Battery capacity meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Batteries gradually lose capacity during use. To assess the real state of the battery and draw conclusions about the advisability of its further use allows the device, which is described in the article.

To control the state of the battery, only a few parameters are available to the user: voltage at its terminals without load, internal resistance, voltage at the terminals at a certain load and its change over time. The last parameter is associated with the capacity of the battery (it is denoted by the Latin letter C). For batteries designed to power electronic devices, the capacity is usually estimated in ampere-hours (Ah) or milliamp-hours (mAh) as the time during which the voltage on the Ni-Cd / Ni-MH battery when discharging it with a stable current decreases to 1 V. The choice of such a value is to some extent conditional, but not accidental. It is believed that by this moment the battery has time to give up about 90% of the energy stored in it, and the rate of decrease in the voltage on the battery increases markedly. It should be noted that the battery capacity determined in this way depends on the selected discharge current. This dependence noticeably weakens only at its values ​​less than 0,5C.

It is convenient to measure the battery capacity in a device capable of discharging it with a stable current of up to 1 V. 1. Its basis is the integrated timer KR1006VI1 (DA1). It contains two comparators (upper and lower levels), a trigger, an output stage and a discharge transistor. Pins 5 and 6 are the inputs of the high-level comparator. The voltage on the first of them is set by the internal divider of the microcircuit and is equal to 2/3 of the supply voltage of the microcircuit, on the second - by a resistive divider R1 - R3, which is powered by a stabilized source of +9 V.

As you can see, power is supplied to the microcircuit through connector X1 from the battery being tested. If it consists of six elements, the comparator should work at a voltage of 6 V, and if out of seven (for example, the Nika battery and the like) - at 7 V. Therefore, the voltage at pin 6 of DA1, set by the divider R1 - R3, in in the first case it should be equal to 4, and in the second - 4,67 V. These values ​​need to be clarified, since they depend on the parameters of the internal divider of a particular microcircuit instance. For definiteness, the variant of the device for the Nika storage battery is further considered.

As long as the battery voltage is above 7 V, the timer output (pin 3) is high (about 1,5 V below the current supply voltage). The discharge current is the sum of the load current (it is maintained unchanged by the current stabilizer on the field-effect transistor VT1) and the current consumed by the microcircuit itself (about 5 mA). Setting a total current of more than 30 mA is undesirable. In the author's version, it is chosen equal to 20 mA. This allows you to discharge the Nika battery with a current of 0,2C, which, on the one hand, reduces the discharge time by half (to about 5 hours), and on the other hand, does not significantly “reduce” the capacity of the tested battery (when discharged with a current of 1C, it may turn out to be 30% lower than low-current discharge).

The load is resistor R4 and LED HL1. The glow of the latter informs that the battery is being discharged and the level of 7 V has not yet been reached. Since the nominal current through the AL307BM LED is 10 mA, the "excess" of the stabilized current (5 mA) flows through the resistor R4.

If more discharge current is needed, the device is supplemented with a transistor VT2 with a resistor R6 (shown in dashed lines). The current through this circuit will be stable, since the voltage at the base of the transistor is almost constant (it is known that the forward voltage drop across the LED does not change much in the region of operating currents). The current in the emitter circuit (and hence the collector) is calculated by the formula I \u0,6d (U - 6) / R. Here U is the voltage at the base of the transistor, V; R is the resistance of the resistor R0,6, Ohm; I - collector current, A; 0,6 - approximate value of the voltage drop at the emitter junction of the transistor (6 V). This formula is estimated, so the value of the discharge current must be clarified when setting up the device by selecting the resistor RXNUMX.

To eliminate possible failures, pin 4 ("Reset") is connected to the positive power rail. The input of the low-level comparator (pin 2) is used to turn on the discharge mode by touching the touch contact E1. Capacitor C1 is connected to the second input of the high-level comparator in order to reduce the likelihood of false alarms from impulse noise penetrating the power circuits.

A piezoelectric sound emitter HPM7AX from JL World (with a built-in generator) is connected to terminal 14 (collector of the timer discharge transistor), which gives a signal when the battery is discharged.

The details of the device are mounted on a printed circuit board, the drawing of which is shown in fig. 2. All parts are installed on it, except for the sound emitter HA1 and connector X1. The board is designed for the use of fixed MLT resistors, SP5-2 wire trimming resistor and KM capacitors. Resistors R2, R4, R5 are installed perpendicular to the board.

An additional regulated voltage source is required to establish the device. It is connected to the device instead of the battery and the voltage is set to 9,4 V. When touching the touch contact E1, the HL1 LED should light up. By selecting resistor R4, they ensure that the total current consumed by the device from an additional source becomes 20 mA. Then the voltage is lowered to 7 V and the voltage at pin 5 of the microcircuit is measured. The same voltage is set with a tuning resistor R3 at its output 6. After that, the device is ready for operation.

In a device with an additional transistor, resistor R6 is selected so that the total discharge current becomes equal to the required value (if VT2 is used without a heat sink, it should not exceed 150 mA). It should be noted that with a collector current of more than 100 mA, the transistor VT2 heats up noticeably. This leads to a change in the base-emitter voltage, and it affects the value of the stabilized current (the value of 0,6 changes in the above formula). Therefore, the discharge current should be set no earlier than 3 ... 4 minutes after the supply voltage is applied. This does not affect the operation of the device in the future, since the “run-out” of the collector current of the transistor VT2 during heating does not exceed a few milliamps and it lasts about 3 minutes.

Then a control experiment is carried out. Turning on the power and setting (according to the voltmeter) at the output of the additional source a voltage of 9 ... 10 V, touch the E1 contact. In this case, the HL1 LED lights up. Then, gradually reducing the output voltage of the additional source, register the value at which the LED went out and an audible signal appeared. If it differs from 7 V, adjust the voltage at the input of the upper level comparator with a tuning resistor R3. At the end of discharging, the device consumes a current of about 5 mA from the battery.

The change in voltage at pin 7 of the microcircuit can be used to disconnect the battery under test from the device at the end of the discharge, as well as to control the timer, which fixes the time of its discharge.

Those who want to get deeper into the issues of battery operation can be recommended to search the libraries for a book [1], as well as visit sites [2 - 5].

Literature

1. Tenkov VV, Center BI Fundamentals of the theory and operation of sealed nickel-cadmium batteries. - Leningrad: Energoatomizdat, 1985.
2. battery.rax.ru
3. landdata.ru/kip
4. cadex.com
5. cc.columbia.edu/~fuat/cuarc/NiCd.html

Author: B.Stepanov, Moscow

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