Battery tester. Encyclopedia of radio electronics and electrical engineering

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

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Leafing through the Radio magazines of past years, you can always get an idea for a new design. In this case, the article [1] under the heading "Abroad" served as the source of the idea. The described device performs a similar function and is designed to quickly check the most common galvanic batteries in household equipment.

The tester measures the residual capacity of the galvanic battery at the time of measurement as a percentage of the nominal. The capacitance of an element is the amount of electricity that it can deliver to a load. When discharging with a constant current I, its value in ampere-hours is equal to the discharge current multiplied by the duration of the discharge of the element to a predetermined minimum voltage. When discharging to a constant load resistance, the current decreases over time due to a decrease in the cell voltage. In this case, the capacity of the element is calculated by the formula

where t is the duration of the discharge; i(t) - discharge current, which changes during the discharge process; R_н - load resistance; U is the voltage of the element, which changes during the discharge process; U_Wed is the cell voltage averaged over the discharge time.

On fig. Figure 1 shows the experimentally measured curve of the discharge of an alkaline element "Energizer" size AA on a resistor with a resistance of 15 ohms. On fig. 2 - obtained by graphical integration of this curve, the dependence of the residual capacity of the element on its voltage. The capacity of a fresh undischarged element turned out to be 2 Ah.

Rice. 1. Discharging curve of an alkaline element "Energizer" size AA on a resistor with a resistance of 15 ohms

Rice. 2. Dependence of the residual capacity of the element on its voltage (click to enlarge)

The scheme of the tester is shown in fig. 3. Using the built-in ADC in the DD1 (ATtiny 13A-SU) microcontroller, it measures the voltage on the tested Gx battery when it is loaded with a resistor selected by the SA2 switch. The measured value is compared with the constants stored in the memory of the microcontroller, the comparison software unit turns on a certain number of LEDs on the linear scale of the element capacity indicator.

Rice. 3. Tester circuit (click to enlarge)

This indicator consists of seven green LEDs and one yellow LED. The number of LEDs on is proportional to the cell capacity: 100% - all HL1-HL8 LEDs are on, 0% - only one yellow HL8 LED is on.

If the cell voltage is less than 1 V, the red LED HL9 turns on. This indicates that the tested battery is not suitable for further use.

When checking, ten measurements of the element voltage are made with pauses between them of 0,2 s. Then the program calculates the average value of the result, which it compares with the constants stored in memory. For correct instrument readings, the voltage divider on the tuning resistor R1 must be adjusted so that the voltage at its movable contact (at the input of the ADC of the microcontroller) is 1 V at a voltage of 1,5 V on the element under test.

The shift register 74HC164 (DD2) switches the LEDs HL1-HL8 of the indicator, the red LED HL9 is connected to the PB1 output of the microcontroller. When the power is turned on, all LEDs flash for 2 s, after they are extinguished, the device is ready for operation. The SB1 button is used to start the measurement subroutine and connects the negative output of the element under test to a common wire. For the duration of the measurement (until the indicator shows the value of the capacity of the element being checked), the button must be kept pressed. Switch SA2 for three positions is used to select a resistor that sets the load current depending on the size of the tested battery. The initial discharge currents are as follows: AA, AAA - 100 mA, C - 250 mA, D - 400 mA.

Trimmer resistor R1 calibrate the device. The procedure for this is as follows. A fresh galvanic cell Gx with a voltage of 1,5 V is connected to the tester. With the switch SA2 in the "AA, AAA" position and the button SB1 pressed, by rotating the trimmer resistor R1 at the input PB4 of the microcontroller, a voltage of 1 V is set relative to pin 4 of the microcontroller.

In the initial state of the SB1 button, pin 3 of the XP1 connector is connected to a common wire, which, when programming the microcontroller, causes a failure or failure of the programmer. To avoid this, during programming it is necessary to disconnect the wire from pin 1 of the SB1 button or keep this button pressed until programming is completed.

As practice has shown, the HL9 LED, connected through a resistor R15 to pin 6 of the XP1 connector, does not affect the correct operation of the STK500 programmer.

The tester is powered by two galvanic cells G1 and G2, size AA. The SA1 power switch has three positions (two of them "On") and is connected between the batteries. It is quite acceptable to use another power source with a stabilized voltage of up to 5 V and a conventional switch.

The printed circuit board of the tester is shown in fig. 4, and the arrangement of elements on it - in Fig. 5. The board is designed for placement in the case from the charger "Varta". Contact lobes are soldered into the rectangular holes provided in it, which, when the board is installed in the case, connect its circuits to the G1, G2 batteries located in the regular mounting places of the case and the tested element Gx. To test larger elements, a terminal block is installed in the upper part of the housing. The appearance of the assembled device is shown in fig. 6.

Rice. 4. PCB tester

Rice. 5. Arrangement of elements on the test board

Rice. 6. Appearance of the assembled tester

Resistors R2 and R7-R15 - size 1206 for surface mounting, R3-R5 - 0,25 W, R6 - 0,5 W for surface mounting. Trimmer resistor R1 is multi-turn.

Oxide capacitors can be used of any type. Capacitor C2 - ceramic KM-6 or similar imported. Instead of discrete LEDs HL1-HL9, you can use a ready-made linear LED scale, for example, DC-7G3HWA. XP1 connector - PLD-6 plug.

To increase the reliability of the result, it is recommended to check the elements at a load resistance close to that with which it is supposed to be operated in the future. Even greater accuracy can be achieved if several blocks of constants are provided in the program, with which the voltage of the element will be compared depending on its type.

When programming the microcontroller, its configuration bits for operation from the internal 4,8 MHz clock generator must be set as follows:

CKSEL = 01;

SUT=10;

CKDIV8 = 1.

When developing the software, the file 16121572.asm was used - a software implementation of the SPI interface for the AT90S1200 model from the book [2].

The microcontroller program can be downloaded from ftp://ftp.radio.ru/pub/2015/06/testbat_v2.zip.

Literature

1. Tester for batteries ("Abroad"). - Radio, 1991, No. 6, p. 86.
2. Trumpert W. AVR-RISC microcontrollers. - Kyiv: "MK-Press", 2006.

Author: N. Salimov

See other articles Section Chargers, batteries, galvanic cells.

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