Measures battery capacity with his own hands. Car battery capacity meter

The article provides a diagram of a meter for the capacity of car batteries. The basis of the circuit is a microcontroller PIC16F873A... All information is displayed on a common cathode LED.

In general, I have composed this scheme and program at the urgent request of one of the site visitors for a long time, but this urgent visitor suddenly disappeared somewhere. Therefore, I spread everything for everyone.

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

Circuit operation

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

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

Setting up the device

Without inserting the programmed microcontroller, we supply power to the properly assembled device. With resistor R11, we set a voltage of 5.12 volts at the output of the stabilizer. We remove the supply voltage from the board and insert the microcontroller. We move the SA1 switch to the upper position, turn off the collector of the VT1 transistor, apply a control voltage of 12 volts to the battery connection connector. We achieve the same reading on the voltmeter indicator using the resistor R9. We put the SA1 switch in the down position, and set the discharge limit voltage, for example, 10.5 volts. In this case, the voltage at the output of OA DA1.2 should be zero. We begin to smoothly reduce the control voltage and in the region of 10.5 volts the comparator should work, while at its output the voltage should increase to about five volts (logical unit). This unit will be fixed by the controller and will give an intermittent sound signal, signaling the end of the battery capacity measurement. At the same time, the HL1 LED will light up.

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

Working with the device.

We take a fully charged battery and connect it to the device. The discharge time starts 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 show the current capacitance values \u200b\u200bover time. The capacity is determined to the nearest tenths. From this it follows that the capacitance readings will change every 6 minutes. After the voltage on the battery decreases to the limit you selected, the LED will light up, a signal will sound. The controller will fix the measured capacity, but the discharge process will not stop, keep this in mind.

Content:

Determination of battery capacity. Physical sense

The capacity of the battery determines the amount of time that the battery can supply power to the payload. Battery capacity is measured in ampere hours. The physical unit itself shows that the capacity of the battery is the product of the battery discharge current (in amperes) by the battery discharge time (in hours).
Battery capacity is a physical quantity that, together with the battery voltage, determines the amount of energy a fully charged battery pack can provide. Do not confuse the concepts of battery capacity and battery charge (charge). The capacity determines the potential of the battery, that is, the amount of time during which the battery can provide 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 will discharge. The lower the temperature, the less capacity the battery has. The battery capacity is a value that depends on the method and conditions of measurement, therefore it must be considered in accordance with the technical documentation for the battery. Typically, the manufacturer specifies a long way to discharge the battery (within 20 hours) at room temperature (20 degrees).

Determination of battery capacity by long-term discharge method

The standard laboratory method for determining battery capacity is the continuous test discharge method. At the beginning, the battery is fully charged and then discharged with a constant low current. At the same time, the battery discharge time is recorded. Battery capacity is calculated as the product of current strength and time. The complexity of the method lies in the need to maintain a constant value of the discharge current; for this, special equipment is used.

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 the load at the rate of 1/20 of the value of the nominal capacity. Time is counted on a regular clock. This method is inaccurate, since the battery voltage decreases during testing, and, therefore, the load current changes. You should also beware of the complete (deep) discharge of the battery, this can lead to battery damage.

Another way to measure the battery capacity is also based on the long-term discharge method. In this case, a special electronic circuit and digital Watchconnected to the circuit. Such a diagram can be found on the pages of amateur radio magazines.

An experienced radio amateur or a professional electronics engineer can assemble it; for each battery, you will have to calculate the necessary load resistance values \u200b\u200bby calculation. The measurement is also carried out within 20 hours.

Determination of battery capacity using a special electronic tester

For a quick determination of the battery capacity, special battery capacity testers can be used. The operation of such devices is based on a series of special measurements. To determine the capacity, the tester sends several probing pulses to the connected battery. Having received a feedback signal, the tester recognizes them and, using a microprocessor, makes the necessary calculations of the battery capacity. The result 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 that does not require any special knowledge to perform measurements. The tester is designed for quick assessment of the technical condition of sealed and unsealed lead-acid batteries with a nominal voltage of 12 V and a nominal 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. The operation of 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 refurbished or replaced.

Batteries are used in many ways everyday life people: vehicles, power tools, uninterruptible power systems, smartphones, laptops, etc.

General information about battery capacity

The main purpose of checking activities for the state of any type of battery is to find out the battery capacity and determine other characteristics. However, existing measuring instruments can only accurately determine the strength of the electric current and the voltage in the storage battery, as well as measure the density of the electrolyte substance.

The capacity is measured indirectly according to a method specific for each type of battery, or using a device for measuring the capacity of a battery, which gives only an approximate result.

Important! External factors such as ambient temperature can affect the accuracy of any measurement in a battery.

The only reliable way to determine the capacity of a battery is to completely discharge it for many hours, accompanied by constant fixation of many parameters. But not every person is ready to go through such a long procedure, because short-term measurements may be enough to establish approximate data on the battery capacity.

Methods for determining the capacity of the car battery:

• the traditional method is a control discharge (a long and voluminous process in terms of procedures);
• measurement of the density and level of electrolyte liquid in the car accumulator;
• by exposing the battery to the load plug;
• capacity tester.

Interesting. The capacity of popular lithium-ion, nickel-cadmium and nickel-metal hydride batteries can be measured with the same control discharge (the battery can fail if all the rules are not followed) or by purchasing special USB testers on Chinese trading platforms, the accuracy and correctness of measurements of which are under great a question.

Control discharge

Long term test discharge is a traditional laboratory method for establishing battery capacity. The essence of the method is that a fully charged battery is discharged by exposure to constant electric currents, the strength of which depends on the parameters of the product.

Meanwhile, hourly measurements of the battery discharge and voltage are taken, which are recorded. The capacity of the battery is calculated by the formula: the product of the electric current strength by the elapsed specific time. Such a measurement can take up to a day of constant monitoring of the battery, which is not very convenient for many ordinary people.

Load fork

Load plug - a device for testing the battery using a controlled load, equipped with a voltmeter, load resistor and two test leads. Such devices are of various types: with analog or digital voltmeter, simple circuit with one load element or complicated devices with several load spirals and an ammeter, there are also load plugs for testing the voltage in individual battery banks.

The essence of the measurements is simple and is described in the instructions for the device. The obtained voltage data must be compared with the following table.

Correspondence table of voltage with battery capacity

Measurement of electrolyte density

You can measure the capacity of the battery components (cans) using a device called a "hydrometer". The essence of the method is that the density of the electrolyte in each battery bank is directly related to its capacitive characteristic.

To measure, it is necessary to open all the covers of the autoaccumulator cans and take electrolyte from each vessel one by one, recording the density data from the device. Further, the density of this substance is compared with the correspondence table of density and capacity.

Correspondence table of electrolyte density and capacity

Measurements using special devices

The idea of \u200b\u200ba load plug was used and improved in electronic portable devices Kulon, which are designed specifically for testing activities in different spectra over lead-acid batteries.

With such devices, you can quickly measure the voltage, determine the approximate capacity of the battery without resorting to a check discharge, and also save the obtained measurements in the device's memory.

Pendant family devices features:

• powered by a battery from which measurements are taken;
• the complete set of devices includes wires with crocodile pliers, which provides a high-quality clamp of wires on all battery terminals;
• a special method for determining the capacity of the battery, which has no analogues;
• it is recommended to independently calibrate the product on a new battery of a similar type to increase the measurement accuracy (the procedure is described by the manufacturer in the operating instructions).

Important! This capacity tester should only be used to establish the capacity in a battery that is fully charged.

There are also other devices from other manufacturers for the same purposes, the method of determining the battery capacity for which is different from each other. For example, SKAT-T-AUTO instruments, PITE testers, Fluke analyzers, Vencon fixtures. All these devices can be used to directly or indirectly measure various parameters.

Knowing the condition of your battery, namely its capacity, you can avoid unpleasant situations on the road. Also, having responded in time to the discrepancy between the measured indicators and those declared by the manufacturer, you can revive or extend the life of the battery by taking a variety of measures.

Video

In order to measure the capacity of a battery, they usually do this: connect a resistor of a certain value to this battery, which discharges this battery, and recording the values \u200b\u200bof the current flowing through the resistor and the voltage across it, they wait until the battery is completely discharged. Based on the data obtained, a discharge graph is plotted, from which the capacity is determined. The only problem is that as the voltage across the battery decreases, the current through the resistor will also decrease, so the data will have to be integrated in time, so the accuracy of this method of measuring the battery capacity leaves much to be desired.

If the battery is discharged not through a resistor, but through a stable current source, then this will allow the battery capacity to be determined with very high accuracy. But there is one problem here - the voltage on the battery (1.2..3.7 V) is not enough for the operation of a stable current source. But this problem can be circumvented by adding an additional voltage source to the measurement circuit.

Figure: 1. Circuit for measuring battery capacity
V1 - investigated battery; V2 - auxiliary voltage source; PV1 - voltmeter;
LM7805 and R1 - stable current source; VD1 - protective diode.

Figure 1 shows circuit diagram installation for measuring the capacity of the battery. Here you can see that the measured battery V1 is connected in series with the current source (it is formed by the LM7805 integrated stabilizer and the resistor R1) and the auxiliary power supply V2. Since V1 and V2 are connected in series, the sum of their voltages is sufficient to operate the current source. Since the minimum voltage required for the operation of the current source is 7 V (of which 5 V is the voltage at the output of the LM7805 microcircuit, i.e. in this case this is the voltage drop across the resistor R1, and 2 V is the minimum allowable voltage drop between input and output LM7805), then the sum of voltages V1 and V2 is enough for the current source to work with some margin.

Instead of the LM7805 stabilizer, you can use another integral stabilizer, for example, LM317 with an output voltage of 1.25 V and a minimum voltage drop of 3 V. Since the minimum operating voltage of the current source will be 4.25 V, the voltage of the second voltage source V2 can be reduced to 5 B. In the case of using the LM317 stabilizer, the stabilization current value will be determined by the formula I \u003d 1.25 / R1

Then for a discharge current of 100 mA, the value of the resistance R1 should be approximately 12.5 ohms.

How to measure battery capacity

First, by selecting the resistor R1, you need to set the discharge current - usually the value of the discharge current is chosen equal to the operating discharge current of the battery. It should also be borne in mind that some models of integrated voltage stabilizers 7805 can consume a small control current of the order of 2 ... 8 mA, so it is recommended to check the current in the circuit with an ammeter. Next, a fully charged battery V1 is installed in the circuit, and by closing the SA1 switch, the time countdown begins until the voltage on the battery drops to a minimum value - for different types of batteries this value is different, for example, for nickel-cadmium (NiCd) - 1, 0 V, for nickel-metal hydride (NiMH) - 1.1 V, for lithium-ion (Li-ion) - 2.5 ... 3 V, for each specific battery model, these data must be viewed in the corresponding documentation.

After reaching the minimum battery voltage, switch SA1 is opened. Remember that discharging the battery below the minimum voltage can damage it. Multiplying the value of the discharge current (in Amperes) by the discharge time (in hours), we obtain the battery capacity (A * h):

C \u003d I * t

Let's consider the practical application of this method of measuring the capacity of a battery using a specific example.

Battery Capacity Measurement NB-11L

The NB-11L battery (Fig. 2) was purchased from DealeXtreme online store for $ 3.7 (SKU: 169532). The battery capacity is indicated on the body - 750 mA * h. On the site, its capacity is indicated already more modestly - 650 mA * h. What is the real capacity of this battery?

Figure: 2. Li-ion battery NB-11L with a capacity of allegedly 750 mAh
Fits CAN.NB-11L 3.7V 750mAh
Use specified charger only

To connect the conductors to the battery contacts, you need two paper clips, which should be bent as shown in Figure 3 and connect them to the "+" and "-" terminals of the battery (Fig. 4.). It is necessary to avoid closing the contacts, it is better to insulate them.

To measure the capacity of the NB-11L battery, its discharge current was taken to be 100 mA. For this, the value of the resistor R1 was chosen a little more than 50 Ohm. The power dissipated across the resistor R1 is determined by the formula P \u003d V 2 / R1where V is the voltage across the resistor R1. In this case, P \u003d 5 2/50 \u003d 0.5 W. The LM7805 stabilizer should be installed on a radiator, but if there is no suitable radiator at hand, then the microcircuit can be partially immersed in a glass of cold water, but so that the leads remain dry (in the case of the TO-220 case).

After installing a fully charged NB-11L battery in the circuit and closing the SA1 switch, the timing began with periodic voltage monitoring by the PV1 voltmeter. The data were entered into a table, which was used to build a graph of the NB-11L battery discharge (Fig. 5).

Figure: 5. Graph of voltage on the NB-11L battery during its discharge with a current of 100 mA

From this it can be seen that after 5 hours of discharge with a current of 0.1 A, the voltage on the battery dropped to 3 volts and began to rapidly fall further.

C \u003d I * t \u003d 0.1 * 5 \u003d 0.5 A \u003d 500 mA * h.

So the actual capacity of the NB-11L battery turned out to be 1.5 times lower than that indicated on it.

A modular version of a clear and accurate meter of ampere-hours of batteries, assembled from computer waste at minimal cost.
This is my response to the article.

A little prelude ...
Under my patronage is a park of 70 computers, different years of release and state. Naturally, an overwhelming number of uninterruptible power supplies are available (in the text - UPS). The organization is budgetary, of course, they don't give money, like - do what you want, but everything should work. After short tests with a load in the form of a 150-watt light bulb, I found that 70% of UPSs do not hold the load for more than 1 minute, APC UPSs sin with switching relay contacts (it goes to the battery, buzzes and beeps, and the output is full zero). Of course, no one gave me all the UPSs to check at once. The solution turned out to be simple: once in half a year - a year I took the computers for cleaning, lubrication, and at the same time the UPS for testing and inspection of giblets.

Of course, UPSs of different brands and capacities (there are old people for 600 watts of 1992 release, the battery died this fall, before that I did resuscitation 4 years ago). If someone is not in the know, in household and office UPS batteries of different types, housings, voltages and capacities are used. A typical representative is the GP1272F2 (12 Volt, 7 A / h). But they also come across at 6V - 4.5 A / h.

Battery prices often exceed half the price of a new UPS. Moreover, dead batteries accumulate in the office (in which I work part-time). The question arose, what is the real capacity before and after lifting from the trash can, how many minutes of operation can you expect from the UPS. And then an article caught my eye I. Nechaeva In the magazine "Radio" 2/2009 about a similar meter.
Of course, I didn’t like some moments, I’m such a bastard.
And so let's start with ...

This is the original diagram from the article.

TTX: discharge current 50, 250, 500 mA, cut-off voltage 2.5-27.5 Volts.
I will list, what didn’t like: the maximum discharge current is only 0.5A (and it is not interesting to wait until 7 Ah is discharged), the cut-off range is too wide and it is easy to knock it down, all the current flows through the button to start, the current stabilizer on the field for the LED is too much, the diode in the control output increases the required drop on current resistors is up to 1.8V and in case of breakdown, 317 skids.

About discharge current: In batteries, it happens that the active mass is, as it were, sealed in a spread (not to be confused with sulfation), while the mobility of the electrolyte decreases and if it is discharged with a low current, it can give up its capacity completely, and when installed in a UPS, the test will not pass. Well then, you need to discharge it with a small current and charge it, i.e. to treat.
The modularity of what I have done is good because you can make 2 or more discharge modules (you can switch 1 and switch current resistors) of different power or even type and 2 cut-off devices for 6 and 12-volt batteries, or 1 with a switch.

Photos of my meter:

We see: cut-off unit, current load, Chinese walkers.
I repeat, I work as a sysAdmin, sometimes I fix motherboards, so there is some kind of dead iron slide.
I'll start in the reverse order: the walkers are slightly modified so that they can run when powered from 1.5 to 25 volts.
Walker modification scheme:

1117 pulled from a dead motherboard.
The 2K resistor is the minimum load for the regulator.

accordingly the scheme:

This is 2 amperes. Since R1 turned out to be more than 0.75 ohms, 2 resistances had to be added (this is R3, two in one in the photo) so that the current was 2 amperes. If someone has not noticed, there are no gaskets between the micro with a transistor on the radiator. You can of course use another circuit, such as in radio 3/2007 page 34, just add the reference voltage.
Current and thermal protection in 317 (real) is.

And the scariest part is the cutter.

Super 3D montage, but only 3 cm cubic, on the seal will be much larger. Polevik, if on a 6V battery, then it is very desirable with logical control.
This part is almost the same as the original one, the start button has been moved from the drain-source to the collector-emitter, the variable is replaced with a fixed divider, a Chinese super-bright LED through a resistor.

Possible variations:the upper arm (according to the original scheme, it is R4), replace it with a resistance + a variable, thus limiting the setting range (required when the discharge current is commensurate with the battery capacity); other ideas are possible.

For formulas Uref \u003d 2.5v for normal 431, and for 431L it is equal to 1.25v.

Fixed Voltage Shutter:

Formula for calculation: Uotc \u003d Uref (1 + R4 / R5)
or R5 \u003d (Uotc-Uref) / (Uref * R4)

Adjustable Voltage Cutoff:

Formula for calculation: Uotc \u003d Uref (1+ (R4 + R6) / R5)
or R5 \u003d (Uotc- Uref) / (Uref * (R4 + R6))

But here it is necessary to count from the variable, on it, at a discharge of 0.1s, (Udelta) 1.15v for 6v akb and 2.30v for 12v akb should fall.
Therefore, the formulas are transformed and the calculation is somewhat different.
Umin look in the table below.
R5 \u003d Uref * R6 / Udelta
R4 \u003d ((Umin -Uref) * R5) / Umin