Design and implementation of automatic tester for rechargeable battery capacity

With the development trend of miniaturization and portability of electronic products , rechargeable batteries are more and more widely used. There are many kinds of batteries on the market, and the quality varies. It is very common to falsely label the battery capacity now, and only a small number of regular manufacturers adopt a realistic attitude. I have seen a mobile phone battery with a capacity of 8000mAH, but the actual capacity can only reach one or two percent of the labeled capacity. With the current technology, it is not possible to achieve such a capacity with the volume of mobile phone batteries, and I am afraid that it may not be realized in the future. With more and more small electronic products, everyone will have more and more demand for batteries, and a large number of rechargeable batteries will be accumulated on hand. A high-quality battery can be used for more than 4 or 5 years, while a low-quality battery is very poor and easy to damage. In view of this situation, it is also necessary for personal use to establish a complete evaluation system as a guide for long-term purchase and use.

Battery capacity is an important indicator of battery quality. There are many ways to test the capacity of rechargeable batteries . According to the discharge curve of the battery, a short-term discharge can be performed to roughly determine the battery capacity. The biggest advantage of this method is that it is fast, but the discharge curve of rechargeable batteries is not universal. The voltage of many inferior batteries is also very stable at the beginning of discharge. Once it enters the middle and late stages, the voltage drops very quickly, so the conclusion drawn by this method will be very inaccurate. The most reliable and accurate method is to discharge with standard current and measure the actual discharge time throughout the process. Different discharge currents can ultimately release different amounts of electricity from rechargeable batteries, and there is a certain gap. The capacity marking of batteries is standardized. The most commonly used are the 10-hour rate discharge capacity and the 20-hour rate discharge capacity. The 10-hour rate discharge capacity means that the battery is discharged at a constant current, and the discharge time can be maintained for about 10 hours until the power is exhausted. This current is called the 10-hour rate current (the standard for measuring the exhaustion of power cannot be based on the voltage at the battery discharge end dropping to zero. Over-discharging of the battery will lead to a reduction in battery capacity, which cannot be restored, or even premature damage and complete failure. Therefore, there are strict regulations on the discharge termination voltage of each battery, which can be found in relevant information. Over-discharge and over-charging are the main reasons for the rechargeable battery not reaching its service life and being scrapped in advance). The biggest disadvantage of the real-time discharge measurement method is that it is time-consuming and labor-intensive. Because it takes a long time, the measurement accuracy is also easily affected by various external factors. During the measurement process, if the 10-hour rate current is used for continuous discharge for at least 5 hours, such a long test requires sufficient patience, energy and ample time. The development of science and technology is very rapid, and today single-chip microcomputers are very popular. Through the single-chip microcomputer program control of the discharge time, deep automatic control, it is easy to accurately measure the actual capacity of the battery and realize automatic control of the entire process. Although the method of simulating actual discharge to measure capacity is a bit wasteful of energy, it is still completely feasible for small-capacity rechargeable batteries below 1A and 2A. It is also necessary to conduct sampling inspections on large-capacity batteries.

The battery capacity tester introduced below uses 89S51 as the control chip. Figure 1 is the circuit schematic of the hardware.

Figure 1 Hardware circuit diagram

This battery capacity tester is composed of two completely independent parts: the discharge circuit and the single-chip microcomputer control timing. The single-chip microcomputer part is time-consuming and labor-intensive to make, and single-chip microcomputers are very popular on the market, so there is no need to make it yourself. Just find a 51 single-chip microcomputer experimental board. The discharge circuit is relatively simple, consisting of only four or five components . The single-chip microcomputer part is mainly responsible for timing the discharge time, and finally obtains a set of reliable data for battery performance considerations.

The essence of this discharge circuit is a simulated thyristor . When we connect the battery to be tested to the corresponding position of the circuit, press the start button. If the battery still has a surplus, the discharge voltage at both ends of the battery will be maintained above the set value, and the transistor VT1 will be saturated instantly, and the battery will be discharged through the resistor R2. This circuit has reliable, accurate and steep switching characteristics, and VT1 absolutely works in two states of saturation and cutoff. By adjusting and setting the critical value of the switch circuit (i.e., the discharge termination voltage of the rechargeable battery) through the adjustable resistor, it can be adapted to the full-process protection discharge of various types of rechargeable batteries. Since personal applications do not require very accurate test results, in principle, it is better to simulate the discharge of the battery faster in actual testing, and only an approximate battery capacity is needed. In order to complete the battery test process faster, the circuit design here uses a two-hour rate current for discharge. Through the horizontal comparison of the measurement results of various batteries, the difference in capacity is still obvious, and it is sufficient to use this as a standard for measuring the quality of the battery. Here we take the test of 1000mAH, 1.2V NiMH battery as an example. A discharge current of 500mA requires a 2Ω discharge resistor, and the battery termination discharge voltage should be controlled above 1V. The discharge termination voltage is adjusted and set by the adjustable resistor R1. Ordinary adjustable resistors have poor accuracy and are prone to drift, which will cause the set termination voltage to fluctuate greatly over time and with changes in the use environment. In order to ensure that the discharge termination voltage is accurate and easy to set, R1 can use the 3296 series precision adjustable potentiometer. The adjustable range of the 3296 multi-turn adjustable precision potentiometer is generally 50T, so the adjustment range of each turn is 2%. For each degree of rotation, the resistance value changes by about 0.005%, so it is easy to adjust to obtain an accurate and stable resistance value.

The setting of the termination voltage must be carried out during the actual discharge process. If the resistance value of the load resistor R2 changes, the set termination voltage will also change and need to be reset. The specific debugging method will not be described in detail, please refer to the relevant information.

This discharge circuit does not require a separate power supply and has no correlation with the type of battery. It can fully adapt to the protective discharge of various types of batteries such as cadmium nickel, nickel metal hydride, lithium batteries, and lead-acid batteries. It only needs to reset the circuit's termination voltage and discharge current according to the battery type and capacity. If the battery capacity is relatively high, the dissipated power of transistors VT1 and VT2 should also be increased accordingly, and don't forget to increase the power of load resistor R2.

Figure 2 is a printed circuit diagram of the discharge circuit, which has a small number of components and is easy to manufacture.

Figure 2 Printed circuit diagram

The discharge time that various batteries can maintain at two-hour current discharge is generally less than 1.5 hours. The microcontroller timing system here uses seconds for timing and 4-digit LED digital tubes for display . The maximum timing time is 9999 seconds, about 2.7 hours.

Figure 1 shows that a single LED digital tube is composed of 8 light-emitting diodes, which are used as the 7-segment font of 8 and a decimal point. A common anode digital tube is used here, and the anodes of the 8 light-emitting diodes are connected in parallel and led out together as an enable control.

In the actual circuit, L1 is the common anode terminal of the first digital tube. The number of output and input interfaces of the single-chip microcomputer is very limited, so the 4-bit LED digital tube driver uses dynamic display. The corresponding cathodes of the 8-segment light-emitting tubes a, b, c, d, e, f, g, dp in the 4 independent digital tubes LED are connected in parallel. They are uniformly driven by the 8-bit output of the single-chip microcomputer P0 port. In order for the digital tube to display the digital, a positive voltage must be applied to the common anode terminal at the same time. Therefore, to make a digital tube among the 4 digits display, just add a positive voltage to the common anode terminal of this digital tube while outputting the font code at the P0 port. Of course, at the same time, the common anode terminals of the other three digital tubes must maintain a low voltage to avoid confusion in the display. The common anode terminal driving current of the digital tube is large, so a triode is used for control. Taking the first digital tube as an example, when the P0 port outputs the font code, P37 outputs a low level, the transistor T4 is turned on, and the common anode terminal L1 obtains a high level, and the number will be displayed on the first digital tube.

The program design uses the P37 port of the single-chip microcomputer as the timing control terminal. The P37 port inputs a low level, the timing program starts, and the 4 digital tubes display the time. Press the start button in the discharge circuit, the discharge process is triggered, VT1 is turned on, the battery terminal voltage drops to both ends of the discharge resistor R2, and the A terminal is high level to the ground. The transistor VT3 is forced to turn on through the resistor R4, and the P37 port level is pulled down, and the single-chip microcomputer timing program starts. After the battery voltage drops to the termination voltage, the discharge circuit is automatically closed, the voltage at the A terminal disappears, VT3 returns to the cut-off state, the timing program stops, and the digital tube continues to display the current duration.

If you want to enter the next test, first press the microcontroller reset button to clear the current timer and wait for the next test to start.

The program design is relatively simple. Its general process is as follows: initialization, P3 port is set, constant a is set as the time counter, the decimal value of a is extracted in turn, and the decimal value of a is transmitted to the P0 port in sequence. P24, P25, P26, and P27 in the P2 port are used as the enable control terminals of the four-digit digital tube in turn. Through the cooperation of the P2 port, the drive of each digital tube can be completed, and the dynamic display of time can be completed. During the program, the value of the P3 port must be continuously detected to determine the timing state: if the battery is in the discharge process, the transistor VT3 is turned on, which will force the voltage of the P37 port to drop to zero, and the value of the P3 port is 127. The microcontroller program detects this result, and the time constant a will automatically add 1, indicating that the discharge time has continued for 1 second. It is more troublesome to accurately calculate this 1 second of time. The timing program is a loop structure, and the time consumed in each cycle is the same. Therefore, in the process of debugging using the keil software, by observing and calculating the time counting register sec, the approximate time required for a cycle can be obtained. Based on this, the constant a timing cycle is controlled to less than 1 second by appropriately changing the number of delay subroutine loops. The remaining small time difference can be corrected by interpolating empty instructions. The timing accuracy only needs to be controlled below one thousandth. When the 51 microcontroller uses an 11.0592MHz crystal oscillator, the instruction cycle is about 1.085 microseconds, so it is not a big problem to control the timing accuracy below one thousandth. There will always be errors, which can only be controlled by precise calculations. The timing accuracy can also be further improved by replacing a higher frequency crystal oscillator to increase the microcontroller clock frequency. If the discharge process is terminated by accident or artificially, the P37 port becomes a high level, and the program loop will continue, but the time constant a stops automatically adding one, and the time display remains unchanged.

After compilation, write it into the microcontroller, connect the discharge circuit part with the 51 microcontroller, and then it can be put into use.

After the battery is connected, press the touch button "Start" to enter a capacity test process. During this period, the removal and connection of the battery will not affect the timing of the MCU. After the battery is discharged, the MCU digital tube display is locked and the total discharge duration is given in seconds. The number of discharge hours can be calculated manually. Of course, you can also improve the program by yourself and display it directly in hours and minutes. As long as the MCU is not powered off, the digital tube will continue to display the current discharge duration. If you want to enter the next measurement process, just press the reset button of the MCU, the digital tube will be cleared, and the MCU program will enter the starting point, and you can enter a new capacity test process.

If a rechargeable battery is idle for a long time, its actual capacity will be affected. The capacity that can be released for the first time when it is reactivated is far from the marked capacity, and the discharge voltage is also very unstable. It takes at least three charge and discharge cycles for the battery to be fully activated and the capacity to be restored to the expected level. Taking this factor into full consideration, the capacity test generally adopts the method of multiple averages, or the discharge duration after three cycles of charge and discharge is used as the basis, which is the appropriate way to measure the battery capacity.

#include "reg51.h"

char

code disp［]={40, 235, 50, 162, 225, 164, 36, 234, 32, 160};

//Glyph code

void delay(unsigned int dt)

{ unsigned int j=0;

for (; dt>0; dt--)

{ for (j=0;j<125;j++)

{;}

}

}

void main()

{ int a, b, c, led1, led2, led3, led4;

P3=255;

a=0;

for(;;)

{b=a;

led1=b%10;

P2=239;

P0=disp［led1］;

delay(6);

P2=255;

b=b/10;

led2=b%10;

P2=223;

P0=disp［led2］;

delay(6);

P2=255;

b=b/10;

led3=b%10;

P2=191;

P0=disp［led3］;

delay(6);

P2=255;

b=b/10;

led4=b%10;

P2=127;

P0=disp［led4］;

delay(6);

P2=255;

for（c=44;c>0;c--）

{

P2=239;

P0=disp［led1］;

delay(5);

P2=255;

P2=223;

P0=disp［led2］;

delay(5);

P2=255;

P2=191;

P0=disp［led3］;

delay(5);

P2=255;

P2=127;

P0=disp［led4］;

delay(5);

P2=255;

}

if (P3 == 127)

delay(3);

if (P3 == 127)

a=a+1;

else a=a;

}

}