A simple automatic charger for Ni-Cd batteries. Scheme, description

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Simple automatic charger for Ni-Cd batteries

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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The article describes a simple device that charges batteries for seven hours and then automatically turns them off, thereby eliminating the possibility of overcharging.

In recent years, with the increase in the number of low-voltage portable household equipment and children's electronic toys, the number of batteries purchased for them has also increased. In such equipment, it is best to use alkaline batteries (their designation contains the letters LR) or, more justified, batteries. Moreover, the latter are much cheaper and more profitable than galvanic cells.

Usually the first question asked by a person whose battery has "run down" is how to return it to service correctly and at the same time faster. And here, when answering, you need to find a reasonable compromise. The fast charging mode requires a sophisticated charging process control device. In addition, haste can damage the battery or reduce its power capacity. Charging with a low current (safe) requires a very long time (more than 12 hours), which is not always convenient. A violation of the temporary regime is also undesirable, although it is safer.

So, you need a device that would charge nickel-cadmium batteries with a safe current and itself monitor the charging time. Based on the foregoing, the proposed charger was made, consisting of a simple current stabilizer and a self-locking timer.

Most modern batteries can be charged with a current equal to 0,2 of their capacity for seven hours. These conditions, as well as a small number of parts, simplicity of design were the basis of the device.

The timer is made on a specialized clock chip K176IE12 (see diagram).

Simple automatic charger for Ni-Cd batteries

The mains frequency was used as the master frequency. After dividing, pulses are generated at the output M with a period of 10,5 hours. While the output is low, and this lasts seven hours as a result of resetting the counters with the "Start" button, transistor VT2 is closed and contacts K1.1 of relay K1 do not interfere with the operation of the current stabilizer, made on the transistor VT1. The charging process is in progress, which is indicated by the HL2 LED "Charging". After seven hours, the output of M will go high. Transistor VT2 opens, relay K1 is activated and with its contacts K1.1 interrupts the operation of the current stabilizer and stops the counter, connecting input Z to the negative power line. In addition, contacts K1.2 include LED HL3 - "End of charging".

In the event that the voltage may fail in the mains (up to 10 ... 12 hours), the device provides protection. It is made on a backup battery for digital automation with a voltage of 9 V (7D-0,125). An alkaline battery of galvanic cells of the same voltage (Krona, Korund and the like) is also suitable. Here the battery works as a buffer and does not require separate maintenance. When using a rechargeable battery, the VD7 diode is not needed. During operation, if the mains voltage fails, clock pulses will not be sent to input Z and the charging time will stop counting. Diode VD9 serves to prevent the discharge of the battery being charged through the collector junction circuit VT1 - resistor R9 when the mains voltage fails. As soon as the mains voltage appears again, charging will resume. After the end of charging, a high level at the output M of the DD1 chip is present all the time, as long as there is mains voltage or if it fails, the GB1 battery does not “sit down” and the counter is reset to zero.

Establishing the device is reduced to the selection of resistors R5 - R7 in the current stabilizer. The output current of the stabilizer depends on their resistance. To charge batteries 7D-0,125 (or similar), the resistance of the current-limiting resistor R5 should be about 35 ... 40 Ohms, AAA batteries - within 18 ... 20 Ohms (R6), AA - about 5 ... 8 Ohm (R7). In each case, a more accurate selection may be required. The main thing is that the charging current does not exceed 0,2 of the battery capacity.

The mains transformer T1 must provide a voltage on the unloaded (idle) secondary winding of at least 20 V, and at a load current of 200 mA - at least 12 V. Relay K1 is selected with a response voltage of not more than the voltage on capacitor C1. In the author's version, the RES9 relay is used, version RS4.529.029-02 or RS4.529.029-11. Transistor VT1 is installed on a heat sink - an aluminum or copper plate with an area of at least 25 cm2.

The charger provides charging of a 7D-0,125 battery or three AA batteries or eight AAA batteries connected in series. After installing the batteries and turning on the device to the mains with the SA1 toggle switch, press the "Start" button. From this moment, the charging time will start. LED HL2 "Charging" will turn on with reliable contact in the connector of the connected batteries. After seven hours, the device will automatically stop charging and signal this by turning on the HL3 LED "End of charging". At the same time, the HL2 LED will turn off.

Author: D.Boev

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