NiMH No.7 batteries

Analyze the technical principles of NiMH No.7 batteries capacity meter

With the gradual strengthening of environmental awareness, countries around the world are competing to develop environmentally friendly vehicles, and my country is also investing in the development of NiMH No.7 batteries-powered electric vehicles. The indispensable instrument for electric vehicles is the NiMH No.7 batteries capacity meter, which is just like the fuel gauge of an ordinary car. It tells the user how much capacity the NiMH No.7 batteries has left and how many kilometers it can travel. In fact, not only electric vehicles need NiMH No.7 batteries capacity meters, but also many occasions where batteries are used have urgent requirements for this. The traditional method of NiMH No.7 batteries monitoring is just a voltmeter, but the voltage cannot accurately reflect the NiMH No.7 batteries capacity. It often happens that the voltage is normal but the capacity is not. What users are often confused about is not knowing how long the NiMH No.7 batteries can last, which affects the use in many critical situations and is prone to misjudgment and accidents. Therefore, it is very necessary to develop an instrument that reflects the NiMH No.7 batteries capacity. At present, similar products have been launched abroad, but perhaps due to technical confidentiality reasons, no implementation method has been introduced.

This article takes electric vehicles as the object of use and proposes a NiMH No.7 batteries capacity meter implemented using the electricity measurement method, which can measure NiMH No.7 batteries capacity under certain conditions. It is based on the principle that the energy charged and released by the NiMH No.7 batteries is calculated and multiplied by the corresponding loss coefficient to indicate the capacity of the NiMH No.7 batteries (this coefficient should take into account the charging efficiency and NiMH No.7 batteries discharge current and other factors on the NiMH No.7 batteries capacity. Influence).

Basic principles of NiMH No.7 batteries capacity meter

In addition to some factors of the NiMH No.7 batteries itself, the capacity of the NiMH No.7 batteries mainly depends on the amount of charge and discharge. Obviously, if the charge and discharge of the NiMH No.7 batteries can always be recorded, the capacity can be measured. We envision equipping a traditional single NiMH No.7 batteries with an instrument called a NiMH No.7 batteries capacity meter to display capacity. The capacity meter dynamically monitors the total charge and discharge of the NiMH No.7 batteries and displays it visually after calculation. Other factors that affect NiMH No.7 batteries capacity are combined into a loss coefficient, and the arithmetic sum of this coefficient multiplied by the charge and discharge amounts is the remaining NiMH No.7 batteries capacity. Since the type, size, and performance of batteries are different, the loss coefficient is different, which is mainly obtained through experiments. Therefore, the coefficient issue is not discussed here, and only the circuit that completes the function of measuring power is studied.

There are many ways to charge and discharge batteries, such as constant current, voltage limiting, pulse, negative pulse, etc. Therefore, the method of simply measuring the capacity by multiplying the current by the time cannot be adapted to other methods except constant current, and the integral method cannot be adapted to negative pressure. Pulse charging is required, and it requires time parameters, which is not suitable. Obviously the NiMH No.7 batteries capacity meter should be designed to accommodate most charging and discharging methods. Regardless of the charging method, the key parameters that affect NiMH No.7 batteries capacity are current and time. In the case of negative pulse charging, there is only negative current at the same time. To this end, we designed a NiMH No.7 batteries capacity meter circuit that works as follows. The principle block diagram is shown in Figure 1.

First, monitor the charging and discharging current of the NiMH No.7 batteries, convert it into a voltage signal, amplify it, send it to a voltage-frequency converter to turn it into a frequency signal, and finally send it to a counter to record the number of pulses, and display the count value in a certain way. It constitutes a NiMH No.7 batteries capacity meter. In fact, the frequency represents the size of the current. The larger the current, the higher the frequency, and the more pulses can be recorded at the same time, and vice versa. The charging and discharging time is also reflected in the counting of pulses. The longer the time, the greater the number of counts. In this way, the calculation of the NiMH No.7 batteries charge and discharge capacity is completed using the counting method.

The combination of the absolute value amplifier and the reversible counter realizes the measurement of the discharge gap during charging (ie, negative pulse charging), and at the same time uses a set of circuits to complete the calculation of both charging and discharging directions. It counts forward when charging and counts reversely (downward) when discharging. The direction of current flow is used to control the counting direction of the reversible counter.

Plan demonstration and key technical solutions

1 current sampling

The purpose of current sampling is to convert the current signal into a voltage signal. There are generally three methods:

(1) Sampling resistance;

(2) Diverter;

(3) Hall devices (including transformers).

From the perspective of the use of electric vehicle batteries, the current is large, so it is obviously not suitable to use a sampling resistor, and the shunt is too heavy and large, so it is not suitable, so Hall devices are more suitable. Its advantage is that the linearity is better than 0.1%, suitable for tracking in a larger range, has good dynamic performance, and the response time is less than 1μs, so that the instantaneous current of car starting can be tracked instantly. In addition, its small size and light weight make it suitable for installation in cars. Its disadvantage is that it is slightly expensive, but it is completely negligible in terms of the price of batteries used in cars. Due to the selection of mature products that can be purchased, the circuit is relatively simple and will not be listed here.

2 absolute value amplifiers

Due to the different directions of charge and discharge currents, an absolute value amplifier is used, which amplifies the positive and negative signals output by the Hall device into positive signals and then sends them to the voltage-to-frequency converter.

There are many design methods for absolute value amplifiers. From the power supply point of view, there are two methods: single power supply and dual power supply. The number of operational amplifiers used is one or more. Since this machine uses a Hall device and a bidirectional current, there is no advantage in a single power supply. However, a single op amp amplifier has too many resistor values, high accuracy requirements, and the load should also be considered, so it is not suitable.

This machine uses an absolute value amplifier composed of two operational amplifiers. The operational amplifier 0p-07 with low offset and low drift is selected. It has high accuracy and its performance is not affected by the load. The accuracy of the absolute value amplifier is required here, not to contribute to the system accuracy. But consider it from another point of view. This is what was mentioned earlier. As far as the NiMH No.7 batteries capacity meter is concerned, the best way to monitor the NiMH No.7 batteries should be to be integrated with the NiMH No.7 batteries and always monitor the NiMH No.7 batteries condition. This requires that when the NiMH No.7 batteries has no charging and discharging current, the output of the amplifier is zero. Otherwise, after long-term storage, the capacity meter will indicate full or discharge due to the error of the amplifier, resulting in misjudgment. The prototype designed with high precision, low offset and low drift has a full-scale error of 1mv and a zero-scale error of less than 1mv. See Figure 2.

3 Voltage to frequency converter

The voltage-to-frequency converter is the core part of the NiMH No.7 batteries capacity meter and is responsible for converting the amplified signal into a frequency signal. Its linearity and accuracy directly affect the entire machine. There are many ways to achieve voltage-to-frequency conversion. From a formal point of view, there are two forms: discrete components and special-purpose integrated chips. Generally, discrete components have higher accuracy, volume, and adjustment complexity than integrated chips, but their prices are lower, while special-purpose chips have better linearity, voltage stability, and Indicators such as accuracy are reduced relative to the acceptable price. Taking into account the issues of volume, full charge and discharge tracking, and performance-price ratio, we chose VFC32 as the voltage-frequency conversion device. The device's better linearity ensures full tracking accuracy and reduces the size with fewer components. The circuit principle is shown in Figure 3.

  Reversible counter

The counter part all uses CMOS circuits. First, it has low power consumption, which is extremely important for relying on the NiMH No.7 batteries itself for power supply; second, its level matches the op amp level and increases the display range. See Figure 4.

One piece of 14-level pulse carry binary counter 4020 and two pieces of 4-bit reversible binary counter 4516 are used to form a 21-level counter. Among them, the upper 7-bit counter value is valid as the counting value and output, while the lower 14-bits are only used for counting and not for output, and 4020 is a one-way counting without subtraction function.

This design has two major advantages:

(1) 4020 is a highly integrated counter that can be used instead of 3 and a half pieces of 4516, which greatly reduces the size.

(2) When used for addition, 4020 can be accurate to the lowest bit; when used for subtraction, the error is the lower fourteen bits, but this fourteen bits is also the maximum error at one time, and there is no accumulation, because the circuit uses asynchronous and synchronous Mixed methods of counting. When subtracting 14 numbers (although 4020 is adding), 4020 outputs an asynchronous pulse 4516 minus "1", just like real subtraction, and the value of 4020 cannot be output, which makes the result very accurate.

5 control circuit

This part includes preset circuit, anti-overflow circuit, and counting direction control circuit.

This prototype has a wide application range. A toggle switch is added to the preset and control circuits of the counter. In this way, the initial value and final value of the counting part can be set by the toggle switch, which can achieve the purpose of detecting the known NiMH No.7 batteries capacitance. Compare convenient.

At the same time, in order to prevent the counter from overflowing in both directions, an anti-overflow circuit is set up so that the counter will no longer count when it reaches zero or full value to prevent errors.

By comparing the current flow direction, the output pulse controls the counting of the reversible counter, forming a direction control circuit.

6 displays

There are two display modes: digital and pointer. In order to ensure an intuitive display and to use ordinary car instruments as much as possible, the original voltmeter on the car that indicates the NiMH No.7 batteries voltage is still used. It is more convenient to set a switch on the voltmeter to switch the voltage and capacity indications.

This requires converting the counter's binary number into a voltage. Obviously it is possible to use D/A conversion, but the complexity of the circuit increases and the cost also increases. Therefore, in order to simplify the circuit, we only borrow the idea of D/A conversion network and use the weighted resistor T-shaped network to convert the 7-bit value of 4516 into an analog output to drive the voltmeter indication, see Figure 5.

Figure 5 shows the circuit schematic

7 Working power supply part

The NiMH No.7 batteries capacity meter is different from other instruments in that it can only use batteries as a power source. Due to changes and fluctuations in NiMH No.7 batteries voltage, direct use is obviously inappropriate. For this reason, a secondary power source must be generated from the NiMH No.7 batteries.

First of all, the Hall device requires a power supply of ±12V, and the circuit control and counting parts also use ±12V. In addition, we consider that in order to make the capacity indication more intuitive and clear, its maximum voltage range should be larger, and at the same time, it can also make full use of its voltmeter for effective indication. . The voltmeter range is 40V, and the maximum NiMH No.7 batteries voltage is 30V, so the maximum capacity indicator is set to 28V, which requires a power supply voltage of 30V.

Due to the large current discharge when the NiMH No.7 batteries is started, the voltage fluctuation is very severe, about 15~30V. In order to adapt to the changes, reduce the power consumption of the capacity meter itself and improve efficiency, all designs use switching power supplies.

First of all, +12V is obtained by using the LM2575 step-down regulator. The input voltage of this chip can reach 40V, the fixed oscillation frequency is 52kHz, and the voltage and current adjustment rates are good, adapting to the requirements of the capacity meter.

[page]-12V is converted by using +12V as input and converted by 34063DC/DC converter. Some power is lost in this way. Our original design used M2575HV (input voltage 60V) to be directly introduced from the NiMH No.7 batteries voltage, but because the 60V LM2575HV could not be purchased, we had to give up. If there is a batch in the future, we can place an order. Fortunately, -12V power is limited and the loss is small. A set of 30V power supply has high voltage and small current. If an ordinary DC converter such as 2575 or other devices is used, the volume is too large, and the magnetic core components are greatly wasted, which is not worth the gain. Therefore, we have been looking for simple methods in the design, and finally decided to use the 555 oscillator to boost the voltage and use the voltage doubler rectification method to increase the 12V to 30V. The effect is excellent, see Figure 6.

Product design and calculation

Setting of voltage/frequency relationship

Voltage 0~10V corresponds to frequency 0~10kHz

Current 0~1000A corresponds to voltage 0~10V

The selection of these values takes into account the optimal values of the Hall element, amplifier, F/V conversion design and the needs of the test prototype.

2 counting digits

4020-14-bit 4516 has two chips with a total of 8 bits. The total is 22 bits. Only 21 bits are used. The counting number is:

221=2.097152×106.

Counting time for 10kHz

T=(221×1/104) seconds=3.49 minutes.

When 10kHz corresponds to 1000A, for a 45Ah NiMH No.7 batteries

T=C/I=45/1000=0.045h=2.7 points<3.49 points,

It can be seen that the timing is enough, and the full-scale timing ampere hours is

(221×1/104)×1000/3600=58.25Ah.

3 Calculation of error

The counting time of the first 14 levels is △T=214, the total time is T=221, and the relative error △T/T=214/221=0.78%.

It can be seen that the errors in the first 14 levels are extremely small, less than 1%, and they only appear once during subtraction and can be ignored. Therefore, it is reasonable to use one piece of 4020 instead of three pieces of 4516.

Performance test results

The whole machine was tested under the condition that the charging and discharging current is 15A, the voltage (representing the capacity) indicates the full capacity is 28.002V, after the NiMH No.7 batteries capacity is exhausted, the voltage (representing the capacity) indicates 0V, and the error between the indicated capacity and the actual capacity is 3%, which is in line with the design. Require.

In the mode where the output capacity is equal to the input capacity multiplied by the loss coefficient, this article takes electric vehicles as the object of use, and makes an in-depth and detailed explanation of the input sampling, absolute value amplification, voltage-to-frequency conversion, display and working power supply, and makes a very detailed explanation. Useful exploration is currently one of the effective methods for measuring NiMH No.7 batteries capacity, and is suitable for batteries with no memory effect and relatively stable performance.

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