Fully automatic charger for NiCd batteries with discharge function

The charger circuit is shown in the figure. A LM358 is used for charge and discharge detection. IC1a is used for charge detection, IC1b is used for discharge detection, and IC2 is LM555, which is connected to work in an unstable state. Every 15 minutes, its ③ pin outputs a positive pulse square wave of about 2 seconds to drive relay J3 to close once, and the battery is temporarily disconnected from the charging power supply through the J3K conversion contact and connected to the in-phase input terminal ⑤ of IC1a to monitor the battery charge. The connection of R15 prevents ⑤ pin of IC1a from being suspended, ensuring stable operation and no false triggering.

When using, install the battery to be charged first. Move the switch K1 to the corresponding position, turn on the power and press AN. The normally closed contact of AN is separated, and the normally open contact is closed, and the residual voltage of the battery is led to the ③ pin of IC1b, and compared with the preset voltage of the ② pin. If the residual voltage of the battery is higher than the preset voltage, the output of IC1b changes from low to high, causing transistor T to turn on quickly, so that relay J2 is attracted, and its conversion contact J2K locks the battery to the ③ pin of IC1b, and discharges through the discharge resistor connected to the switch K1C. When the battery terminal voltage drops to the preset voltage of the ② pin, IC1b flips, J2 is released to cut off the discharge circuit, and the charging power is turned on, and the circuit automatically enters the charging state.

When charging, IC2 drives relay J3 to make the circuit detect the battery voltage once every 15 minutes. When the voltage is higher than the preset voltage of pin ⑥, it means that the battery is full, and IC1a outputs a high level to trigger the one-way thyristor SCR, so that J1 is attracted, and a pair of normally closed contacts of J1 are separated at the same time, cutting off the charging circuit and the power supply of the control part, and the charging is over. However, a current of about 20mA flows through R26 to keep the battery in trickle charge. In order to charge battery packs with different numbers of cells, a three-pole, three-position switch is used to preset multiple comparison voltages for IC1 and switch the corresponding discharge resistors. When the switch is set to "1", a battery pack with three cells can be charged. When it is set to "2", a battery pack with two cells can be charged. When charging one battery, the switch should be set to "3". At the same time, turn W1 to control the charging current within the appropriate range.