The market offers a wide choice of battery testers, all providing correct predictions when the battery is dead, but so does the user. The challenge is tracking the capacity of a working battery that is in normal use but will eventually need replacement.
Most testers measure voltage and internal resistance, also known as impedance, but capacity estimation is beyond reach. In spite of this limitation, some makers of battery testers promote the ability to estimate capacity. This confuses the industry into believing that multifaceted results can be attained with basic impedance methods. The term “capacity” as energy storage capability is not always clearly understood.
Battery rapid-testing was easier in the past. Early testers simply measured the internal resistance of a battery that increased with age. Modern batteries have improved and the one-size-fits-all approach no longer applies. Electrolyte additives and advancements in electrode materials have reduce corrosion and the resistance stays low through most the battery life. These improvements apply to lithium-ion and lead acid batteries.
Figure 1 shows the relationship between capacity and internal resistance of modern Li-ion batteries as a function of cycle count. The batteries tested reflect a predictable capacity drop while the resistance stays low throughout most of the service life. Older batteries would have shown a pronounced rise in resistance and likely cross the capacity line, forming the letter ”X.”
Figure 1: Relationship between capacity and resistance as part of cycling
Resistance does not reveal the state-of-health of a modern Li-ion; the internal resistance stays flat.
Average of 5 Li-ion batteries cycled at 1C:
Charge: 1,500mA to 4.2V, 25C
Discharge: 1,500 to 2.75V, 25C
Improvements in battery technology have not eliminated the gradual loss of capacity and batteries have a defined life span. This also reflects in the auto industry, especially in starter batteries. The short lifespan is mostly caused by the start-stop function and energy-robbing auxiliary loads of modern cars. ADAC road assistance in Germany says that 42 percent of all vehicle breakdowns are battery related. According to the ADAC 2013 report, problems with starter battery have risen four times between 1996 and 2010. The report states that each third breakdown involves a discharged or defective battery and that few packs reach the age of five years. (ADAC stands for Allgemeiner Deutscher Automobil-Club, a German automobile club founded in 1903.)
Checking the capacity by discharging a fully charged battery would provide the most reliable estimation but this is impractical for larger systems and requires removing the battery from service. There is also the stress factor. A lead acid permanently loses roughly two percent of its capacity with each full cycle. A starter battery is made for short deliveries of high current and loses as much as eight percent when fully discharged. Starter batteries should not be deep-cycled.
A luxury German carmaker reports that half of all batteries replaced under warranty exhibited no fault. A leading battery maker states that only 5 to 7 percent of warranty batteries have a factory fault. This implies that most failures are user-induced and a replacement could be prevented. Japan says that battery breakdown is the largest single complaint by new car owners. Starter batteries in gridlocked traffic do not always receive sufficient charge and fail because of sulfation at no fault to the manufacturer.
A battery that cranks the engine well does not always relate to good health. This gives the motorists a false sense of security by assuming that all is well. Capacity determines the health but this value is difficult to measure on the fly. Not knowing the capacity causes many batteries to be changed too soon, but most are replaced too late.
Most testers for automotive batteries measure cold cranking amp (CCA). CCA relates to internal resistance that tends to stay high while the capacity drops in a linear and predictable way with use and age. CCA reading could indeed predict state-of-health on older batteries to some degree but this is no longer accurate with the improvements. Cadex tested 175 aging starter batteries and found that the correlation between CCA and capacity is only 0.55. A perfect match would be 1. Basing battery SoH on CCA is alike tossing a coin.
Battery rapid-testing is developing on several fronts and involves time domain with discharge pulses and frequency domain with frequency scans. Cadex developed Electrochemical Dynamic Response (EDR) to check small Li-ion cells by observing voltage recovery on applied small pulses. A fast recovery denotes good ion flow; a sluggish reaction indicates capacity loss. An analogy is a dried-out felt pen that still writes but needs rest to replenish the ink. Some older battery may also show elevated voltage deflection due to higher internal resistance and EDR evaluates this also. Figure 2 compares a good battery with quick recovery against a faded one with a sluggish response.
Figure 2: Electrochemical Dynamic Response
EDR measures the ion flow between the positive and negative electrodes. A good battery responds quickly; a faded one is sluggish.
EDR is superior to the rudimentary pulse method in that it can estimate SoH of Li-ion, but the technology is ill-suited for larger batteries. EDR is also sensitive to the choice of electrolytes and cathode materials. This drawback can be solved by characterizing a battery, but the test becomes battery specific, requiring so-called look-up tables that must be derived from batteries with various performance levels.
Technologies using frequency domain are mostly based on electrochemical impedance spectroscopy (EIS). Scientists believe that the future of battery rapid-testing is in EIS. The battery is scanned with a frequency from less than one Hertz to several kilohertz. High frequency reveals resistive qualities, also known as bird-eye view, but the unique characteristics of a battery are hidden in the low frequencies.
Cadex advanced EIS further and developed multi-model electrochemical impedance spectroscopy, or Spectro™ for short. The battery is scanned to produce a Nyquist plot that characterizes the individual components of the Randles model. Randles breaks the battery it down to ohmic and reactive components. Electrochemical models, or matrices, are then fitted to estimate capacity and CCA values, calculations that involve 40 million transactions to derive at the end result. Spectro™ and its matrices are similar to devices that read letters, fingerprints, eye retinas and facial features.
Spectro™ can operate on a battery-specific or generic matrix. Both are created by scanning several batteries of different performance levels. The battery-specific matrices have the advantage of displaying the capacity in a numeric readout (percent of the full capacity). The generic matrix may include a group of starter batteries ranging from 40–100Ah and offers the result in a pass/fail classification. Many service personnel appreciate this method as it present a clear yes/no assessment and eliminates customer interference.
The Spectro™ technology is embedded in the Spectro CA-12, a handheld battery tester that estimates the capacity and CCA in less than 30 seconds noninvasively. The battery must have a minimal charge of 60 percent. Best results are attained by testing a “working” battery taken from regular service. New batteries that lack formatting, or packs that had been in prolonged storage, may not exhibit the same symptoms.
Another promising application to estimate capacity is integrating the Spectro™ technology in a Battery Management System (BMS). Standby batteries are often installed and forgotten. Voltage and impedance readings cannot estimate the capacity and very few large batteries are discharged for the purpose of verifying sufficient energy storage. A capacity-estimating BMS can tell the user when to replace a battery.
Technologies measuring battery state-of-health can also be integrated into a battery charger. Such a system will not only charge a battery but also assure that the pack delivers a capacity of 80 to 100 percent when the ready light illuminates. Health validation in a charger provides quality control at no extra effort and tells the fleet managers when to show faded packs the exit door.
Batteries do not exhibit visible changes as they wear down and age; they look the same when fully charged or empty, new or old and in need of replacement. A car tire, in comparison, distorts when low on air and indicates end-of-life when the treads are worn.
Batteries should receive the same treatment as a critical part in an aircraft, medical device or industrial machine where wear-and-tear falls under strict maintenance guidelines. Authorities struggle to implement such a procedure for batteries; lack of suitable test technology makes this difficult. Bad batteries can hide and enjoy immunity among the peers. New technologies being developed will eventually invade the long-held exclusive status and making batteries accountable. Cadex is part of this development.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Buchmann has studied the behavior of rechargeable batteries in practical, everyday applications, has written award-winning articles including the best-selling book “Batteries in a Portable World,” now in its third edition. Cadex specializes in the design and manufacturing of battery chargers, analyzers and monitoring devices. For more information on batteries, visit www.batteryuniversity.com; product information is on www.cadex.com.
Last updated 2014-11-20
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