A device for charging small lead-acid (gel) batteries. The design has a number of very valuable functions that are rarely found in other chargers. First of all, it is that the device prevents overcharging the battery. When the battery is charged, the charge current automatically decreases. If a 230 V mains failure occurs during charging, the device will not discharge the battery. The charger can work as a power source. 12v, 1...30АhCharacteristic
charging 12 V lead acid batteries
battery capacity: 1 ... 30 Ah
charging system alarm: two-color LED diode
sound signal of wrong polarity
power source: 11 ... 13 V (the power of the transformer should be at least 50% than the power obtained by multiplying the charging current and voltage of 15 V)
Circuit description
The device is used to charge small lead-acid batteries. The device can also work with the battery as a buffer power source, providing uninterrupted power to devices.
The presented circuit has a number of very valuable functions rarely found in chargers. Primarily:
Prevents battery overcharging. When the battery is fully charged, the charging current drops to a negligible value, so even multi-day charging is not terrible.
Loss of mains voltage during charging will not lead to a rapid discharge of the battery - then the discharge current is about 2.5 mA.
The charger is not afraid of reverse battery connection. Most chargers with a reverse battery connection can be damaged by the huge current flowing through the output circuits.
The circuit is also not afraid of a short circuit of the output terminals of the charger.
A two-color LED indicates the operating status, and smooth changes from red to green reflect the charging process.
All these functions are implemented in a very simple scheme containing several popular and cheap elements.
Pay attention to the correct soldering of the terminals of the buzzer Y1, which is designed to work with a battery connected in the opposite direction. The charging current should be selected depending on the capacity of the rechargeable battery. It is very easy. It is necessary to solder 1-ohm resistors (indicated on the RS circuit) to obtain the necessary current. One RS resistor with a value of 1 Ω provides a charging current of approximately 0.15 A. For example, for a 2 Ah gel cell, the maximum charging current is 0.6 A (0.3 · 2), so you need to solder four RS resistors per 1 Ohm. After assembling and checking the entire battery, connect the battery to terminals P, N. When charging a discharged battery, the current on it will be immediately large, but after a while it will stabilize to a value determined by the active resistance PR1. Use potentiometer PR1 to adjust the final charge voltage. If the charger will operate in uninterrupted power mode (constantly on and connected to the battery), then PR1 should set the final voltage to about 13.8 V (13.5 ... 13.8 V), which corresponds to a value of 2.25 ... 2.3 In per element, guaranteeing the expected durability. During cyclic operation (alternate charging and discharging), the final voltage on the battery should be about 15 V (14.4 ... 15 V). This voltage value is not critical here. The higher the voltage, the faster the battery will charge. However, leaving the battery continuously under a voltage of more than 15 V. can shorten its life. Soldering the corresponding number of RS resistors per 1 Ohm and setting the potentiometer PR1 are the only necessary settings.
For transistor T1, you need to choose the appropriate radiator. Its size will depend on the charging current and voltage of the transformer. At low currents, a piece of aluminum sheet may suffice.
12 1-ohm resistors will give a maximum current of 2A. For this, the radiator must be appropriate. During operation, such a radiator can be very hot - up to +90 ... 100 C. When choosing a charging current, remember that it should not exceed numerically 0.3 C (C is the battery capacity in ampere-hours). At 0.3 C, a full charge will be approximately 6 hours. For example, for a battery with a capacity of 10 ampere-hours (10 A · h), the charging current should not exceed 3 A (0.3 · 10 A · h). Some manufacturers give a maximum charging current of 0.25C. Of course, the charging current can be reduced to 0.1 C or even 0.05 C, but then the charging time will drastically increase, even up to tens of hours. The value of the charging current determines the resulting resistance of the resistors RS1 ... RS4.The circuit diagram (Fig. 1) shows four RS resistors. Four groups are provided on the sample board to solder up to 12 resistors. This method was used intentionally because it makes it very easy to select a charging current. The power transformer used must have a nominal (alternating) voltage of 12 ... 15 V. Its power will depend on the required charging current. The power of the transformer must be at least 50% higher than the power obtained by multiplying the charging current and a voltage of 15 V. For example, for a charging current of 0.6 A, the product of 0.6 A · 15 V is 9 W, so the power of the transformer should not be less than 13.5 watts (150% * 9 watts).
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