BU-902: How to Measure Internal Resistance

Learn what resistance readings tell about a battery.

The resistance of a battery provides valuable information about its performance as it detects hidden trouble spots. High internal resistance are often the triggering point to replace an aging battery; however, resistance measurement is not the only performance indicator as the value between batches of lead acid batteries can vary by 5–10 percent, especially with stationary batteries. Because of this wide tolerance, the resistance method works best when comparing the readings of 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.

There is a notion that internal resistance is related to capacity, and this is false. The resistance of modern lead acid and lithium-ion batteries stays flat through most of the service life. Advancements in electrolyte additives are reducing internal corrosion issues that were in part responsible for the rise in resistance. Figure 1 shows capacity fade with cycling in relation to the internal resistance of Li-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 and 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


What Is Impedance?

Before exploring the different methods of measuring the internal resistance of a battery, let’s examine what electrical resistance means, and establish the difference between pure resistance (R) and impedance (Z) that includes reactive elements such as coils and capacitors. Both readings are obtained in Ohms (Ω), a measurement that goes back to 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 is expressed in 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 occurs 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 are reactive and have impedance. Impedance consists of a capacitive reactance component (capacitor) and an inductive reactance component (coil). Capacitive resistance decreases with higher frequency while inductive resistance increases. An analogy is an oil damper that stiffens when forcing fast back-and-forth movements. [BU-105, Watts and Volt-amps (VA)]

A battery as power source has resistive, capacitive and inductive resistance, and the term impedance includes all three in one. This can best be illustrated with the Randles model (Figure 2) that reflects resistors R1 and R2 as well as capacitor 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 diagram and electrical values differ for every battery.

R1 = Internal resistant; R2 = Charge transfer;
C1 = Double layer capacitor

Measuring a battery by resistance is almost as old as the battery itself and several methods have developed over time, all of which are still in use.


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. High-end test instruments claim resistance readings in the 10 micro-ohm range. Many garages use the carbon pile to measure starter batteries and an experienced mechanic gets a reasonably good assessment of the battery.

The DC load method has limitations in that it blends R1 and R2 of the Randles model into one combined resistor and 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. In addition, the DC load method gets similar readings with good battery that is partially charged and a marginal battery that is fully charged. State-of-charge and capacity estimations are not possible.


 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. The load test is the preferred method for batteries powering DC loads.

Two-tier DC load

Figure 4: Two-tier DC load

The two-tier DC load follows the IEC 61951-1:2005 standards and provides lifelike test conditions for many DC battery applications.

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 converges, 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 car garages to measure CCA (cold cranking amps) of a starter battery. This single-frequency method 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 readings 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 they are so far apart, the user needs to consider the application.

The pulse DC load method provides valuable readings for a DC application such as driving a motor or powering a light, while the 1000Hz method better reflects the performance requirements of a digital load, such as portable computing and mobile phones that rely to a large extent on the capacitive characteristics of a battery. Most resistance values indicate the test method used. Figure 6 illustrates the 100Hz method.

1000-hertz method

Figure 6: 1000-hertz method

The IEC 1000-hertz provides reactive resistance readings. It is the preferred method to take impedance snapshots of batteries powering digital devices.

Courtesy of Cadex


Electrochemical Impedance Spectroscopy (EIS)

Research laboratories have been using EIS for many years to evaluate battery characteristics. High equipment cost, slow test times and the need for trained professionals to decipher the large volume of data have limited this technology to laboratory environments. EIS reads each component of the Randles model individually; however, analyzing the value at different frequencies and correlating the data is a large task. Digital signal processor (DSP) and fuzzy logic provide assistance in correlating the data. Figure 7 illustrates the individual battery components R1, R2 and C, which EIS 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

*  Resistance (R) is a purely resistive value at zero frequency (direct current or DC)
    Impedance (Z) includes reactance when measured with an alternative current (AC)

Last Updated 2015-06-18

 


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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?

On March 27, 2015 at 12:14am
Larry wrote:

Does anyone have idea about which instrument (e.g. AC milliohmmeter, LCR meter, dedicated battery testers like Hioki’s BT3536/4560) can measure the AC IR/Z of a battery best? Why? Thanks.

On May 9, 2015 at 1:33am
dhanjay kumar wrote:

dear Sir/madam,
i m from India , i want to know how to improve a battery operated vehicle ,when not showing ground in vehicle.
please guide.

On June 25, 2015 at 6:42am
Sam wrote:

This idea that resistance remains consistent throughout a cells life is nonsense.