Charger for recharging batteries. Scheme, description

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Charger for recharging batteries. Encyclopedia of radio electronics and electrical engineering

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

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It is known that the operation and storage of semi-charged batteries is one of the main reasons for the reduction of their service life. A stored charged battery after a while, as a result of self-discharge, passes into a semi-charged state. For new lead and miniature nickel-cadmium batteries, self-discharge is 0,5 ... 2% of their capacity per day, and for used ones it is much higher. To increase the life of the batteries, they should be constantly kept in a fully charged state, compensating for self-discharge with a relatively small current from a low-power charger.

It is considered optimal to consider such a charging mode when the charging current is numerically equal to 0,1 of the nominal battery capacity. However, some battery manufacturers now recommend a twenty-hour charging mode with a current numerically equal to 5% of the rated capacity in order to increase their service life. In other words, charging the battery with a current that is significantly less than the optimal one has a positive effect on its service life, but it requires a correspondingly longer time.

Thus, in a number of practical cases, complex and heavy chargers, often equipped with automatic control, can be replaced by simple, small-sized and economical ones. One such device is described below.

It can be used for recharging car batteries up to 100 Ah, for near-optimal charging of motorcycle batteries, and (with a simple modification) as a laboratory power supply.

The charger is based on a transistor push-pull voltage converter with autotransformer coupling and can operate in two modes - a current source and a voltage source. When the output current is less than a certain limit value, it works as usual - in source, voltage mode. If you try to increase the load current above this value, the output voltage will decrease sharply - the device will switch to the current source mode. The mode of the current source (having a large internal resistance) is provided by the inclusion of a ballast capacitor in the primary circuit of the converter.

Battery charger
Fig. 1

Schematic diagram of the charger is shown in fig. 1. The mains voltage through the ballast capacitor C1 is supplied to the rectifier bridge VD1. Capacitor C2 smooths out the ripple, and the zener diode VD2 stabilizes the rectified voltage. The voltage converter is assembled on transistors VT1, VT2 and transformer T1. Diode bridge VD3 rectifies the voltage taken from the secondary winding of the transformer. Capacitor C3 - smoothing.

The converter operates at a frequency of 5...10 kHz. The zener diode VD2 simultaneously protects the converter transistors from overvoltage at idle, as well as when the device output is closed, when the voltage at the output of the VD1 bridge rises. The latter is due to the fact that when the output circuit is closed, the generation of the converter can be disrupted, while the load current of the rectifier decreases, and its output voltage increases. In such cases, the zener diode VD2 limits the voltage at the output of the bridge VD1.

The experimentally taken load characteristic of the charger is shown in fig. 2. With an increase in load current to 0,35 ... 0,4 A, the output voltage changes slightly, and with a further increase in current, it sharply decreases. If an undercharged battery is connected to the output of the device, the voltage at the output of the bridge VD1 decreases, the zener diode VD2 exits the stabilization mode and, since capacitor C1 with a large reactance is included in the input circuit, the device operates in the current source mode.

If the charging current has decreased, then the device smoothly switches to the voltage source mode. This makes it possible to use the charger as a low-power laboratory power supply. When the load current is less than 0,3 A, the level of ripple at the operating frequency of the converter does not exceed 16 mV, and the output resistance of the source decreases to a few ohms. The dependence of the output resistance on the load current is shown in fig. 2.

The charger fits easily into a box measuring 155x80x70 mm. The box should be made of insulating material.

Transformer T1 is wound on an annular magnetic circuit of size K40x25x11 made of ferrite 1500NM1. The primary winding contains 2x160 turns of wire PEV-2 0,49, the secondary - 72 turns of wire PEV-2 0,8. The windings are insulated between each other with two layers of varnished fabric.

Zener diode VD2 is mounted on a heat sink with a usable area of 25 cm². Converter transistors do not need additional heat sinks, since they operate in the key mode. Capacitor C1 - paper, designed for a rated voltage of at least 400 V.

If it is necessary to use a device for charging small-sized batteries with a capacity of up to units of ampere-hours and regenerating galvanic cells, it is advisable to provide adjustment of the charging current. To do this, instead of one capacitor C1, a set of capacitors of a smaller capacity, switched by a switch, should be provided. With sufficient accuracy for practice, the maximum charging current - the closing current of the output target - is proportional to the capacitance of the ballast capacitor (at 4 μF, the current is 0,46 A).

If you need to reduce the output voltage of the laboratory power supply, it is enough to replace the Zener diode VD2 with another one with a lower stabilization voltage.

Adjustment begins with checking the correct installation. Then they make sure that the device is working when the output circuit is closed. The closing current must be at least 0,45 ... 0,46 A. Otherwise, resistors Rl, R2 should be selected in order to ensure reliable saturation of transistors VT1, VT2. A larger closing current corresponds to a smaller resistance of the resistors.

Author: N. Khukhtikov, Sergiev Posad, Moscow Region; Publication: cxem.net

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