A device that measures battery capacity. Instruments for testing lead-acid batteries. What is the error of Coulomb measurement

The article provides a diagram of a car battery capacity meter. The basis of the circuit is a microcontroller PIC16F873A. All information is displayed on an LED indicator with a common cathode.

In general, I composed this diagram and program at the urgent request of one of the site visitors a long time ago, but this insistent visitor suddenly disappeared somewhere. That’s why I post everything for everyone.

In principle, the circuit consists of already tested working fragments from different devices, so I did not implement this device into hardware. The operation of the meter was simulated in PROTEUS 7.7 SP2.

Circuit operation

On transistor VT1 and op-amp DA1.1 - LM358N, an electronic load equivalent is assembled with stabilization of the incoming discharge current of the battery under test.

The discharge current level is set using trimming resistor R5. Low-resistance resistor R7 is a current sensor for the DA1.1 amplifier, and the signal for the microcontroller ADC - a digital ammeter - is also taken from it. A comparator for limiting the battery discharge voltage is assembled at op-amp DA1.2. The controlled voltage from the discharged battery through the voltage divider R8 and R9 is supplied to the inverting input of the op-amp DA1.2. The division coefficient of this divider is 1:10, the same voltage through switch SA1, pins 1-3 is supplied for digitization to the RA1 input of the DD1 microcontroller. This is a digital voltmeter. The non-inverting input of op-amp DA1.2 is supplied with a reference voltage from the divider R2 and R3. Resistor R9 adjusts the readings digital voltmeter. Resistor R3 sets the battery discharge limitation voltage. The magnitude of this voltage can be viewed by moving switch SA1 to the lower position according to the diagram. Transistor VT2 is switching amplifier sound signal for the end of battery discharge. By changing the value of resistor R13, you can change the sound volume of loudspeaker BA1. The DA2 chip is a microcontroller supply voltage stabilizer, and since the controller supply voltage is selected as the reference voltage when digitizing signals in the program, the value of this voltage must be adjusted by resistor R11 at 5.12V. LED HL1 is an indicator of the end of the measurement process.

Device setup

Without inserting the programmed microcontroller, we supply power to the correctly assembled device. Using resistor R11 we set the voltage at the stabilizer output to 5.12 volts. Remove the supply voltage from the board and insert the microcontroller. We move switch SA1 to the upper position, turns off the collector of transistor VT1, and applies a control voltage of 12 volts to the battery connection connector. We achieve the same reading on the voltmeter indicator using resistor R9. We move switch SA1 to the lower position and set the discharge limitation voltage, for example, 10.5 volts. In this case, the voltage at the output of op-amp DA1.2 should be equal to zero. We begin to smoothly reduce the control voltage and around 10.5 volts the comparator should operate, and the voltage at its output should increase to approximately five volts (logical unit). The controller will record this unit and give an intermittent sound signal, signaling the end of battery capacity measurement. At the same time, the HL1 LED will light up.

We include a control ammeter in the battery discharge circuit, set the required current (the discharge current of car batteries is chosen in accordance with the formula C/10, where C is the battery capacity) of the discharge with resistor R5 and compare our readings with the control ones. The accuracy of our ammeter mainly depends on the accuracy of the value of the current sensor resistor R7. If the readings are too high, then the value of resistor R7 will need to be reduced.

Working with the device.

Take a fully charged battery and connect it to the device. The countdown of the discharge time begins immediately. On the left indicator according to the diagram we will see the value of the discharge current, on the middle one - the voltage on the discharged battery, provided that SA1 is in the upper position. The right indicator will display the current capacity values ​​over time. Capacity is determined accurate to tenths. It follows from this that the capacity readings will change every 6 minutes. After the voltage on the battery decreases to the limit you have chosen, the LED will light up and a signal will sound. The controller will record the measured capacity, but the discharge process will not stop, keep this in mind.

The number of auxiliary equipment and tools used in service centers, large ATP. Not only new tools are appearing, but also electronics that significantly facilitate diagnostics. Developed by talented engineers, the battery tester allows you to determine the performance of the power source and the remaining resource in a matter of seconds. Appliances are used stable demand, because with their help the process of checking batteries is noticeably accelerated.

Features of measuring equipment

Note that the devices are compact and do not take up much space. Manufacturers provide detailed instructions, thanks to which it is easy to understand the algorithm of the product. A capacity tester equipped with options deserves a more careful study; the information presented will certainly help you make a quicker choice:

1. Let's start with the most common areas of application. When shipments from the manufacturer arrive at warehouse sites and stores, it becomes possible to instantly identify defective items. Regular recording of operating parameters during battery operation allows timely detection of deviations from optimal characteristics and determination of the approximate remaining life.
2. The equipment is equipped with USB, which makes it possible to create a large database. Processing the received signal takes only three seconds. This time is enough to remove information.
3. A modern, advanced battery capacity tester is capable of taking readings from the battery in different conditions with minimal error. temperature conditions. In this case, the capacity assessment is as accurate as possible, regardless of the temperature of the energy source.
4. The designers took care to create reliable protection against interference, so the degree of error in any conditions is minimal.

Justified expenses

This is not the first year that devices capable of quickly checking batteries have been sold. During this time, it was possible to collect user data indicating the undoubted benefits of electronics. Even in a small enterprise, a battery tester will pay for itself in six months. The product is indispensable if you have at least a dozen pieces of equipment in your garage.

How and why is battery capacity measured?

Charge Q, as the amount of electricity, is measured in coulombs (C), the electrical capacity of capacitors C is in farads, microfarads (μF), but for some reason it is measured not in farads, but in ampere-hours (milliamp-hours).

What would that mean? One ampere is a coulomb in one second; we know from a physics course that if an electric charge equal to 1 coulomb passes through a conductor in 1 second, then a current of 1 ampere flows through the conductor.

So what is an ampere hour then? Ampere-hour (Ah) is the battery capacity at which, based on a reduced current of 1 ampere, the battery will be discharged in 1 hour to the minimum permissible voltage.

1 ampere hour is 3600 coulombs. Suppose we want to obtain a bank of capacitors that is equivalent in discharge characteristics, albeit over a short section, to a 12-volt battery with a capacity of 55 ampere-hours. 55 amperes for an hour is 55 * 3600 coulombs.

Let's take a voltage change from 13 to 11 volts, then since Q = C (U1-U2), then C = 55 * 3600/2 = 99000 F. Almost 100 kilofarads equivalent electrical capacity car battery, if its discharge characteristics were the same as that of a capacitor.

There is a video on the Internet where six supercapacitors of 3000 F, 2.7 V each, connected in series, replace the starter battery of a car. It turns out 500 F at about 16 V.

Let's estimate what current and for how long such an assembly can produce. Let the operating range be taken again from 13 to 11 volts. For how long can you count on a current of 200 A (with a margin)? I = C(U1-U2)/t, then t = C(U1-U2)/I = 500*2/200 = 5 seconds. Enough to start the engine.

Content:

Determination of battery capacity. Physical meaning

The capacity of the battery determines the amount of time during which the battery can supply energy to the payload. Battery capacity is measured in ampere-hours. Herself physical unit shows that the battery capacity is the product of the battery discharge current (in amperes) and the battery discharge time (in hours).
The battery capacity is physical quantity, which, together with the battery voltage, determines the amount of energy that a fully charged battery can provide. The concepts of battery capacity and battery charge (charge) should not be confused. Capacity determines the potential of the battery, that is, the amount of time during which the battery can supply power to the load if the battery is fully charged.

The actual capacity of the battery is determined by several factors: the magnitude of the applied load, the temperature of the battery. The more load is applied, the faster the battery discharges. The lower the temperature, the less capacity the battery has. Battery capacity is a value that depends on the measurement method and conditions, so it must be considered in accordance with the technical documentation for the battery. Typically, the manufacturer specifies a long-term method for discharging the battery (within 20 hours) at room temperature (20 degrees).

Determining battery capacity using long-term discharge method

Standard laboratory method Determining the battery capacity is a method of long-term control discharge. First, the battery is fully charged and then discharged with a constant low current. At the same time, they keep track of battery discharge time. Battery capacity is calculated as the product of current and time. The complexity of the method lies in the need to maintain a constant value of the discharge current; special equipment is used for this.

A common way to measure battery capacity is to discharge the battery using a constant load. In this case, one or more car lamps, choosing a load based on 1/20 of the nominal capacity. Time is recorded using a regular clock. This method is inaccurate, since the battery voltage decreases during testing, and, consequently, the load current changes. You should also be wary of a complete (deep) discharge of the battery, this can lead to battery failure.

Another way to measure battery capacity is also based on the long-discharge method. In this case, use a special electronic circuit And Digital Watch, connected to the circuit. Such a diagram can be found on the pages of amateur radio magazines.

It can be assembled by an experienced radio amateur or a professional electronics engineer; for each battery you will have to select the required load resistance values ​​by calculation. The measurement is also carried out for 20 hours.

Determining battery capacity using a special electronic tester

To quickly determine the battery capacity, you can use special battery capacity testers. The operation of such devices is based on a series of special measurements. To determine the capacity, the tester sends several probe pulses to the connected battery. Having received the return signal, the tester recognizes them and, using a microprocessor, makes the necessary calculations of the battery capacity. The result obtained is displayed on the electronic display of the device.

One of such devices is the SKAT-T-AUTO battery capacity tester.

The SKAT-T-AUTO battery capacity tester is a fully automatic device and does not require special knowledge to carry out measurements. The tester is designed for quick assessment technical condition sealed and non-sealed lead-acid batteries with a rated voltage of 12 V and a rated capacity of 1.0 to 120 Ah.

The battery capacity tester allows you to determine the battery capacity with the accuracy required for battery operation in just 15 seconds. Working with the device is very simple. You need to disconnect the battery from the device in which it is installed, connect it to the tester using special clamps and press just one button.

After determining the residual capacity of the battery, it is compared with the nominal capacity of the new battery indicated in the product passport. If the residual capacity of the battery is less than 50%, then it must be taken out of service and the battery must be restored or replaced.

It is no secret that over time, the capacity of rechargeable batteries becomes smaller, and they can no longer supply the device with the amount of current that they could supply before. In this regard, many users often have a question about how to measure the battery capacity, or more precisely, how to find out the indicator of its residual potential, with which you can understand whether the battery will need to be replaced in the near future.

If you start from the concept that the capacity indicator is the amount of energy or current given by the battery over a certain period of time, it simply will not work. If we're talking about on how to find out the real battery capacity in the form AA batteries, here you will first have to measure the current, and then use some simple calculations - so that the indicator is as accurate as possible. As for any Android-based mobile phone, using a small USB tester.

A simple test of battery capacity with a USB tester and subsequent clarification

The USB tester for measuring battery capacity has very rich functionality - it can be used to measure the battery capacity of a tablet, smartphone, etc. Based on what this device shows, you can get an idea about the wear of the batteries: is it worth changing the battery, or do you need to purchase a new device.

With just one control button, you can measure various indicators, including the battery capacity of a particular device. The button switches tester memory cells and operating modes. If there is a sufficient voltage level in the connected device, the tester turns on.

The meter usually displays the battery capacity indicator in the lower left corner. The measurement accuracy of the tester is not 100% , and therefore it is recommended to use a simple mathematical formula based on the following example.

Let's say you have a certain device (phone, power bank or tablet), which, being completely discharged, charged in a period of time equal to 3 hours. If, out of curiosity, you measured the current using a tester, and its value was, for example, 1.15 A, the actual battery capacity of your device is calculated by multiplying these two numbers together. 1.15 amperes is 1150 milliamps, multiply this number by 3 and get 3450 mAh. This is how real capacitance is measured. If the current “capacity” on your device indicated by the manufacturer is several times greater than the actual one, this is just a standard advertising ploy that should not be trusted.

How to determine battery capacity using a multimeter

It is impossible to determine the battery capacity itself with a multimeter. To be more precise, this device will help in determining the actual capacity indicators.

In order to find out the capacity of the 18650 battery, as well as other batteries, so-called “smart charging device" But their cost is quite high. It’s not worth buying such chargers simply to determine the capacity of a couple of batteries. This indicator will easily determine the usual calculation method with the preliminary use of a multimeter. However, when making calculations, certain subtleties must be observed.

Checking the battery capacity indicator with a multimeter is not just measuring its real indicator, the calculations of which are made using basic mathematical calculations. It is necessary to measure the current level supplied by the battery (any battery) and calculate exact amount time during which the battery could continuously and efficiently produce electrochemical energy. It is important to remember that all measurements will not be 100% accurate. But they are the ones that best reflect the true essence of the matter.

They have their own scale of ranks. It shows how much U depends on the charge rate. This is necessary to know: the current flow through the resistance depends on the voltage level. To ensure that this dependence does not affect the measurements, an additional device should be assembled - linear current stabilizer (2.7-3 volts).

Using a linear stabilizer

Using this stabilizer, set the current indicator, calculating it from a 2.7 volt U battery. Then, using the U stabilizer, connect any resistor device (it can be made independently or purchased from an online store). Measure the current that passes through the circuit and set a stopwatch. Next, we periodically check and monitor the voltage at the battery terminals. When it reaches the number 2.7 volts, the stopwatch will need to be quickly turned off and the time obtained recorded.

So, how to measure the capacity of a 18650 battery and other chemical current sources? Real indicator mwe deduce by doing multiplying the current flowing through the circuit through resistance by the same time (in hours) that was originally spent . This is the most precise measurement containers. In the absence of technical capabilities, it will be more difficult to design a voltage stabilizer, carry out calculations and measurements. Try to find a way out of the situation using a variable resistor.

Using a Variable Resistor

In order to conduct a high-quality capacity test, you can use a smaller battery. For example, 14500, the actual capacity of which is 300 mAh. Let's take a variable resistor of 100 Ohms. Important point: if a resistor is used direct current, the process will be complicated by the fact that it will be necessary to frequently record the results of its readings and carry out calculations of the spent capacity for certain sections of the scale .

It is possible to average the indicators as much as possible, focusing the calculation on the “arithmetic average” number of the current. In order to understand how to measure the battery capacity, it is recommended to use a variable resistor with a dosed decrease in the resistance value throughout the entire time the battery is being discharged. It is important that the current level is approximately the same throughout the entire process.

Now switch the multimeter to the voltmeter position (measure U) and measure U at the terminals of your battery. Let's say it has an incomplete charge level, say 4 volts. Next, discharge it by applying a current of 450-500 milliamps, lowering the resistance level from time to time and controlling the voltage. When it drops to 2.7 volts, turn off the stopwatch. To completely discharge the battery using 500 mA current , it takes about half an hour, more precisely, 25 minutes. Now let's multiply this current by the amount of time measured in hours. So, the real capacity indicator is 200 mAh.

Thus, it becomes clear how to find out the battery capacity using the most accurate method - not just by measurements, but by mathematical calculations that can most accurately reflect the actual state of the battery and help the user navigate what its potential is in reality.