Why do Different Test Methods Provide Dissimilar Readings?

During the last 20 years, three basic battery rapid test methods have emerged: DC load, AC conductance and multi-frequency electro-chemical impedance spectroscopy (EIS). All methods are resistance based, a characteristic that reveals the battery's ability to deliver load current. Internal resistance provides useful information in detecting problems and indicating when a battery should be replaced. However, the battery often drops below the critical 80% level set by IEEE before the condition can effectively be detected. Neither does resistance alone provide a linear correlation to the battery's capacity. Rather, the increase of cell resistance relates to aging. 

When measuring the internal resistance of brand new VRLA cells from the same batch, variations of 8% between cells are common. Manufacturing process and materials used contribute to the discrepancies. Rather than relying on an absolute resistance reading, service technicians are asked to take a snapshot of the cell resistances when the battery is installed and then measure the subtle changes as the cells age. A 25% increase in resistance over the baseline indicates a performance drop from 100% to about 80%. Battery manufacturers honor warranty replacements if the internal resistance increases by 50%. 

Before analyzing the different test methods, let's briefly brush up on internal resistance and impedance, terms that are often used incorrectly when addressing the conductivity of a battery. 

Resistance is purely resistive and has no reactance. There is no trailing phase shift because the voltage and current are in unison. A heating element is such a pure resistive load. It works equally well with direct current (DC) and alternating current (AC). 
Most electrical loads, including the battery, contain a component of reactance. The reactive part of the load varies with frequency. For example, the capacitive reactance of a capacitor decreases with rising frequency. A capacitor is an insulator to DC and no current can pass through. The inductor, on the other hand, acts in the opposite way and its reactance increases with rising frequency. DC presents an electrical short. A battery combines ohmic resistance, as well as capacitive and inductive reactance. The term impedance represents all three types. 

The battery may be viewed as a set of electrical elements. Figure 1 illustrates Randles' basic lead-acid battery model in terms of resistors and a capacitor (R1, R2 and C). The inductive reactance is commonly omitted because it plays a negligible role in a battery at low frequency. 

 

Figure 1: Randles model of a lead acid battery. 
The overall battery resistance consists of pure ohmic resistance, as well as inductive and capacitive reactance. The values of these components are different for every battery tested.

Battery rapid test methods and how they work 

Let's now look at the different battery test methods and evaluate their strengths and limitations. It is important to know that each method provides a different internal resistance reading when measured on the same battery. Neither reading is right or wrong. For example, a cell may read higher resistance readings with the DC load method than with a 1000-hertz AC signal. This simply implies that the battery performs better on an AC than DC load. Manufacturers accept all variations as long as the readings are taken with the same type of instrument. 

DC load method: The pure ohmic measurement is one of the oldest and most reliable test methods. The instrument applies a load lasting a few seconds. The load current ranges from 25-70 amperes, depending on battery size. The drop in voltage divided by the current provides the resistance value. The readings are very accurate and repeatable. Manufacturers claim resistance readings in the 10 micro-ohm range. During the test, the unit heats up and some cooling will be needed between measurements on continuous use. 

 

The DC load blends R1 and R2 of the Randles model into one combined resistor and ignores the capacitor. C is a very important component of a battery and represents 1.5 farads per 100 Ah cell capacity.
Figure 2:DC load method. 
The true integrity of the Randles model cannot be seen. R1 and R2 appear as one ohmic value.

AC conductance method: Instead of a DC load, the instrument injects an AC signal into the battery. A frequency of between 80-100 hertz is chosen to minimize the reactance. At this frequency, the inductive and capacitive reactance converges, resulting in a minimal voltage lag. Manufacturers of AC conductance equipment claim battery resistance readings to the 50 micro-ohm range. AC conductance gained momentum in 1992; the instruments are small and do not heat up during use. 

The single frequency technology sees the components of the Randles model as one complex impedance, called the modulus of Z. The majority of the contribution is coming from the conductance of the first resistor.
Figure 3: AC conductance method. 
The individual components of the Randles model cannot be distinguished and appear as a blur.

Multi-frequency electro-chemical impedance spectroscopy (EIS): Cadex Electronics has developed a rapid-test method based on EIS. Called Spectro™, the instrument injects 24 excitation frequencies ranging from 20-2000 Hertz. The sinusoidal signals are regulated at 10mV/cell to remain within the thermal battery voltage of lead acid. This allows consistent readings for small and large batteries..

With multi-frequency impedance Spectroscopy, all three resistance values of the Randles model can be established. 
A patented process evaluates the fine nuances between each frequency to enable an in-depth battery analysis.
Figure 4: Spectro™ method. 
R1, R2 and C can be measured separately, enabling the estimation of battery conductivity and capacity

Spectro™ is the most complex of the three methods. The 20-second test processes 40 million transactions. The instrument is capable of reading to a very low micro-ohms level. With stored matrices as reference, Spectro™ is capable of providing battery capacity in Ah, conductivity (CCA) and state-of-charge.

The EIS concept is not new. In the past, EIS systems were hooked up to dedicated computers and diverse laboratory equipment. Trained electrochemists were required to interpret the data. Advancements in data analysis automated this process and high-speed signal processors shrunk the technology into a handheld device.

Capacity measurements

DC load and AC conductance have one major limitation in that these methods cannot measure capacity. With the growing demand of auxiliary power on cars and trucks and the need to assess performance of stationary batteries non-invasively, testers are needed that can estimate battery capacity. Cadex has succeeded in doing this with car batteries. The company is working on applying this technology to stationary batteries.

Figure 5 reveals the reserve capacity (RC) readings of 24 car batteries, arranged from low to high on the horizontal axis. The batteries were first tested according to the SAE J537 standard, which includes a full charge, a rest period and a 25A discharge to 1.75V/cell during which the reserve capacity was measured (black diamonds). The tests were then repeated with Spectro™ (purple squares) using battery-specific matrices. The derived results approach laboratory standards, as the chart reveals

 

Figure 5: Reserve capacity of 24 batteries with a model-specific matrix. 
The black diamonds show capacity readings derived by a 25A discharge; the purple squares represent the Spectro™ readings.

Some people claim a close relationship between battery conductivity (ohmic values) and capacity. Others say that internal ohmic readings are of little practical use and have no relation to capacity. To demonstrate the relationship between resistance and capacity, Cadex Electronics has carried out an extensive test involving 175 automotive batteries in which the cold cranking amps (CCA) were compared with the RC readings. CCA represents the conductivity of the battery and is closely related with the internal resistance. 

Figure 6 shows the test results. The CCA readings are plotted on the vertical Y-axis and the RC on the horizontal X-axis. For ease of reading, the batteries are plotted as a percentage of their nominal value and are arranged from low-to-high on the X-ax
is. 

Figure 6: CCA as a function of reserve capacity (RC).
Internal resistance (represented by CCA) and capacity do not follow the red line closely and fail to provide accurate capacity readings.

Note: The CCA and RC readings were obtained according to SAE J537 standards. CCA is defined as a discharge of a fully charged battery at -18°C at the CCA-rated current. If the voltage remains at or above 7.2V after 30 seconds, the battery passes. The RC is based on a full charge, rest period and a discharge at 25A to 1.75V/cell.
If the internal resistance (CCA) were linear with capacity, then the blue diamonds would be in close proximity of the red reference line. In reality, CCA and RC wander off left and right. For example, the 90% CCA battery produces an RC of only 38%, whereas the 71% CCA delivers a whopping 112% capacity (green dotted line). 

An important need is fulfilled

Cadex has packaged the EIS technology into an elegant hand-held tester that is currently being beta-tested in the USA, Canada, Europe and Japan. 

Being able to obtain battery capacity makes the EIS technology one of the most sought-after test systems for automotive, marine, aviation, defense, wheeled mobility, traction and UPS batteries. Capacity fading due to aging and other deficiencies can be tracked and a timely replacement scheduled.
 

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Comments

On February 10, 2011 at 8:19am
Carlos Olaya wrote:

Which is the difference between the Impedance result from the Spectro device and the Sulfation result from the EC2000 device (Global Energy Innovation)

On November 29, 2015 at 5:46am
John Adank wrote:

I’ve noticed most all the auto repair places switching to the conductance type battery testers like the ones from Midtronics.  Places like Walmart, SAMs club Fleet Farm and many of the auto repair dealerships and garages now use these.  One mechanic told me NAPA gave them the $1,000 Midtronics tester fro free as long as they sell 5 batteries per month.  What I’m seeing with these units is people just hooking them up and printing off the receipt and giving it to you.  They don’t explain anything or can’t and just read off whatever the slip says.  Sometimes they don’t even take the battery temperature with the unit.  Case in point, my daughter recently took her car to a Tires Plus for a tire rotation.  We did not request them to check her battery but they did.  They used a Midtronics tester and called me later and told me the battery was low and needed to be replaced.  I asked low on what?  Water, CCA?  The guy couldn’t tell me.  He just said it was low.  The print out stated that it measured 156CCA out of 700CCA and the voltage was 12.5volts.  The graph stated to replace battery and this is what they told me to do.  They would install a new Interstate flooded battery for $165.  The battery in her vehicle was a standard flooded battery purchased in 2011.

I brought the car home and removed the battery cell caps to check the fluid levels.  All 6 cells were low in water.  I filled up all six cells with distilled water to the proper levels and cleaned both battery terminals.  I then charged up the battery using my Optima Digital 1200 battery charger.  After the battery was fully charged, I hooked it up to my Schumacher BT-100 load tester, removed the service charge and load tested the battery.  The battery tested good.  Time will tell how long the battery lasts but for now it’s working fine.  It’s winter here so if it’s going to fail, it should fail in the cold temps.

I guess my question is this.  The Midtronics tester stated to replace the battery but if the service person would have topped off the cells with water and charged the battery would he have received different results?  What I’m seeing is your and adults using these conductAnce testers and not knowing what it is they’re testing.  They can’t explain or trouble shoot the issues.  They just print off the receipt and repeat what it says.  It’s basically used as a dummy tester.  Just hook it up and print off the receipt.  There’s a reason why battery companies are requiring these units.  Yes, it’s safer because you won’t blow up a battery but is it as accurate in the hands of a novice technition than a load tester?  In my experience I would say no..  These units will sell more batteries but one has to wonder if the battery is really in need of replacement or just water and a good terminal cleaning.  I was going to purchase one of these Conductance testers from Midtronics but now I’m not so convinced they’re the way to go.  Any thoughts?

On April 17, 2016 at 11:55pm
Ahmed Sabry wrote:

I need to know if DC Electronic Loads and Battery Testers are the same thing or not. In other words, can I call DC Electronic Load as a battery tester.

On December 8, 2016 at 3:15pm
Javier Del Rio wrote:

Dear all; most important thing about ac conductance testers and particularly Midtronics is: for a man like me working on batteries for more than 20 years, cca readings reveals important information that makes us understand battery evolution.
in John’s example, he says that recovered a battery with 156A cca out of 700A cca nominal and the voltage was 12,5volts. With these two data (CCA & Voltage) you can imagine a lot about this battery.
1º battery voltage is too low, with these voltage we can imaging that something wrong is happening in our car.
2º cca levels are to low in comparison with nominal cca, this only occurs when a battery is very aged or in bad maintenance conditions.
To make a fair analysis on Midtronics decision we need an important data that we don´t know: CCA readings after maintenance process.
I´m quite sure that CCA readings on john´s example possibly reachs no more than 400A CCA level.
With this CCA value, battery life expectancy is too low for sure. If you analyze time wasted on battery maintenance, maybe you realize that Midtronics decision was reliable on general terms.
The problem with AC analysers is very simple: understand readings.
Even for inexperience people AC analysers help a lot, and of course, for experience people open a wide world for better understand batteries.

On December 9, 2016 at 1:35pm
John Adank wrote:

So in your opinion, the conductance battery testers like the Midtronics EXP700 that was used to test my daughters battery gives more information and a better analysis then say a carbon pile tester?  I’m just wondering because I’ve onsidered purchasing one of these conductance units but I’ve been hesitant after my daughters battery test.  The battery is still in her vehicle and working fine.  I guess the thing to do would have been to tKe the battery back to Tires Plus for testing after I had cleaned it up and added water to the cells to see what the Midtronics unit gave for results.  Do you feel the conductance battery testers provide more accurate results of the current and future performance of the battery?

Thank you,
John

On December 31, 2016 at 3:47pm
Bud M wrote:

John,
I believe the article above shows that the EIS (Spectro) device is clearly the superior way to measure battery health. The biggest problem with an EIS device is that they are too expensive for most of us (at around $2000). Even most pros can’t justify that sort of price and that’s why they’re using cheaper conductance type testers (like the Solar or Midtronics available on Amazon from around $50).

In turn, a conductance tester, appears to be superior to all other testers.

Your guy in Tires plus gave you a conductance test result that showed your daughter’s car battery was 156 CCA. Let’s assume the CCA rating on the battery was 700 when new and that he did the test correctly, then we see that the current CCA is way lower than when new.  Now we are told that CCA is defined as discharge of a fully charged battery at -18°C at the CCA-rated current for 30 seconds. If the voltage remains at or above 7.2V after 30 seconds, the battery passes.
This means that your daughter’s battery when new could supply 700A for 30 seconds under the above conditions. But now it can only supply 156A. Her starter motor needs about this much to start the car after it’s been sitting overnight. So this is getting marginal. The battery appears to have no reserve for the time when the car doesn’t easily start first time.  If the battery was fully topped up with water and fully charged, the Tires Plus guy would have a more solid point. As it was, he did neither of these things and really just wanted to sell you a new battery.
Nonetheless he was still helpful to you. He alerted you to a problem and you attempted to fix it at home. Now you need to retest. If the battery is still showing 156 CCA I think you should seriously consider replacement. The point is that the CCA reading from the conductance tester is way more helpful than say a voltmeter reading which tells you that the battery has 13V -  ie it looks good but you are left guessing whether it can give you those 13V when you need them.

You are obviously a guy who thinks for himself.  I recommend to you an excellent youtube video at https://youtu.be/Oi8sUE9XCgA .  I think that after you view the video, you will see some sense in purchasing a conductance tester.