Automatic charger. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Batteries, chargers

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As you know, lead-acid batteries last much longer if they are constantly charged. For this purpose, the industry produces several models of simple household chargers, but many of the readers cannot afford their cost. Below is a homemade charger, the manufacture of which is quite within the power of medium-skilled radio amateurs.

In most cases, the charger is a source of constant or pulsating current, consisting of a mains transformer, a rectifier and a ballast element that limits the battery charging current. On the ballast element (most often its role is played by a rheostat, an incandescent lamp or a powerful transistor), significant power is lost, released in the form of heat.

During the charging process, it is necessary to constantly monitor and adjust the charging current, which changes due to changes in the battery voltage, mains voltage instability and other reasons, which is extremely inconvenient.

Quite a lot of different designs of chargers are described on the pages of amateur radio literature. Nevertheless, I would like to bring to the attention of readers another version of an automated charger that is free from the above disadvantages and allows charging lead-acid batteries with a capacity of 10 to 160 Ah.

It provides a stable pulsating current equal to (average value in amperes) 5 ... 10% of the battery capacity value (in ampere hours). Charging lasts 10 ... 12 hours until the battery voltage reaches 14,6 ... 14,9 V at an electrolyte density of 1,27 ... 1,29 g / cm3.

The charger consists of a mains transformer T2 (see schematic diagram), a powerful rectifier based on diodes VD8, VD9 and trinistors VS1, VS2, a low-power source made on the elements VD6, VD7, R17, VD5, VD4, C4, C5 and feeding the electronic assembly . The electronic assembly, in turn, includes a trinistor control device assembled on a unijunction transistor VT2 and a pulse transformer T1, a charging current stabilizer on the DA2 op amp, an automatic battery voltage control system on the DA1 comparator and a protection device against erroneous connection of the load in reverse polarity, made on relay K1.

(click to enlarge)

Thanks to the use of automation devices that stabilize the charging current and control the degree of charge of the battery by the voltage on it, the need for constant monitoring of the charging process is completely eliminated.

From the current-measuring resistor R18, a voltage proportional to the charging current is supplied to the inverting input of the op-amp DA2 through the resistor R14. From the R12R13 divider, the voltage required to set the initial bias and compensate for the technological spread of the operational amplifier parameters is applied to the same input, which is necessary for its unipolar power supply. This allows you to use almost any OS in the node.

Resistor R9 sets the required value of the charging current.

Thanks to the capacitor C3, op-amp DA2, in addition to comparing the input signals, also performs the function of integrating their difference with a large time constant. The fact is that the voltage falling across the resistor R18 is not constant, but pulsating. With an increase for any reason, the charging current increases the voltage across the resistor R18, and hence at the inverting input of the op-amp DA2. The voltage at its output decreases, the charging of the capacitor C3 slows down and the opening of the rectifier trinistors is delayed. As a result, the charging current returns to its original value.

The voltage at the terminals of the battery being charged is monitored by an automatic control system assembled on the DA1 comparator. The voltage is supplied to its inverting input from the divider R2R3. As soon as it exceeds the threshold level set by the divider R1R4R5, a high level will appear at the output with an open emitter (pin 2) of the comparator. Transistor VT1 opens and shunts capacitor C6. For this reason, the flow of control pulses to the trinistors VS1, VS2 will stop, and they will close, and the "green" LED HL1 that turns on will signal the end of charging.

If, after some time, the voltage on the battery drops to 11 ... 11,5 V, the comparator switches to its original state, the transistor VT1 closes and the charging process starts again. The threshold voltage corresponding to the termination of charging is set by resistor R1. The C1R7VD2 circuit allows you to more accurately measure the voltage at the battery terminals, since it eliminates the influence of the output voltage of the charger.

If the battery is connected by mistake to the charger in reverse polarity, the VD11 diode will open, relay K1 will operate and bypass capacitor C1.1 with its contacts K6. Therefore, the SCRs will not open when the device is powered on. The error will be indicated by the HL2 LED turning on.

It should be noted that such protection is effective only when the battery is connected to the switched off charger - this should be remembered when using it. If you use a more powerful automotive relay K1, you should include its break contacts in the negative circuit break at point B (see diagram) - the protection will be more reliable.

Fuse FU2 is used to open the charging circuit in case of emergency. Since the charger is, in fact, a source of stable current, it withstands short-term output closures, but a long stay in this mode is unacceptable due to overheating of the elements by a large pulse current.

Structurally, the charger is made in a metal casing of suitable dimensions (which must be grounded during operation of the device), although it can be mounted directly into the electrical distribution panel of a garage or workshop. Rectifier elements VS1 and VD8, VS2 and VD9 are installed in pairs on two heat sinks. Resistor R18 is made of wire with a diameter of 0,5 ... 0,8 mm with high resistivity (constantan, manganin, nichrome).

Replacing KU202E trinistors and D231 diodes with T122-16 and D112-16, respectively, will increase the maximum allowable charging current and device reliability. At the same time, the T2 network transformer must also be selected more powerful. Instead of K553UD1, almost any general-purpose op-amp is suitable, for example, from the K140 or 153 series. An op-amp can also be used as a DA1 comparator.

Relay K1 - RES10, passport RS4.529.031-08. Ammeter RA1 - any magnetoelectric with a total deflection current of 10 A.

Transformer T1 - serial TI-4 or home-made, wound on a ring of size K20x12x6 from M3000NM ferrite. The primary winding contains 60, and the secondary - 40 turns of PELSHO wire with a diameter of 0,1 mm. The windings should be securely isolated from one another and from the magnetic circuit with varnished cloth.

Network transformer T2 - industrial or home-made with a power of at least 180 W with a voltage on the secondary winding of 18 ... 20 Veff at a current of at least 10 A. In the case of independent manufacture of the transformer, it is easier to convert it from a network TC-180 or TC-200 from a lamp TV . All secondary windings should be removed from it and a new winding should be wound - 65 turns of PEV-2 1,5 wire.

The wires from the charger to the battery must be double insulated, with a cross section of at least 2,5 mm2, and terminate with clamps that ensure reliable contact with the battery terminals.

If, when repeating the charger, there were difficulties with acquiring a KT117A unijunction transistor or doubts about its performance, the easiest way to solve the problem is to replace this device with an analog assembled from two bipolar transistors (see B. Erofeev's article "Economical touch light switch" in "Radio", 2001, No. 10, pp. 29, 30).

The device is not critical to the spread of the parameters of the elements, but requires adjustment. This will require a serviceable charged battery, load equivalents - two wire resistors with a resistance of 1 and 3 ohms with a dissipation power of at least 100 W (pieces of a nichrome spiral, wire resistors, etc.), as well as an acid hydrometer for measuring electrolyte density.

First, they establish a system for stabilizing the charging current. A load with a resistance of 3 ohms is connected to the output of the device. The diode VD3 is disconnected from the collector circuit of the transistor VT1 and the device is powered. Resistor R12 at the top position of the resistor R9 engine according to the scheme achieves a current in the load equal to 1 A.

Next, a load with a resistance of 1 ohm is connected to the output of the device and, selecting resistors R10, R11 and R13 (carefully so as not to overload the charger!), They achieve a change in current through the load within 1 ... 10 A when the engine of the resistor R9 rotates.

Then they set up an automatic voltage control system on the battery. Solder in place the output of the diode VD3. Attach a battery to the output of the device and turn on the power. When the electrolyte density reaches 1,27 ... 1,29 g / cm3, the resistor R1 slider is slowly rotated until the HL1 LED lights up and the charging current is turned off. By adjusting the resistor R5, the charging current is switched on again when the voltage at the battery terminals drops to 11 ... 11,5 V (the battery must be discharged for this).

If you make a scale for the variable resistor R9 and calibrate it when adjusting, you can abandon the PA1 ammeter.

In conclusion, advice: in no case should lead acid batteries be charged in a city apartment due to the release of aggressive toxic gases during the charging process and the impossibility of grounding the device.

Author: V.Sorokoumov, Sergiev Posad

See other articles Section Automobile. Batteries, chargers.

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