Digital battery capacity and internal resistance meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed device is designed to measure the capacitance and internal resistance of Ni-Cd and Ni-MH batteries. There is a sound indication of excessively low battery voltage, as well as the end of its discharge.

The battery capacity measurement is based on discharging it with a steady current, measuring the discharge time, and multiplying these values. When measuring internal resistance, the device measures the voltage of the battery without load, then under load with a current of 1 A and, based on these data, calculates the internal resistance of the battery.

Fig. 1

The scheme of the device is shown in fig. 1. Its basis is the ATmedav microcontroller (DD1). The keyboard with a single-wire interface consists of six buttons SB1-SB6. Information about the measured parameters of the battery is displayed on the nine-digit LED indicator HG1. To discharge the connected battery, a voltage-controlled current source (ITUN) was used on the op-amp DA2, transistor VT1, resistors R9, R10, R19-R21, R23 and capacitors C7, C9.

If the voltage of the connected battery is below 1 V, the instrument keypad is locked and the BF1 capsule emits three intermittent sound pulses at a frequency of 600 Hz. If the battery voltage is above 1 V, the BF1 capsule emits two intermittent sound pulses at a frequency of 3000 Hz when the battery is connected, and also when it is discharged to the set voltage

Fig. 2

Fig. 3

Fig. 4

After connecting the battery, the voltage is set to which it must be discharged by pressing the buttons SB3 and SB4. The setting step with a short press is 0,1 V. While holding the button, the first ten step values ​​are 0,1 V, then - 1 V. Then, by pressing the buttons SB1 and SB2, the discharge current is set. If these buttons are held for less than five seconds, the current value does not change and its current value is displayed, as shown in the photo fig. 2 (symbol i in lower position). If the buttons SB1 and SB2 are held for more than five seconds, the current value will change in variable steps: first 50 mA, then 150 mA. In this case, the i symbol will be displayed in the upper position, as shown in the photo fig. 3.

The maximum value of the discharge current is 2,55 A As soon as the discharge current reaches a value greater than zero (when the battery voltage is greater than or equal to the set threshold), the sound signal will disappear and the HL1 LED will start flashing at a frequency of 0,25 Hz. When the SB5 button is pressed, the voltage is measured and stored without load, then under load, the internal resistance in ohms is calculated, which is displayed in the lower digits of the indicator with the symbol r, as shown in the photo fig. 4.

When the SB6 button is pressed, the current battery voltage is displayed in the high-order digits of the HG1 indicator. When no button is pressed, the high digits of the HG1 indicator show the voltage to which the battery must be discharged, and the low digits show the capacity in the format XX,XX ampere-hours. Insignificant zeros of tens of volts and ampere-hours are canceled by software.

Fig. 5

Most of the parts are mounted on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in fig. 5 The thin rectangles show surface mount components R7, R8 and C5 mounted on the conductor side.

To ensure the linearity of the ITUN current throughout the entire range, it is necessary to use the op amp DA2 with the lowest possible zero bias voltage and the transistor VT1 with a small threshold voltage. In the author's copy, the zero bias voltage of the op-amp DA2 is about 4 mV and the transistor VT1 with a threshold voltage of 1,85 V at a drain current of 1 A, the non-linearity of the ITUN current did not exceed 10%. The minimum value of the ITUN current is no more than 2 mA. Transistor VT1 is installed without a heat sink. For its cooling, a fan from a computer processor is used. The fan and the device are powered by an unstabilized power adapter with an output voltage of 9..12 V and a load current of at least 0,5 A.

Establishment consists in the selection of resistors R6 and R9. By selecting the resistor R6, the indications of the senior digits of the indicator HG1 are set according to the exemplary voltmeter. Then, by pressing the buttons SB1 and SB2, the required value of the discharge current is displayed on the HG1 indicator, the ITUN current is measured with a reference ammeter and, by selecting the resistor R9, the measured current is set equal to the readings of the HG1 indicator.

PS In the absence of self-excitation of the clock generator of the microcontroller, its conclusions 9 and 10 should be connected to a common wire through capacitors of the same capacity 12 ... 22 pF.

Microcontroller programs can be downloaded from ftp://ftp.radio.ru/pub/2012/03/accmeter.zip.

Author: M. Ozolin

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