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# Battery Self Discharge Indicator(BC557)

2016-05-09 14:40
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This article describes the Battery Self Discharge Indicator (BC557). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: BC557.

If the lead acid battery is not using for long time, self discharge takes place at the rate of 4 % per week at 27 degree. For example, a 125 Ah tubular battery self discharge at the rate of 5 Amps current per week if it is not in charge / discharge cycles. This add on circuit can be used to detect the self discharge of the 12 volt lead acid battery below the safe level of 11.6 volts. It is ideal to monitor the charge level in the battery, it the battery is not using for long period. The circuit uses only a few components and its working is simple. APNPtransistor T1 act as a switch to light theLED, if the battery voltage drops below the safe level. The base bias of T1 is controlled by a Zener diode ZD. Its rating is 10 volt 1 W.

#### Figure 1 Battery Self Discharge Indicator Circuit Diagram

The Zener diodes usually requires 1.6 volts excess than its rated value to enter into the “Avalanche state”. So, as long as the battery voltage is above 11.6 (10 1.6), Zener conducts and keeps the base of T1 high. Since T1 is aPNPtransistor, it will not conduct till its base becomes negative. SoLEDremains dark. When the voltage in the battery reduces below 11 volts, Zener turns off and the base of T1 becomes negative. T1 then conducts andLEDlights. So the battery can be charged again to keep it in top condition. Preset VR can be used to set the exact point at whichLEDturns on.