How to measure the internal resistance of a battery. Battery internal resistance - what is it and how to measure it Measuring the internal resistance of li ion
What is the internal resistance of a battery and what is it used for?
The impedance of a lead-acid battery is the sum of polarization resistance and ohmic resistance. Ohmic resistance is the sum of the resistances of the battery separators, electrodes, positive and negative terminals, connections between cells and electrolyte.
The resistance of the electrodes is influenced by their design, porosity, geometry, lattice design, state of the active substance, the presence of alloying components, and the quality of the electrical contact of the lattice and coating. The resistance values of the negative electrode arrays and the sponge lead (Pb) on them are approximately the same. At the same time, the resistance of lead peroxide (PbO2), which is applied to the positive electrode grid, is 10 thousand times greater.
During the discharge of a lead-acid battery, lead sulfate (PbSO4) is released on the surface of the electrodes. This is a poor conductor, which significantly increases the resistance of the electrode plates. In addition, lead sulfate is deposited in the pores of the plate coating and significantly reduces the diffusion of sulfuric acid from the electrolyte into them. As a result, by the end of the discharge cycle of a lead-acid battery, its resistance increases by 2-3 times. During the charging process, lead sulfate dissolves and the battery resistance returns to its original value.
The resistance of the electrolyte has a significant influence on the resistance of a lead-acid battery. This value, in turn, strongly depends on the concentration and temperature of the electrolyte. As the temperature decreases, the resistance of the electrolyte increases and reaches infinity when it freezes.
With an electrolyte density of 1.225 g/cm3 and a temperature of +15 C, it has a minimum resistance value. As the density decreases or increases, the resistance increases, which means the internal resistance of the battery also increases.
The resistance of separators changes depending on changes in their thickness and porosity. The amount of current that flows through the battery affects the polarization resistance. A few words about polarization and the reasons why it occurs. The first reason is that the electrode potentials change in the electrolyte and on the surface of the electrodes (electric double layer). The second reason is that when current passes, the electrolyte concentration changes in the immediate vicinity of the electrodes. This leads to a change in the electrode potentials. When the circuit opens and the current disappears, the electrode potentials return to their original values.
One of the features of lead-acid batteries is their low internal resistance compared to other types of batteries. Thanks to this, they can deliver high current (up to 2 thousand amperes) in a short time. Therefore, their main area of application is starter batteries in vehicles with internal combustion engines.
It is also worth noting that the internal resistance of the battery at alternating or direct current strongly depends on its frequency. There are a number of studies whose authors observed the internal resistance of a lead-acid battery at a current frequency of several hundred hertz.
How can you estimate the internal resistance of a battery?
As an example, consider a 55 Ah car lead-acid battery with a nominal voltage of 12 volts. A fully charged battery has a voltage of 12.6-12.9 volts. Let's assume that a resistor with a resistance of 1 ohm is connected to the battery. Let the voltage of the open battery be 12.9 volts. Then the current should theoretically be 12.9 V / 1 Ohm = 12.9 amperes. But in reality it will be below 12.5 volts. Why is this happening? This is explained by the fact that in an electrolyte the rate of diffusion of ions is not infinitely large.
The image shows the battery as a 2-pole power source. It has an electromotive force (EMF), which corresponds to the open circuit voltage, and internal resistance. In the diagram they are designated E and Rin. When the circuit is closed, the emf of the battery partially drops across the resistor, as well as through the internal resistance itself. That is, what happens in the circuit can be described by the following formula.
E = (R + Rin) * I.
In the images below you can see the values of the EMF of a car battery in an open circuit and the voltage when connecting a load in the form of two car light bulbs connected in parallel.
As already mentioned, the internal resistance of the battery is a conventional value. A lead-acid battery is a nonlinear device, the internal resistance of which varies depending on temperature, load, state of charge, electrolyte concentration and other above-mentioned parameters. So, to carry out accurate calculations of the battery, discharge curves are used, and not the value of internal resistance.
In this case, the value of internal resistance can be used in calculations of electrical circuits with batteries. Naturally, the value of internal resistance is always taken taking into account the factors on which it depends (charge or discharge, direct or alternating current, current frequency, etc.).
So, based on the formula above, you can calculate the internal resistance of a battery with an emf of 12.6 volts when discharged with a constant current of 2 amperes.
r = (E ─ U) / I = (12.9 V – 12.5 V) / 2 A = 0.2 Ohm.
By the way, some chargers allow you to measure the internal resistance of the battery. For example, below you can see the value of the internal resistance of a charged car battery, measured by charging the SkyRC iMax B6 mini. True, it is unknown on what principle the device calculates this value.
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If you take a new lithium-ion battery, say size 18650, with a nominal capacity of 2500mAh, bring its voltage to exactly 3.7 volts, and then connect it to an active load in the form of a 10-watt resistor with a nominal value of R = 1 Ohm, then what is the value of the constant is the current we expect to measure through this resistor?
What will happen there at the very first moment, until the battery practically begins to discharge? In accordance with Ohm's law, it would seem that there should be 3.7A, since i=U/R=3.7/1 = 3.7[A]. In fact, the current will be slightly less, namely in the region of I = 3.6A. Why will this happen?
The reason is that not only the resistor, but also the battery itself has a certain internal resistance, since chemical processes inside it cannot occur instantly. If you imagine a battery in the form of a real two-terminal network, then 3.7V will be its EMF, in addition to which there will also be an internal resistance r, equal, for our example, to approximately 0.028 Ohm.
Indeed, if you measure the voltage across a resistor of R = 1 Ohm attached to the battery, it will turn out to be approximately 3.6 V, and 0.1 V will therefore drop at the internal resistance r of the battery. This means that if a resistor has a resistance of 1 ohm, the voltage measured across it was 3.6 V, therefore the current through the resistor is equal to I = 3.6A. Then, if u = 0.1V went to the battery, and our circuit is closed, in series, then the current through the battery is I = 3.6A, therefore, according to Ohm’s law, its internal resistance will be equal to r = u / I = 0.1/3.6 = 0.0277 Ohm.
What determines the internal resistance of a battery?
In reality, the internal resistance of batteries of different types is not constant all the time. It is dynamic and depends on several parameters: on the load current, on the battery capacity, on the degree of charge of the battery, as well as on the temperature of the electrolyte inside the battery.
The higher the load current, the lower, as a rule, the internal resistance of the battery, since the charge transfer processes inside the electrolyte are more intense in this case, more ions are involved in the process, and the ions move more actively in the electrolyte from electrode to electrode. If the load is relatively small, then the intensity of chemical processes on the electrodes and in the battery electrolyte will also be less, which means the internal resistance will seem greater.
Batteries with a larger capacity have a larger electrode area, which means the area of interaction between the electrodes and the electrolyte is larger. Therefore, more ions are involved in the charge transfer process, more ions create a current. A similar principle is demonstrated - the larger the capacitance, the more charge can be used in the vicinity of a given voltage. So, the higher the battery capacity, the lower its internal resistance.
Now let's talk about temperature. Each battery has its own safe operating temperature range, within which the following applies. The higher the temperature of the battery, the faster the diffusion of ions inside the electrolyte occurs, therefore, at a higher operating temperature, the internal resistance of the battery will be lower.
The first lithium batteries, which did not have protection against overheating, even exploded because of this, since the oxygen formed due to the rapid disintegration of the anode (as a result of a rapid reaction on it) was released too actively. One way or another, batteries are characterized by an almost linear dependence of internal resistance on temperature in the range of acceptable operating temperatures.
As the battery discharges, its active capacity decreases, since the amount of active substance in the plates that can still participate in creating current becomes less and less. Therefore, the current becomes less and less, and accordingly the internal resistance increases. The more charged the battery, the lower its internal resistance. This means that as the battery discharges, its internal resistance becomes greater.
A multimeter is a multifunctional device for measuring various parameters of electric current, so it can also be used to check the battery charge. To perform this work, you can use different types of multimeters. The cost of the product does not matter, the main thing is that the digital or analog measuring device is in good condition. How to check the battery with a multimeter will be discussed below.
What parameters can be checked?
Using a multimeter, you can measure voltage with high accuracy. By the magnitude of the electrical voltage, you can determine whether the battery is charged or the element needs to be charged with direct current.
Using a multimeter, you can check the voltage not only of acid batteries, but also of cell phone batteries. To check the mobile phone's battery charge level, the device is switched to the mode for measuring direct current up to 20 V. In this mode, the digital device allows you to measure voltage with an accuracy of hundredths of a volt.
The screwdriver battery can also be easily checked with a multimeter. The rated voltage of the device, in this case, can be found out from the documentation of the power tool, and if the voltage is less than this value, then the battery must be charged.
The battery capacity can also be checked with a multimeter. For this purpose, you can use several methods.
You can check current leakage using a multimeter. If it is necessary to measure this parameter on a car, then in addition to the current leakage on the body, the leakage in the vehicle’s on-board network is also checked.
In this way, you can prevent rapid discharge of the battery and increase its service life.
How to measure voltage
If it is necessary to check only the battery voltage, then the multimeter is switched to DC mode. If you need to check a source of electricity whose voltage does not exceed 20 volts, then in this sector the mode switch is set to the 20 V position.
Then the black probe of the multimeter should be connected to the negative terminal, and the red one to the positive terminal of the battery; the device display, at this moment, will show the DC voltage.
Typically, a serviceable and fully charged car battery has a voltage of 12.7 V. If at this voltage the electrolyte density is normal, then the source of electricity can be used for its intended purpose.
The voltage of lithium-ion batteries of cell phones, as well as alkaline or gel batteries, which are used to start the engines of various motorcycles, diesel generators and other devices that require a certain charge of electricity to start operating, is measured in a similar way.
How to measure capacitance
The multimeter can also be used as a tester to measure battery capacity. The battery capacity can be measured using a test battery discharge. To check the capacity, you will first need to fully charge the battery. Then you need to make sure that the battery is fully charged by measuring the voltage and density of the electrolyte.
Next, you need to connect a load of known power, for example a 24 W incandescent lamp, and note the exact start time of this experiment. When the battery voltage drops to 50% percent of the previously set reading of a fully charged battery, the light should be turned off.
Capacity measurement, which is expressed in a/h, is carried out by multiplying the current in the circuit with a connected load by the number of hours during which the control discharge of the battery was carried out. If you get a value that is as close as possible to the nominal a/h value, then the battery is in excellent condition.
Check internal resistance
To check the battery for serviceability using a multimeter, you need to measure the internal resistance of the battery. You can check the functionality of the power source using a multimeter and a powerful 12 V light bulb. You need to check the battery in the following sequence:
If the measurement difference does not exceed 0.05 V, then the battery is in good condition.
In the case where the voltage drop is greater, the internal resistance of the power source will be higher, which indirectly will indicate a significant deterioration in the technical condition of the battery.
In this way, it is possible to fairly accurately check the power source for serviceability.
How to check leakage current
The battery can discharge on its own, even if its terminals are not connected to electrical consumers. The amount of self-discharge is indicated in the documentation for the battery and is a natural process. The loss of electricity can be especially noticeable in acid batteries.
In addition to natural electrical leakage, there may be areas of the circuit that are wet or have thin insulation. In this case, even when all electricity consumers are turned off, an additional current leak occurs, which can lead to a complete discharge of the battery, and in some cases, to a fire in the damaged area.
Especially, this phenomenon can be dangerous in the on-board network of a car, in which the negative conductor is the entire body and components, which may contain a sufficient amount of flammable substances to form an open flame even from a small spark or electric arc.
To identify such “unauthorized” consumption of electricity, it is necessary to turn off the car’s ignition, as well as turn off devices operating in “standby mode,” such as a radio and alarm system.
You can measure the current on the battery using a multimeter only if the measuring device is switched to the current measurement mode, indicated by the “10 A” icon. To do this, the circular switch is switched to the appropriate mode, and the red plug is placed in the socket marked “10 ADC”.
The red probe of the multimeter is connected to the “+” of the battery, and the black probe is connected to the disconnected terminal. At this moment, there should be absolutely no readings from the device. If the multimeter shows any value, then the leakage current is significant, and it is necessary to carry out a detailed diagnosis of the vehicle’s on-board network.
Leakage is measured in other electronic systems in a similar way. When carrying out diagnostics, care should be taken, and if you suspect a significant electrical leakage, which is manifested by sparking when disconnecting or connecting a terminal, you should refuse to measure the leakage current with a multimeter.
If you neglect this rule, you can “burn” the device, which is not designed to test large current values.
How to check the battery charge with a multimeter and not damage the fragile electronic “stuffing” of the device?
To ensure that the battery test is not the last thing for the tester, you must select the correct diagnostic mode. If you need to check the amperage, it is strictly forbidden to do this without an additional load, which should not exceed a power of 120 W.
When selecting the DC measurement mode, you should be careful not to mistakenly turn the multimeter into the resistance measurement mode, which is located, in most multimeter models, next to the DC current measurement switch position.
They have a larger working surface of the plates and more space for electrolyte diffusion inside the battery. Therefore, the internal resistance of large batteries is less than the internal resistance of smaller batteries.
And measurements of the internal resistance of batteries at direct and alternating current show that the internal resistance of the battery is highly dependent on frequency. Below is a graph of battery conductivity versus frequency, taken from the work of Australian researchers.
It follows from the graph that the internal resistance of a lead battery has a minimum at frequencies of the order of hundreds of hertz.
At high temperatures, the rate of diffusion of electrolyte ions is higher than at low temperatures. This dependence is linear. It determines the dependence of the internal resistance of the battery on temperature. 
At higher temperatures, the internal resistance of the battery is lower than at low temperatures.
During battery discharge, the amount of active mass on the battery plates decreases, which leads to a decrease in the active surface of the plates. 
Therefore, the internal resistance of a charged battery is less than the internal resistance of a discharged battery.
4. Can the internal resistance of the battery be used for ?
Devices for testing batteries have been known for quite some time, the principle of operation of which is based on the connection between the internal resistance of the battery and. Some devices (load forks and similar devices) offer to evaluate the condition of the battery by measuring the voltage of the battery under load (which is similar to measuring the internal resistance of a battery at direct current). The use of others (alternating current battery internal resistance meters) is based on the connection of internal resistance with the state of the battery. The third type of devices (spectrum meters) allows you to compare the spectra of internal resistance of batteries running on alternating current of different frequencies and draw conclusions about the condition of the battery based on them.
In itself, the internal resistance (or conductivity) of the battery allows only a qualitative assessment of the condition of the battery. In addition, manufacturers of such devices do not indicate at what frequency the conductivity is measured and with what current the test is performed. And, as we already know, the internal resistance of the battery depends on both frequency and current. Consequently, conductivity measurements do not provide quantitative information that would allow the user of the device to determine how long the battery will last the next time it is discharged to the load. This drawback is due to the fact that there is no clear relationship between the internal resistance of the battery.
The most modern ones are based on analysis of the oscillogram of the battery response to a signal of a special shape. They quickly assess, which allows you to monitor wear and tear, calculate the duration of the battery discharge for a given state and make a forecast of the remaining life of the lead battery.
Category: Battery support Published 09/12/2016 15:51Internal resistance provides valuable information about the battery that can indicate when it is reaching the end of its life. This is especially true for electrochemical systems based on nickel. Resistance is not the only indicator of performance; it may well vary by 5-10 percent between batches lead acid batteries, especially for stationary use. Because of this wide tolerance, the resistance-based method works best when comparing readings taken from a particular battery at the time of its assembly with subsequent time periods. Service teams already recommend taking readings of each element or battery as a whole during installation in order to further monitor their aging process.
There is an opinion that internal resistance is related to capacitance, but this is not true. Resistance of modern lead-acid and lithium-ion batteries remains at the same level for most of its service life. Special additives to the electrolyte have reduced the problem of internal corrosion, which correlates with internal resistance. Figure 1 shows the reduction in capacity during cycling in relation to the internal resistance of a lithium-ion battery.
Figure 1: Relationship between capacitance and resistance relative to the number of charge/discharge cycles. Resistance does not reveal the health status of a battery and often remains the same during use and aging.
Cyclic tests of lithium-ion batteries were carried out at C-rating 1C:
Charging: 1.500mA to 4.2V at 25°C
Discharge: 1.500mA to 2.75V at 25°C
What is resistance?
Before exploring the different methods for measuring the internal resistance of electric batteries, let's look at what electrical resistance is and the difference between simple resistance (R) and impedance (Z). R is the resistance of a substance to the passage of electric current, and Z includes the reactive component inherent in devices such as coils and capacitors. Both are measured in ohms (Ohm), a unit of measurement named after the German physicist Georg Simon Ohm, who lived from 1798 to 1854. (1 ohm results in a voltage drop of 1V at 1A current). Electrical conductivity can also be measured in siemens (S). The combination of resistance and impedance is known as reactance. Let me explain.
The electrical resistance of a normal load, such as a heating element, has no reactive component. The voltage and current flow in it in unison - there are no shifts in their phases. Electrical resistance, caused by the opposition of the material through which current flows, is essentially the same for direct (DC) and alternating (AC) currents. The power factor is unity, which provides the most accurate measurement of power consumption.
Most electrical loads are still reactive, and can include capacitive (capacitor) and inductive (coil) reactance. Capacitive reactance decreases with increasing AC frequency, while inductive reactance increases. An analogy for inductive reactance is an oil shock absorber that becomes stiff when moving quickly back and forth.
An electric battery has resistance, capacitance, and induction, all three of these parameters are combined in the concept of impedance. Impedance is best illustrated in the Randle circuit (Figure 2), which contains resistors R1 and R2 and capacitor C. Inductive reactance is usually omitted because it plays a minor role in electric batteries, especially at low frequencies.
Figure 2: Randle equivalent circuit for a lead-acid battery. The total resistance of a battery consists of active resistance, as well as inductive and capacitive resistance. The circuit and electrical values are different for each battery.
R1 - equivalent series resistance
R2 - charge transfer resistance
C - double layer capacitor
Attempts to measure the internal resistance of an electric battery are almost as old as the battery itself, and over the years several methods have been developed that are still in use today.
Direct Current Load Resistance Measuring Method (DC Load)
Ohmic measurements are one of the oldest and most reliable testing methods. Their meaning is a short-term (a second or a little more) discharge of the battery. The load current for a small battery is 1A or less, and for a large battery, such as a starter battery, it is 50A or more. The voltmeter measures the open circuit voltage without a load, and then takes a second measurement with a load connected. Next, using Ohm's law, the resistance value (potential difference divided by current) is calculated.
The DC load sensing method works well for large stationary batteries and the ohmic readings taken are accurate and repeatable. High-quality test instruments allow you to take resistance readings in the range of 10 μΩ. Many garages use film-carbon resistor testers to measure the resistance of starter batteries, giving experienced auto mechanics an excellent tool for assessing the required parameter.
However, this method has a limitation in that it combines resistors R1 and R2 from the Randle circuit into one resistor and ignores the capacitor (see Figure 3). “C” is the equivalent circuit component of an electric battery, taking a value of 1.5 farads for every 100Ah. Essentially, the DC load sensing method sees the battery as a resistor and can only take into account the active component of the electrochemical current source. In addition, this method will obtain similar readings from a good battery that is partially charged and from a weak one that is fully charged. Determining the degree of performance and assessing capacity in this case is not possible.
Figure 3: DC load measurement method. The method does not show full compliance with Randle's scheme. R1 and R2 work as one active resistance.
There is an alternative method - two-level DC load measurement, when two consecutive discharge loads with different current strengths and durations are applied. First, the battery is discharged with a low current for 10 seconds, and then with a higher current for three (see Figure 4); Afterwards, the resistance value is calculated according to Ohm's law. Analyzing the voltage under two different load conditions provides additional information about the battery, but the resulting values are strictly resistive and do not reveal performance or capacity parameters. Load connection methods are preferred for batteries supplying DC loads.
Method for measuring electrical conductivity with alternating current (AC Cunductance)
Electrical conductivity measurements for the evaluation of starter batteries were first introduced in 1975 by Keith Champlin, demonstrating a linear correlation between load testing and conductivity. When connecting an AC load with a frequency of approximately 90Hz, the capacitive and inductive reactance corresponds to a 70-90Ah lead-acid battery, resulting in a slight voltage phase delay that minimizes reactance. (The frequency increases for a smaller battery and correspondingly decreases for a larger one). AC electrical conductivity meters are commonly used in automotive garages to measure inrush current. The single-frequency method (Figure 5) sees the Randle circuit components as one complex impedance called the Z modulus.
Figure 5: AC electrical conductivity measurement method. The individual components of the Randle circuit are combined into one element and cannot be measured individually.
Another common method is testing using a frequency of 1000Hz. This frequency excites the battery and the resistance can be calculated using Ohm's law. It should be noted that methods using AC voltage show different values compared to methods based on DC voltage when measuring reactance, and both approaches are valid.
For example, an 18650 size lithium-ion cell has a resistance of about 36mOhm with a 1000Hz AC load and about 110mOhm with a DC load. Since both of the above indications are fair, but far from each other, the consumer must take into account the specifics of the battery's operation. The DC method provides valuable data for applications with DC loads such as heating elements or incandescent lamps, while the 1000Hz method better reflects performance requirements optimized for powering various digital devices such as laptops or mobile phones. for which, first of all, the capacitive characteristics of batteries are important. Figure 6 shows the 1000Hz method.
Figure 6: 100Hz method. This method provides reactance values. This is the preferred method for reading the impedance of batteries powering digital devices.
Electrochemical Impedance Spectrocsopy (EIS)
Research laboratories have been using the EIS method for many years to evaluate the performance of electric batteries. But the high cost of equipment, the long duration of testing and the need for qualified specialists to decipher large amounts of data have limited the use of this technology to laboratory conditions. The EIS is capable of deriving R1, R2, and C values from the Randle circuit (Figure 7), but correlating this data into inrush current (cold cranking current) or capacitance estimation requires complex modeling (See BU-904: How to Measure Capacitance).
Figure 7: Spectro™ method. R1, R2 and C are measured separately, allowing for the most efficient assessment of health and capacity.