Portable radio battery tester. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Portable radio battery tester. 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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While working as a communications engineer, I encountered the following problem. At the enterprise where I work, several dozen portable radio stations are in operation. They are equipped with 7,2V Ni-Cd, Ni-MH or Li-ion rechargeable batteries. It happens that you need to estimate the actual capacity of these batteries, but the equipment for this is not provided. To solve this problem, I designed and manufactured a battery tester based on the PIC16F688 microcontroller.

The principle of operation of the tester is based on discharging the battery with a fixed current, measuring its duration and then calculating the capacity. Information about the battery capacity is displayed on the LCD.

The tester does not charge, but only discharges the battery (charging is done in a standard charger). It is powered by the voltage of the battery being tested, so it does not require an external power source. To check the battery, simply put it with the contacts down on the device. Discharging will start automatically. If the battery is not charged, the device will display the requirement: "Charge the battery". There are no controls or switches.

During discharging, the device displays the current value of the battery voltage on the LCD. At the end of the discharge, the indicator displays the battery capacity in milliamp-hours and the red LED flashes.

The tester circuit is shown in fig. 1. A stable current source is assembled on the field effect transistor VT2 and the operational amplifier DA1.1, which can be adjusted by the tuning resistor R3. After assembling the tester, it is necessary to set this current to 1 A as accurately as possible, the accuracy of the instrument readings depends on this.

Rice. 1. Tester circuit (click to enlarge)

Transistor VT1, after the initial configuration of the microcontroller pins, is open and connects the gate of transistor VT2 to a common wire. When the program sets a low level at the RA5 pin of the microcontroller, the circuit connecting the gate of the transistor VT2 to the common wire opens, which turns on the source of a stable discharge current.

The XP1 connector of the tester is used only for programming the microcontroller. The battery voltage is supplied to the input of the analog-to-digital converter (output AN3 of the microcontroller) through the resistive divider R6R7. A selection of resistor R8 sets the optimal LCD contrast. Instead of the HL1 LED, you can install a piezoelectric sound emitter with a built-in generator. In this case, to set the desired sound volume, you will have to select a resistor R10.

The LM2940CSX-5.0 linear stabilizer can be replaced with another one with an output voltage of 5 V and with a small allowable voltage drop between input and output - no more than 0,8 V. For example, KF1158EN501A. Since the stabilizer works with a small load current, heat sink is not required.

The microcontroller turns on the discharge current with pulses lasting 4 s. The pauses between them are 2 s. This mode of discharging the battery simulates the real conditions of its operation. When the discharge current is on, the voltage on the battery is measured and compared with a threshold value of 6 V. When it reaches the specified threshold, the battery discharge ends and its capacity is calculated.

Structurally, the device is assembled on a metal plate, in which a hole is cut out for a board made of insulating material with spring contacts (XS1 connector, according to the diagram in Fig. 1). The board is fixed on the reverse side of the plate relative to the battery and serves to connect the battery to the tester.

On the same plate, two metal corners are installed, forming a seat for the tested battery. After installation, it is fixed with an elastic rubber band, which ensures a reliable connection with the contacts on the board.

A drawing of the printed circuit board of the device and the location of parts on it is shown in fig. 2. This board is also fixed on the metal plate mentioned above. The VT2 field effect transistor is equipped with a heat sink designed for a power dissipation of 8 watts.

Rice. 2. Drawing of the printed circuit board of the device and the location of parts on it

However, you should not install this heat sink too close to the battery under test, so that it does not heat it during discharging.

The HG1 indicator is fixed on a metal plate next to the seat for the battery and connected to the printed circuit board of the device with a bundle of nine wires.

PCB file in Sprint Layout 5.0 format and microcontroller program: ftp://ftp.radio.ru/pub/2015/02/testerAB.zip

Author: S. Tomilov

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