Lead Acid and Tubular battery sometimes require boost charging with high current to remove the memory effect. In Inverter battery, the charger delivers low current since the charger is always on to charge the battery. If the line voltage is low, the charging will not be complete and battery gets only partial charge. This leads to memory effect and the battery remembers this partial charge as full charge in the subsequent charging. The best method to remove the memory effect is boost charging with high current for a short period. It is better to boost charge the inverter battery once in a month using an external charger after removing the battery connection from the inverter. This charger will do a quick charging and is ideal for Automobile battery and Inverter battery.
Charging current is obtained from a 14-0-14 Three Ampere step-down transformer. Low voltage AC is rectified by the 6 Ampere diodes D1 and D2. The rectified DC is made ripple free by the smoothing capacitor C1. The 14 volt DC passes through the Ampere meter which indicates the flow of current into the battery. When the battery is partially discharged, the meter needle will be close to the 2 amps position which indicates that the battery is drawing 2 amps current. As the battery attains full voltage, the meter needle shifts to 0 level which indicates that no more current is flowing into the battery. Thus by observing the meter reading, it is easy to determine whether the battery is fully charged or not.
LED indicators are also included for easy indication of the charging status. This can be omitted if needed. The circuit is based on the switching of two NPN transistors (BC547) to drive the corresponding LED. Zener diode ZD is connected to the base of T1 so as to switch on T1 when the Zener conducts. This happens only when the battery voltage is above 12 volts. Green LED lights when the battery voltage is normal or battery attains full charge. Resistor R1 and Preset VR adjust the base bias of T1 for smooth switching. When T1 conducts, base of T2 will be pulled to ground and T2 turns off and Red LED extinguishes. If the battery voltage is above 12 volts (that is the normal terminal voltage of 13.8), Zener conducts and Green LED lights and Red LED remains off.
When the battery voltage is below 12 volts, Zener remains non conducting and Green LED remains off but Red LED lights. When the battery is connected to the charger and current is flowing into the battery, Green LED goes off and Red LED remains on. When the battery attains full charge, Green LED lights and Red LED turns off. If the battery dead and not accepting charges, Green LED never lights, even after a prolonged charging. This indicates that the battery is not good for use.
The same charger can be modified slightly to charge 6 volt and 9 volt batteries. The table below shows the values of transformer and Zener for modification.
Transformer Zener Output Voltage Battery
6-0-6 – 3 Amps 4.7 V 6 V 4.5 V 7.5 AH battery
9-0-9 – 3 Amps 6.1V 9 V 6 V 7.5 AH Battery
12-0-12 3 Amps 9.1 V 12 V 9 Volt Battery