How to Measure Internal Resistance
The resistance of a battery provides useful information about its performance and detects hidden trouble spots. High resistance values are often the triggering point to replace an aging battery, and determining resistance is especially useful in checking stationary batteries. However, resistance comparison alone is not effective, because the value between batches of lead acid batteries can vary by eight percent. Because of this relatively wide tolerance, the resistance method only works effectively when comparing the values for a given battery from birth to retirement. Service crews are asked to take a snapshot of each cell at time of installation and then measure the subtle changes as the cells age. A 25 percent increase in resistance over the original reading hints to an overall performance drop of 20 percent.
Manufacturers of stationary batteries typically honor the warranty if the internal resistance increases by 50 percent. Their preference is to get true capacity readings by applying a full discharge. It is their belief that only a discharge can provide reliable readings and they ask users to perform the service once a year. While this advice has merit, a full discharge requires a temporary disconnection of the battery from the system, and on a large battery such a test takes an entire day to complete. In the real world, very few battery installations receive this type of service and most measurements are based on battery resistance readings.
Measuring the internal resistance is done by reading the voltage drop on a load current or by AC impedance. The results are in ohmic values. There is a notion that internal resistance is related to capacity, and this is false. The resistance of many batteries stays flat through most of the service life. Figure 1 shows the capacity fade and internal resistance of lithium-ion cells.
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Figure 1: Relationship between capacity and resistance as part of cycling Resistance does not reveal the state-of-health of a battery. The internal resistance often stays flat with use and aging.
Cycle test on Li-ion batteries at 1C: Courtesy of Cadex |
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To estimate capacity and state-of-charge on the fly involves impedance trending by scanning a battery with frequencies ranging from less than one hertz to several thousand hertz. Read more about Testing Lead Acid Batteries.
What Is Impedance?
Before exploring the different methods of measuring the internal resistance of a battery, let’s examine what electrical resistance means, and let’s differentiate between a pure resistance (R) and impedance (Z) that includes reactive elements such as coils and capacitors. Both values are given in Ohms (W), a measure formulated by the German physicist Georg Simon Ohm, who lived from 1798 to 1854. (One Ohm produces a voltage drop of 1V with a current flow of 1A.) The difference between resistance and impedance lies in the reactance. Let me explain.
The electrical resistance of a pure load, such as a heating element, has no reactance. Voltage and current flow in unison and there is no advancing or trailing phase shift that would occur with a reactive load, such as an electric motor or a florescent light fixture. The ohmic resistance on a pure resistive load is the same with direct current (DC) as is with alternating current (AC). The Power Factor (pf) is 1, which provides the most accurate metering of the power consumed.
Most electrical loads, as well as a battery as power source, have reactance. They consist of capacitive reactance (capacitor) and inductive reactance (coil). The resistor of a reactance varies with the frequency of the electrical power. The capacitive resistance decreases with higher frequency while the inductive resistance increases. (To explain resistance change with frequency, we compare an oil damper that has a stiffer resistance when moved fast. A battery has resistive, capacitive and inductive resistance, and the term impedance includes all three in one.
Impedance can best be illustrated with the Randles model. Figure 2 illustrates the basic model of a lead acid battery, which reflects resistors and a capacitor (R1, R2 and C). The inductive reactance is commonly omitted because it plays a negligible role in a battery, especially at a low frequency.
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Figure 2: The overall battery resistance consists of ohmic resistance, as well as inductive and capacitive reactance. The schematic and electrical values differ for every battery. |
Now that we have learned the basics of internal battery resistance and how they can be applied to rapid-test batteries at different frequencies, this section examines current and future battery test methods. It also discusses advantages and shortfalls.
DC Load Method
Ohmic measurement is one of the oldest and most reliable test methods. The battery receives a brief discharge lasting a few seconds. A small pack gets an ampere or less and a starter battery is loaded with 50A and more. A voltmeter measures the voltage drop and Ohm’s law calculates the resistance value (voltage divided by current equals resistance).
DC load measurements work well to check large stationary batteries, and the ohmic readings are very accurate and repeatable. Manufacturers of test instruments claim resistance readings in the 10 micro-ohm range. Many garages use the carbon pile to measure starter batteries, and with experience mechanics familiar with this loading device get a reasonably good assessment of the battery. The invasive test is in many ways more reliable than non-invasive methods.
The DC load method has a limitation in that it blends R1 and R2 of the Randles modelinto one combined resistorand ignores the capacitor(see Figure 3). “C” is an important component of a battery that represents 1.5 farads per 100Ah capacity. In essence, the DC method sees the battery as a resistor and can only provide ohmic references.
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Figure 3: DC load method The true integrity of the Randles model cannot be seen. R1 and R2 appear as one ohmic value. Courtesy of Cadex |
The two-tier DC load method offers an alternative method by applying two sequential discharge loads of different currents and time durations. The battery first discharges at a low current for 10 seconds, followed by a higher current for three seconds (see Figure 4), and Ohm’s law calculates the resistance values. Evaluating the voltage signature under the two load conditions offers additional information about the battery, but the values are strictly resistive and do not reveal SoC and capacity estimations.
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Figure 4: Two-tier DC load The two-tier DC loadfollows the IEC 60285 and IEC 61436standards and provides lifelike test conditions for many battery applications. The load test is the preferred method for batteries powering DC loads. Courtesy of Cadex |
AC Conductance
The AC conductance method replaces the DC load and injects an alternating current into the battery. At a set frequency of between 80 and 90 hertz, the capacitive and inductive reactance converge, resulting in a negligible voltage lag that minimizes the reactance. Manufacturers of AC conductance equipment claim battery resistance readings in the 50 micro-ohm range, and these instruments are commonly used in North American car garages. The single-frequency technology as illustrated in Figure 5 sees the components of the Randles model as one complex impedance called the modulus of Z.
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Figure 5: AC conductance method The individual components of the Randles model are molten together and cannot be distinguished. Courtesy of Cadex
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Smaller batteries often use the popular 1000-hertz (Hz) ohm test method. A 1000Hz signal excites the battery, and the Ohm’s law calculates the resistance. It is important to note that the AC method shows different values to the DC load, and both are correct. For example, Li-ion in an 18650 cell produces about 36mOhm with a 1000Hz AC signal and roughly 110mOhm with a DC load. Since both readings are correct, and yet are so far apart, the user needs to consider the application. The pulse DC load method provides the best indication for a DC application such as driving a motor or powering a light, while the 1000Hz method better reflects the performance of a digital load, such as a cellular phone that relies to a large extent on the capacitor characteristics of a battery. Figure 6 illustrates the 100Hz method.
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Figure 6: 1000-hertz method The IEC 1000-hertz is the preferred method to take impedance snapshots of batteries powering digital devices. Courtesy of Cadex |
Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy (EIS) enables more than resistance readings; it can estimate state-of-charge and capacity. Research laboratories have been using EIS for many years to evaluate battery characteristics, but high equipment cost, slow test times and the need for trained professionals to decipher large volumes of data have limited this technology to laboratory environments. EIS is able to read each component of the Randles model individually; however, analyzing the value at different frequencies and correlating the data is an enormous task. Fuzzy logic and advanced digital signal processor (DSP) technology have simplified this task. Figure 7 illustrates the battery component, which EIS technology is capable of reading.
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Figure 7: Spectro™ method R1, R2 and C are measured separately, which enables state-of-charge and capacity measurements. Courtesy of Cadex |
Comments
When you say “The pulse DC load method provides the best indication for a DC application such as driving a motor”, are you referring to the two-tier method? Or is there a separte pulss DC load method that is different from the two-tier method?
Thanks.
Ok, so just how can I test the internal resistance of a lead acid battery? I have a standard digital multimeter to use for this task.
plz inform me the simplest method to measure the internal resistance of lead acid battery ......so i want to do it practically
thanks
To Measure the internal resistance:
Buy a high wattage (10W) precision resistor of low value, say 0.1 ohm.
Put the resistor in series with the battery charger + cable and one terminal of resistor, connect battery charger - cable to battery -, connect second terminal of resistor to battery +, (A) Read the voltage across the resistor terminals. (B)Read the voltage across the battery terminals.
A) Divide the voltage across the resistor by the value of the resistor (0.1 ohms) to get the exact current flowing into the battery. This value will be used for step B
B) Divide the voltage across the battery terminals by the current found in step A. The answer will be roughly (a few percent, of a 70% accurate measurement is pretty good) the internal resistance of the battery.
Second Method: Get an ESR Meter (Equivalent Series Resistance) ($100-$300), these are essentially AC Ohmmeters with fixed or variable frequency. Measure resistance of battery (Equivalent Series Resistance) which is a direct reading with no other meters needed. These meters can be used on batteries from AAA to 9V alkaline with very good indication of health as well.
Dear sir,
I am from India working in Amara raja batteires ltd.,my query is -
1)can we detect any abnormality in the batteries during manufacturing process by using internal resistance meter or conductance meter
2) which method is the best method to verify defect in our internal process
3)we are planning to study internal resistance behaviour on the batteries , pl suggest us the best meter
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Thank you,
Garth.
pls tell me how to check short cct of leadd acid battries.
This article addresses the theory very well, but I was expecting to read something more practical, as applied to lead acid starting batteries. For instance, how can I measure the internal DC resistance of a lead acid battery using only a resistor and a regular 5 amp battery charger?