BU-902: 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.

Relationship between capacity and resistance as part of cycling

 

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:
Charge: 1,500mA to 4.2V, 25°C
Discharge: 1,500 to 2.75V, 25°C

Courtesy of Cadex

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.

Most electrical loads, as well as batteries providing power, have internal impedance. Impedance consists of a capacitive reactance component (capacitor) and an inductive reactance component (coil). Capacitive reactance decreases with increasing frequency, while inductive reactance increases with increasing frequency. (To explain resistance change with frequency, we compare an oil damper that has a stiffer resistance when moved fast.). Read more about Watts and Volt-Amps (VA).

A battery has resistive, capacitive and inductive resistance, and the term impedance includes all three in one. The inductive component only shows up at high frequencies when the inductive reactance of the conductors inside the battery becomes a factor in the overall impedance.

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.

Randles model of a lead acid battery

Figure 2:
Randles model of a lead acid battery

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.


 DC load method

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.

Two-tier DC load

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.

AC conductance method

Figure 5: AC conductance method

The individual components of the Randles model are molten together and cannot be distinguished.

Courtesy of Cadex

 

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.

1000-hertz method

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.

Spectro™ method

Figure 7: Spectro™ method

R1, R2 and C are measured separately, which enables state-of-charge and capacity measurements. 

Courtesy of Cadex


Comments

On March 23, 2011 at 1:44pm
Santiago Lopez wrote:

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?

On July 29, 2011 at 11:07am
Justin wrote:

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.

On September 28, 2011 at 8:32am
Kevan wrote:

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.

On October 18, 2011 at 4:35am
wajid iqbal wrote:

plz inform me the simplest method to measure the internal resistance of lead acid battery ......so i want to do it practically
thanks

On November 5, 2011 at 10:32am
Troy Mikkelson wrote:

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.

On March 13, 2012 at 10:24pm
SURYA KUMAR wrote:

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

On March 26, 2012 at 6:37pm
Garth Bowen wrote:

Hello, I just want to send you a note of appreciation in regards to your site. I’ve been an electrician for over forty years and recently became involved in work on small off-grid gnerator/inverter/battery/P.V. installations and maintenance. I, fortunatly have some background in lead-acid battery service and maintenance but this is over thirty years old and, needless to say, I’ve needed upgrading. I have found your site to be of immense value in gaining back that ground. Thank you for taking the time and trouble to compile and establish this site..

If you use this note on your site or for any purpose please exclude my last name and e-mail address.

Thank you,
Garth.

On May 10, 2012 at 11:58pm
saman wrote:

pls tell me how to check short cct of leadd acid battries.

On May 23, 2012 at 5:17am
Ans wrote:

Randles model shows the capacitor in the impedence model of battery.
can you please tell me why capacitor has been shown in the model, as battery has to supply only DC current. Capacitive reactence phenomenon is present only in AC urrent not in DC current.

It will be highly appreciated if you can refer to me with some technical literature regarding this.
thanx

On July 29, 2012 at 8:48am
Robert wrote:

For the DC method: What value should be used for the 2 currents? Example 10 seconds 0.1 C and then 3 seconds 1 C?

On November 14, 2012 at 2:05am
bratt wrote:

@Troy Mikkelson: your first method is completely wrong IMO. You cannot use charging current for calculating internal resistance as SoC influences the current the most. During charging the current significantly falls down at the same voltage supplied, so it would indicate that resistance grows up?

On November 23, 2012 at 12:01am
yuvaraj s wrote:

how will current accuracy of battery analyzeraffects the Li-ion batteries?

On December 16, 2012 at 3:25am
chandru wrote:

can you tell Do ‘s and Don’t for tubular batteries?

On March 20, 2013 at 8:49am
Les Springs wrote:

@Bratt and Troy,
Troy’s method is okay (except that I would use the battery itself as the power source).  Charging current decreases at a constant charging voltage because the battery’s voltage is increasing (the battery is charging up).  When the battery’s voltage is close to the charger’s voltage, current will be very, very, low (but not zero).  It’s faster to use a constant-current charger, that increases its voltage until rated volts is attained.
A good test of a battery’s condition, or internal resistance, is taking the difference between no-load and loaded terminal voltage, divided by the test current.
I submit these comments as an electrician, electronics technician, and electrical engineer with two degrees and 45 yrs experience.  Use the comments as you see fit, or not.

On July 1, 2013 at 2:40am
Anil Malapni wrote:

How to test Electrical Resistance for Tubular Bags

On July 7, 2013 at 5:09pm
Lloyd Johnson wrote:

I have been measuring battery internal impedance for many years with a simple method. You switch a current source load on the battery on and off at 40-100 Hz. Read the AC voltage on the battery terminals with a regular DVM. Works very well. If you size the current correctly, voltmeter reads out directly in milliohms.

On October 11, 2013 at 11:26pm
N M SHETH wrote:

Looyd Johnson your method of testing sounds interesting, can u please explain in detail.

On October 14, 2013 at 12:06pm
Lloyd Johnson wrote:

I use a 555 timer running at 100 Hz drving a Power Fet with a current source in series as a load. I apply a fixed load and then swich in the current source load at 100 Hz. If you size the current source correctly, the AC voltmeter reads out as 1 mv = 1 milliohm. This method may not conform to the standard methods, but I have confirmed it is an accurate measure of internal impedance. Works equally well on Alkaline, Lead Acid, NIMH, Li-Ion, NiCad.

On October 15, 2013 at 7:49pm
Jones wrote:

measure internal resistance of 12 volt lead-acid battery

1) get a low beam incandescent (not halogen) sealed beam (*must* be sealed beam for safety!!) auto headlight from an auto junkyard

2) buy 2 digital multimeters (DVM) at Harbor Freight for $2.99 each (they go on sale often)

3) set DVM1 to the 20VDC range and connect it directly across the battery terminals

4) set DVM2 to the 10 amp setting and wire it in series with the headlight.  connect this series circuit across the battery terminals

5) the headlight should be illuminated and you should be reading roughly 3 amps on DVM2, and roughly 12 volts on DVM1.

6) wait till the voltage reading on DVM1 stabilizes, and record that voltage and the amps reading on DVM2. 

7) disconnect the headlight and immediately read the voltage on DVM1 again.

8) the internal resistance of the battery in ohms is equal to the difference in the two DVM1 voltage readings divided by the DVM2 current reading.

On October 25, 2013 at 5:23am
kuldeep wrote:

Lloyd johnson
Jones:
could u please tell me which method is better for testing VRLA type batteries:
a)  Impedance Method
b) Resistance Method
and why???

On November 20, 2013 at 10:34pm
Karen Priban wrote:

Balancing a lithium battery pack for Electric Vehicle is difficult with large differences between battery cells resistance. I’m looking for a way to measure each cell to purchase batteries with equal resistance.  can you give me more information on how to use an ESR meter?

On October 30, 2014 at 8:34am
Muni sankar Naidu wrote:

Internal resistance values will change with respect to the battery SOC, age, operating tempature etc and hence both IR , impedance and conductance methods and not reliable test methods while comparing with load test..

On November 6, 2014 at 12:06pm
George Moranian wrote:

Lloyd Johnson
How do you determine the correct load for this test? As you state “If you size the current source correctly” How do you determine this?

George

On December 10, 2014 at 9:49am
Robert Dierker wrote:

It sure seems Li-Ion battery capacity goes down really fast with the number of cycles.  How do they overcome this in hybrid cars?