BATTERY_CAPACITY_INDICATOR

Published:2009/7/14 1:52:00 Author:May | From:SeekIC

Zener diode D1 creates a reference voltage to which the battery voltage is compared. The diode specified has a breakdown voltage of 5.1 V. That rating will work fine with most six- or seven-cell NiCd battery packs, as well as with 12-V lead-acid batteries. The circuit can be customized for a particular battery by selecting a unit for D1 that has a voltage rating about 1 V below the completely discharged voltage of the battery pack that you wish to measure. Transistor Q1, an emitter-follower amplifier, greatly increases the sensitivity of the circuit over what it would be if R7 were connected directly to the wiper of R6. A further advantage to that arrangement is that it reduces the current drain that flows through RI, R2, and R6. By amplifying the current flowing through the resistors, the resistance value can be increased to a very high value, lowering the total current draw of the circuit. Resistor R6 adjusts the meter to read 0 mA when the battery is completely discharged and R7 adjusts the meter to read 1 mA when the battery is fully charged. If the circuit were accidentally connected backward to the battery, current would flow through D1 and M1. The transistor would become re-verse-biased, allowing a complete path back to the battery. That situation would allow excessive current to flow through D1, M1, and Q1, destroying them in the process. D2, R1, and R2 are included to prevent any current flow in case the battery is reversed.



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