Tester for Ni-Cd and Ni-MH AA batteries. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Tester for Ni-Cd and Ni-MH AA batteries. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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An unpleasant situation happens when, when going out into nature with a camera, it turns out that recently charged batteries “sit down” very quickly. This can be avoided if they are tested in advance by determining the capacity. Check the batteries will help the device offered to the attention of readers. Its peculiarity is that the test results are displayed on the LCD indicator from the cell phone.

The tester is designed to simultaneously test four Ni-Cd or Ni-MH batteries with a nominal voltage of 1,2 V. With it, you can select batteries with similar parameters to make up a battery, train newly purchased batteries, or other preventive actions where they are needed. discharging with safe current. Each battery has a separate discharge indicator. The principle of operation of the device is simple - by discharging the battery through a known resistance to a voltage of 1 V, the discharge time is controlled. The results obtained are displayed on an alphanumeric LCD display.

Tester for Ni-Cd and Ni-MH AA batteries
Fig. 1

The scheme of the device is shown in fig. 1. The analog part consists of four identical discharge nodes A1-A4. An exemplary voltage source is assembled on the resistor R1 and the HL1 LED, in addition, this LED acts as an indicator for turning on the supply voltage. Voltage comparators are assembled on the operational amplifiers DA1.1, DA1.2, DA2.1 and DA2.2, which compare the voltage on the battery with the exemplary one. A trimming resistor R2 sets a voltage of 1 V at the non-inverting inputs of all op-amps, corresponding to the voltage of a discharged battery.

But until it is discharged, its voltage exceeds 1 V and a low logic level is formed at the output of the DA1.1 op-amp, so the 1VT1 transistor is closed, and 1VT2 is open and the battery connected to the 1X1 contacts is discharged through the 1R4 resistor and the 1VT2 transistor. In this state, the 1HL1 LED does not light, which indicates the process of discharging the battery. For the elements indicated in the diagram, the maximum discharge current is about 250 mA.

When the battery is discharged to a voltage of 1 V or less, the comparator on the op amp DA1.1 switches, the transistor 1VT1 opens, and 1VT2 closes, the discharging process stops, and the 1HL1 LED lights up, signaling this.

Since it is inconvenient to constantly monitor the LEDs, a digital control unit was introduced into the device, which records the duration of the discharge of each battery. This node is assembled on a DD1 microcontroller (PIC16F628A) and an LCD from a NOKIA 3410 cell phone, which is connected to the XS1 socket.

The LCD indicator requires a supply voltage of about 2,5 V (at a current of up to 1 mA), which is formed by a resistive divider R4R5. Resistors R6-R10 ensure the matching of the levels of the output signals of the microcontroller with the LCD indicator. Capacitor C4 (its capacitance can be from 1 to 10 microfarads) is part of the filter of the internal power supply of the LCD indicator. The resonator ZQ2 is designed for the built-in generator of the internal timer of the microcontroller DD1, which counts the duration of the discharge.

After the supply voltage is applied, the LCD is initialized, and if it was successful, the message "Ok" is displayed on it. Then, the tested batteries (or one battery) are installed in the holder and the SB1 "Start" button is pressed - the countdown of the discharge duration will begin. When the battery voltage drops to 1 V, the countdown stops and the result is displayed on the LCD. Pressing the SB1 button again will restart the process.

Tester for Ni-Cd and Ni-MH AA batteries
Fig. 2

Most of the parts of the analog part of the device are placed on a printed circuit board made of one-sided foil fiberglass 1,5 ... 2 mm thick, the drawing of which is shown in Fig. 2. Here, elements for surface mounting are mainly used - fixed resistors RN-12 and capacitors (1C1-4C1) - size 0805. We will replace the IRF740 field effect transistor with IRFZ44, IRL2505 and the like. The tuning resistor is SP5-2, but SPZ-19 is also suitable, fixed resistors 1R4, 2R4, 3R4, 4R4 (MLT, C2-23) and field-effect transistors are installed on the side of the board free from printed conductors.

Tester for Ni-Cd and Ni-MH AA batteries
Fig. 3

The elements of the digital assembly and the LCD indicator are mounted on a printed circuit board made of double-sided foil fiberglass with a thickness of 1,5 ... 2 mm, the drawing of which is shown in fig. 3. Resistors for surface mounting of size 0805 are also used here, a quartz resonator ZQ1 - HC-49S, ZQ2 - "clock". The microcontroller is installed in the panel. The appearance of the assembled board is shown in fig. four.

Tester for Ni-Cd and Ni-MH AA batteries
Fig. 4

On the third board (with the same dimensions as the first and second) the battery holders are fixed. This board can be made of non-foil fiberglass. If you use foil, then contact pads are cut out on it, to which connecting conductors are soldered. All three boards are assembled into a single structure with the help of screws with nuts and metal (or plastic) racks (Fig. 5). The connections between them are made with an insulated mounting wire.

Tester for Ni-Cd and Ni-MH AA batteries
Fig. 5

The discharge current can be changed by selecting a resistor 1R4 (2R4, 3R4, 4R4), but it should not be increased by more than 0,5 A. do not install, but install the HL1 LED on the board. To power the device, you can use a stabilized power supply with an output voltage of 1 V and a current of up to 1 mA when using 2HL2-4HL1 LEDs or 5 mA without them. Suitable, for example, stabilized memory from a cell phone.

The microcontroller program can be downloaded hence.

Author: N. Nistratov, Rostov-on-Don; Publication: radioradar.net

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