The charger described switches off the charging voltage when the battery reaches its full nominal voltage and switches it on again when the battery voltage drops below a predetermined level. Part of the battery voltage is taken from across potential divider Ri-R2-R3-R4 and compared with a reference voltage in IC2b.
As long as the battery voltage is 0 V, only a small voltage drop is caused across R5 by the input current of the opamp, so that 1C2(. toggles at 0 V. The relay, therefore,
remains (1c-energized. At the same time, the output of IC2b is high, but this has no effect whatsoever owing to AND gate D4-D5. When a battery is connected, its small residual voltage ensures that IC2, toggles. diodes D4 and D5 are reverse-biased, a reference voltage is applied to the non-invert-ing input of 1C2d, and the relay is energized. The battery is then charged until its voltage has reached the nominal level. Because of potential divider R1-R2-R3-R4, there is a voltage of more than 3.45 V at the inverting input of IC2b, which causes this opamp to toggle so that its output becomes low (0), the relay is de-energized, and the charging voltage is removed from the battery.
The (reference) voltage at the output of ICIa is set to 3.45 V. Potential divider D3-R6-R7-Pi provides a certain hysteresis to comparator IC2b. When the battery voltage drops below the level set with P1. IC2b toggles again and the charging voltage will be reapplied to the battery.
Calibration is carried out with a voltmeter connected to the output of IC2a, after which P2 should be adjusted for a reading of 3.45 V. Next, turn P1 to full resistance. Replace the battery by a regulated, variable power supply and set its output to 6.2-6.4 V (S1 in position 6 V) or 12.4-12.8 V (S1 in position 12 V), that is, the voltage at which charging should commence. Adjust P1 till the relay is energized.