideal to keep 80% charge in it. That means, charging requires when 20% discharge occurs. But the condition of NimH and Li-Ion batteries used in Mobile phone, Digital Camera etc is different. These batteries attain full charge even if they are discharged completely.
How the circuit works?
It is a simple Voltage controlled indicator. A Zener diode (ZD) is used as a voltage controlled switch. In the circuit, the Cathode of Zener is connected to the Positive terminal of battery. To its Anode, a Red LED with a 100 Ohms series resistor is connected. The other end of resistor is connected to the Negative terminal of the battery.
Now let us see what happens to this circuit. As you know, the Zener conducts only when it gets a higher voltage than its value. For example, a 5 Volt zener conducts when it gets 6 volts and below 5 volts, it remains off. So here a Red LED is selected as the indicator. It has a forward voltage drop of 1.7 volts. This makes the trick to fix the “charging reminding point”. That means
Zener value + 1.7 V is the reminding point. When the voltage in the battery is above this value, Zener conducts and LED remains lit. When the battery voltage reduces below this, LED turns off to indicate the need of charging. For example, take the example of a 12 volt Lead acid battery. It attains full charge at 13.8 volts. So take a 10 V Zener.
10 V (Zener voltage) + 1.7 (LED forward voltage) = 11.7 V
So till battery retains voltage above 11.7 volts, LED remains on. When it turns off, it indicates that, the voltage reduced below 11.7 volts and needs charging.
You can select the Zener value in another way. Minus 1.7 V from the full charge voltage. It will give the Zener value. But this has one problem. The battery voltage may vary from 12 V to 13.8 V. If we take the Zener value like this, it may change the charge remind point. So fixing the “reminding point” is best as per the method explained. Note that, 1.7 V is for Red LED only. If you use Green LED, it will be 2 volts.
This simple technique is used in Inverters for Auto charging and , as Auto cut off. When the battery voltage reduces below 11.5 or 11 V, a relay turns on to charge the battery.
The only drawback of the circuit is that, the LED consumes some current even when the load is off. No problem. Battery needs both charging and discharging to maintain its health.