Automatic charger for lead-acid batteries. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Automatic charger for lead-acid 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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The charger is a 14,2 V parametric voltage regulator with a field-effect transistor control element. The gate circuit of a powerful field-effect transistor VT1 is powered by a separate 30 V source.

Automatic lead acid battery charger

To obtain an output voltage of 14,2 V, it is necessary to apply a stabilized voltage of about 1 V to the gate of the transistor VT18, since the cutoff voltage of the field-effect transistor IRFZ48N reaches 4 V. The voltage at the gate forms a parallel regulator DA1, fed through a resistor R2 from a source of 30 V. Stabistor VD3 introduced to compensate for changes in the EMF of a fully charged battery when the external temperature changes.

If a discharged battery is connected to the charger (the indicator of a deeply discharged battery is the EMF less than 11 V at its terminals), then the transistor VT1 will switch from the active stabilization mode to a fully open state due to the large difference between the voltage at the gate and at the source: 18 V -11V=7V, that's 3V more than the cutoff voltage 7V-4V=3V.

Three volts to open the IRFZ48N transistor is enough. The open channel resistance of this transistor will become negligible. Therefore, the charging current will be limited only by the resistor R3 and will become equal to 10m. This is the calculated value of the current. In practice, it will not exceed 10 A due to the voltage drop on the secondary winding of the transformer and on the diodes of the VD2 bridge, while the current will pulsate at twice the mains frequency. If the charging current still exceeds the recommended value (0,1 of the battery capacity), then it will not damage the battery, as it will soon begin to decline rapidly. As the battery voltage approaches the stabilization voltage of 14,2 V, the charging current will decrease until it stops altogether. In this state, the device can be left for a long time without the risk of overcharging the battery.

The HL1 lamp indicates that the device is connected to the network, and HL2 signals, firstly, the health of the FU2 fuse and, secondly, the connection of a rechargeable battery. In addition, the HL2 lamp serves as a small load, making it easy to accurately set the output voltage.

The device must use a network transformer with an overall power of at least 150 W. Winding II must provide a voltage of 17 ... 20 V at a load current of 10 A, and winding III - 5 ... 7 V at 50 ... 100mA. Transistor IRFZ48N can be replaced with IRFZ46N. If the device is used to charge batteries with a capacity of not more than 55 Ah, then the IRFZ44N transistor (or domestic. KP812A1) is suitable.

We will replace the GBPC15005 rectifier bridge with four D242A, D243A or similar diodes. Instead of KD243A, it is possible to use a KD102A or KD103A diode. Resistor R3 is made of nichrome wire with a diameter of at least 1 mm. It is wound on a ceramic rod, and each of the leads is clamped under an M4 screw with a nut and a soldering lug. The resistor should be mounted so that nothing interferes with its natural cooling by air flow.

The stabistor KS119A will replace four KD522A diodes connected in series according to. Instead of TL431, its domestic counterpart KR142EN19A will do. Resistor R6 should be selected from the SP5 series.

Transistor VT1 must be installed on a heat sink with a usable area of 100...150 cm2.

Thermal power during charging will be distributed between the transistor and resistor R3 as follows: at the initial moment, when the transistor is open, all thermal power will be released on resistor R3; by the middle of the charging cycle, the power will be distributed equally between them, and for the transistor this will be the maximum heating (20 ... 25 W), and by the end the charging current will decrease so much that both the resistor and the transistor will remain cold.

After assembling the device, it is only necessary to set the threshold voltage of 6 V at the output with a trimming resistor R14,2 before connecting the battery.

The device described in the article is simple and easy to use. However, it must be borne in mind that not all battery instances have an EMF equal to 14,2 V when charged. Moreover, it does not remain constant during the service life due to destructive changes in the battery plates. This means that if the charger is adjusted as recommended by the author, some batteries will be undercharged, while others will be overcharged and may "boil". The EMF also depends on the temperature of the battery.

Therefore, for each instance of the battery, it is necessary to first determine the optimal value of its EMF by controlled charging until the first signs of "boiling" and, taking into account the temperature, set this value in the charger. It is also advisable in the future to periodically (at least once a year) check the EMF and adjust the setting of the threshold voltage of the charger.

Author: Kostitsyn V.

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