Intelligent charger. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Intelligent charger

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

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Ni-Cd batteries are widely used to power modern wearable equipment. To charge them, many devices are produced, similar devices and radio amateurs are assembled. However, most industrial and amateur designs are designed for simple recharging of batteries. Often they are not able to fully charge them due to the inherent disadvantage of Ni-Cd cells - the so-called "memory effect". It consists in the fact that if you charge an incompletely discharged battery, then it will give energy only to the level from which charging began. In order for this effect not to appear, the battery must be completely discharged (to about 1 V), and then charged to a voltage of about 1,4 V. The microcontroller device described below automatically solves this problem. The battery that has not completely given up its capacity is first completely discharged, then charged to a predetermined level, checks its ability to work normally, and then disconnects it from the device.

The proposed device is designed for simultaneous independent charging of four Ni-Cd batteries with a capacity of 600, 800 and 1200 mAh, but can also be used to charge other types of batteries. The ability to change the device operation algorithm programmatically provides the necessary flexibility and ease of working with it.

Schematic diagram of the charger is shown in Fig.1. Functionally, it consists of a control unit and four identical charge-discharge cells.

(click to enlarge)

The control unit contains MK DD1, switch DD2, comparator DA1, exemplary voltage generator (VT13, VT14), battery fault sound signaling unit (VT15) and buffer DD3. MK controls the operation of the device as a whole, ensuring the independent operation of all four charging nodes. Switching the voltages coming from the batteries to the non-inverting input of the comparator DA1 is carried out by the switch DD2. Reference voltages are formed depending on the code determined by the signals E0 and E1 specified by the microcontroller. Buffer DD3 provides decoupling of port P1 of the microcontroller from charge-discharge cells.

Each such cell consists of a current stabilizer DA2 (hereinafter, the positional designations of the elements of cell A1 are indicated), current-setting resistors R3 - R5, transistor switches (VT1 - VT3), switching node states (charge-discharge-control) and LEDs HL1 (red). glow) and HL2 (green), indicating the state of the node (red - charging, green - discharging). Switches SA1 and SA2 allow you to set the required charging current (in this case 60, 80 or 120 mA).

Let's consider the operation of the device in more detail. When the power is turned on, the program analyzes the state of the battery G1, in turn comparing the voltage on it (signal K1) with the reference voltages generated by the shaper on transistors VT13, VT14. If the voltage on the battery is less than 0,7 V, it "concludes" that the cell is empty, and proceeds to analyze the state of the next one. If the voltage on the battery is more than 1 V (usual case), MK DD1 issues (through buffer DD3) signals R1=1, Z1=1. In this case, the HL2 LED lights up and transistors VT1, VT3 open. The first of them blocks the charging channel (DA2, R3-R5, VT2), and the second connects resistor R9 in parallel with the battery. The discharging process starts.

In the discharging and charging modes, the voltage on the batteries is measured once every 4 s. The measurement cycle (signal Z1=1, R1=0) is approximately 1 s, i.e. the time to service one battery, including the delay, is 1 s. At this time, the battery voltage is measured, and depending on its value, the microcontroller decides whether to continue discharging (charging) the battery or turn it off (if charging is completed). This is clearly seen by the glow of the LEDs. Periodic lighting of the green LED (HL2) indicates that the battery of this cell is in the discharge mode, and the red one (HL1) is in the charging mode.

But back to the discharge mode. The signal K1 (voltage on the battery being discharged) through the switch DD2 is fed to the non-inverting input of the comparator DA1, where it is compared with the reference voltage (about 1 V) supplied to the inverting input from the shaper on transistors VT13 and VT14 (the first of them is open, and the second is closed). At the moment the specified voltage value is reached, the comparator issues a signal about the completion of the discharge process and the MK switches the device to charging mode (signals R1 and Z1 take the values of log. 0). In this case, the HL1 LED lights up, the transistors VT1, VT3 close, and VT2 opens.

In the process of prototyping the device and testing it in operation with batteries of different capacities and different companies, it was found that the maximum battery charge corresponds to an exemplary voltage of approximately 1,45 V (taking into account losses in the measuring circuits). If necessary, it can be changed in one direction or another with a tuning resistor R44.

When the voltage on the G1 battery reaches approximately 1,45 V, charging stops. Then for some time (approximately 8 ... 10 s) the cell switches to the discharge mode (the HL2 LED lights up) with the control of the battery voltage. If it has not changed significantly during this time, charging ends (both LEDs do not light up). If the voltage drops sharply (up to 1 ... 1,1 V), which indicates a battery malfunction, then an audible signal is emitted, and the HL2 LED starts flashing.

The device has a forced charging mode. It is used when the battery is discharged to a voltage of less than 1 V or it needs to be urgently recharged (bypassing the discharge process to 1 V). Switching on for forced charging is carried out by the SB1 button (it is held pressed until the HL1 LED lights up).

The choice of charging currents equal to 0,1 of the battery capacity is carried out by switches SA1 and SA2 by shunting resistor R4 with resistors R3 and R5. In the positions of the switches shown in the diagram, the charging current is determined by the resistance of the resistor R4 and is equal to 60 mA. Closing the contacts of the SA1 switch leads to an increase in the charging current up to 80 mA, and both (SA1 and SA2) - up to 110 ... 120 mA. The maximum output current of the 78L05 voltage regulators is 100 mA, however, in current regulator mode, it passes 120 mA with relatively little heating (in extreme cases, you can put a small heat sink on it).

Charger parts are mounted on a printed circuit board made of double-sided foil fiberglass (Fig. 2).

Intelligent charger

The board is designed for the use of constant MLT resistors, trimmers SDR-19a, capacitors K50-35 (C1, C4), KD-1 (C2, C3) and KM (others), a two-pin section from the PLS-40 (XP1) plug, B38 buttons or B32 (SB1), miniature sliding switches VDMZ-2V (SA1-SA8). In the frequency-setting circuit of the built-in MK oscillator, a quartz resonator with a frequency of 3,58 MHz is used, but any other with a frequency of 3 to 8 MHz is also acceptable (in this case, some constants will have to be changed in the program). As a BF1 sound emitter, you can use telephones of the TM-2V type or a ZP-31 piezo emitter. To connect MK DD1 use a 20-pin panel.

Codes "firmware" ROM MK are shown in the table.

(click to enlarge)

Program source code

Most resistors are installed perpendicular to the board. Wire jumpers are inserted into the holes marked on the bottom (in Fig. 2) drawing with four points, connecting the printed conductors on different sides of the board.

Setting up the device comes down to setting the reference voltages and the required values of the charging and discharging currents. Reference voltages (see the table in the lower left part of Fig. 1) are set by trimming resistors R42, R43, R44 and the selection of resistor R41. Do this without MK, temporarily removing it from the panel. Two conductors are inserted into its sockets 2 and 3 (or soldered to the corresponding pads of the board) and connected through resistors with a resistance of 10 kOhm to a voltage source of +5 V. Then power is supplied to the board and, connecting the named panel contacts in various combinations with a common wire (codes 00, 01, 10, 11), using tuned resistors, set the voltages indicated on the diagram at point K (pin 4 of the DA1 chip; E0 is the most significant bit, E1 is the least significant).

The required charging currents are set by selecting resistors R3 - R5. To do this, a battery discharged to 1 V is installed in any cell, a strip of double-sided foil fiberglass (or getinax) with pieces of mounting wire soldered to the foil is inserted between its positive terminal and the corresponding contact, and a milliammeter with a measurement limit of 150 ... 300 is connected to the free ends of the latter. ma. Resistor R4 is temporarily replaced with a tuned resistor with a resistance of 270 ... 330 Ohm (preferably a multi-turn wire one) and, by turning on the forced charging mode with the SB1 button, such a resistance of the part of the resistor introduced into the circuit is selected at which the charging current is 6 mA (for a battery with a capacity of 600 mA h). Then a constant resistor of close resistance is soldered in its place, replaced with a tuning resistor R3 and, by closing the contacts of the switch SA1, the current is increased to 80 mA (for batteries with a capacity of 800 mAh). Finally, with the contacts of both switches SA1 and SA2 closed, the resistance of the resistor R5 is selected, corresponding to the charging current of 120 mA (for batteries with a capacity of 1200 mAh). Similarly, the resistors of the charging circuits and the remaining three cells are selected.

The discharge current (about 60 mA at a battery voltage of 1,2 V) is set by selecting the resistor R9. To speed up the discharge of batteries with a capacity of 800 and 1200 mAh (in the first case, with a current of 80, and in the second - 120 mA), two more resistors can be introduced into the collector circuit of the transistor VT3, connected in parallel with R9 using switches similar to SA1, SA2 (of course, the same in this case, changes must also be made to the bit circuits of the remaining cells).

In conclusion, it should be noted that the described device is capable of charging batteries with a larger capacity. To do this, it is necessary to replace DA2-DA5 with stabilizers for a higher current (300 ... 400 mA), and key transistors with more powerful ones.

Authors: M. Demenev, I. Koroleva

See other articles Section Chargers, batteries, galvanic cells.

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