A Look at Emerging Technologies Capable of Estimating Battery Reserve Capacity

by Isidor Buchmann
Cadex Electronics Inc.

Ac conductance testing was introduced in 1992 as a new way of measuring the cold cranking amps (CCA) of a car battery. This non-invasive method was hailed as a major breakthrough and, to a large degree, eliminates load testing to measure battery performance. The test only takes a few seconds; the readings are displayed in digital numbers and a message spells out the condition of the battery. There are no sparks at the battery terminals and the instrument remains cool.

However, single-frequency ac conductance has limitations. It does not measure CCA according to SAE standards, but offers an approximation relating to the battery's power output capability. This relative power figure often varies with state-of-charge and other battery conditions. At times, a good battery fails and a faulty one passes by error; the most significant drawback is its inability to read the reserve capacity (RC). Despite these shortcomings, ac conductance has become an accepted standard for predicting battery life and determining when to replace an old battery before it becomes a nuisance.

What are the main differences between CCA and RC? A high CCA reading ensures good battery conductivity and provides strong cranking ability. High CCA goes hand-in-hand with a low internal battery resistance. Figure 1 compares high CCA with a large, open tap that allows unrestricted flow.

Figure 1. Battery with high CCA and 100-percent reserve capacity. A high CCA battery can be compared to a large, open tap that allows unrestricted flow.

Figure 1. Battery with high CCA and 100-percent reserve capacity. A high CCA battery can be compared to a large, open tap that allows unrestricted flow.

Reserve capacity governs the amount of energy the battery can store. A new battery is rated at a nominal capacity of 100 percent. As the battery ages, the reserve capacity drops and the battery eventually needs replacing when the reserve capacity falls below 70 percent. The RC reading always refers to a fully charged battery; the state-of-charge (SOC) should not affect the measurement.

A battery may provide a good CCA reading and start a car well, but be low on reserve capacity. This battery would be run down in no time when drawing auxiliary power. Figure 2 illustrates such a battery. The so-called "rock content" that builds up as the battery ages is permanent and cannot be reversed.

Figure 2. Battery with high CCA, but low reserve capacity. The cranking on this battery is good, but running on auxiliary power will drain the battery quickly.

Figure 2. Battery with high CCA, but low reserve capacity. The cranking on this battery is good, but running on auxiliary power will drain the battery quickly.

Figure 3 illustrates a battery with good reserve capacity, but low CCA. This battery has a difficult task turning the starter and needs replacing even though it could be used for low load applications.

Figure 3. Battery with low CCA, but high reserve capacity. The low CCA of this battery provides poor cranking although the reserve capacity is high.

Figure 3. Battery with low CCA, but high reserve capacity. The low CCA of this battery provides poor cranking although the reserve capacity is high.

With increased demand for auxiliary power on vehicles, measuring energy reserve is more relevant than CCA. The slogan goes: "Starting is easy, but can I steer and brake?" Modern battery testers must adapt to this new requirement and also include RC measurements. European car manufacturers place heavy emphasis on reserve capacity, while in North America CCA is still the accepted standard to assess battery performance. Most modern battery testers also provide SOC readings.

Measuring reserve capacity is more complex than CCA. Many methods have been tried, including multi-frequency conductance, but most have failed. One of the main obstacles is processing large volumes of data received when scanning a battery with multiple frequencies. Collecting the data is easy; making practical use of the information is the problem. Lack of high-speed microprocessors and processing difficulties have stalled the developments of advanced battery testers. Because of this, no major improvements have been made in this field during the last 12 years. This may change soon.

Cadex Electronics has invented a method that enables the processing of a large volume of data received through multi-frequency electro-chemical impedance spectroscopy (EIS). Trademarked Spectro™, the system injects 24 excitation frequencies ranging from 20 Hz to 2,000 Hz, and reveals CCA, reserve capacity and SOC on a single measurement. The signals are regulated at 10 mV to remain within the thermal battery voltage of lead acid. This permits stable readings for small and large batteries. The test takes 20 seconds, during which about 40 million transactions are completed.

Normally, EIS requires dedicated equipment and a computer to analyze the obtained data. To permit such analyses in a hand-held unit, high-speed digital signal processing is used. Spectro™ has primarily been demonstrated on 12V lead-acid batteries, automotive in particular. The large pool of available car batteries provides an excellent platform to verify the technology. The same technology can also be used on nickel and lithium-based batteries.

Figure 4a. Displays CCA, RC and SOC. During the 20-second test time, the digital signal processor completes 40 million transactions. Figure 4b. Displays CCA, RC and SOC. During the 20-second test time, the digital signal processor completes 40 million transactions.

Figure 4. Displays CCA, RC and SOC. During the 20-second test time, the digital signal processor completes 40 million transactions.

The battery needs to be charged for testing. The typical test band is 50 percent to 100 percent SOC. Early tests provide stable results over a wide temperature range. There is good immunity to electrical noise. Parasitic loads of up to 30A have been tried without notable side effects. The tester tolerates some acid stratification, but chemical additives may affect the readings.

Early Test Results on Reserve Capacity

Verifying the accuracy and repeatability of a new invention takes much time and effort. To test Spectro™, Cadex assembled a test bed of 91 car batteries with diverse performance levels. The preparation consisted of a fully saturated charge, followed by a 24-hour rest period and a 25A discharge to 10.50V (1.75V/cell), during which the reserve capacity was measured. This procedure produced a +/-15-percent variation in capacity readings across the full population. When comparing the capacity obtained through a conventional discharge and by non-invasive means, one must take into account the vulnerability of lead acid.

Figure 5. RC comparison of 38 batteries with a generic matrix. The black diamonds show the RC obtained with a full discharge; the purple squares represent Spectro estimations.

Figure 5. RC comparison of 38 batteries with a generic matrix. The black diamonds show the RC obtained with a full discharge; the purple squares represent Spectro™ estimations.

How can the RC readings be further improved? Best results are achieved by sorting the batteries according to architecture and CCA rating. We developed a model specific matrix and tested a group of same-model batteries. Figure 6 shows the reserve capacity readings derived through a conventional full discharge and Spectro™. With specific matrices, the readings approach laboratory standards in terms of accuracy.

Figure 6. RC comparison of 24 batteries with a model-specific matrix. The purple squares follow the black diamonds very closely. Specific matrices approach reading within laboratory standards.

Figure 6. RC comparison of 24 batteries with a model-specific matrix. The purple squares follow the black diamonds very closely. Specific matrices approach reading within laboratory standards.

Summary

Technology has advanced to a point where measuring battery performance through non-invasive means will become the acceptable standard. Applying a full discharge for the purpose of obtaining the reserve capacity will be a thing of the past. Multi-frequency electrochemical impedance spectroscopy with improved data processing algorithms will make this possible.

Scanning a battery with multi-frequencies EIS not only makes RC estimations possible; it also improves the CCA readings. Rather than measuring vague numbers that only simulate the ability to deliver power, as is the case with single frequency AC conductance, EIS properly executed can provide actual CCA equivalents. In addition, the wealth of information available with a multi-frequency scan will also improve state-of-charge estimations.

Ac conductance will continue to fill an important role in testing batteries in a service sector. However, serious battery users will welcome the introduction of more advanced instruments with open arms. Typical applications of the new EIS technology will be evaluating warranty returns in the automotive industries, assessing the state-of-life of stationary batteries and verifying the reserve capacity for batteries in defense applications. EIS will also become indispensable in checking batteries for wheelchairs, golf carts, robots, boats and forklifts, provided the appropriate matrix is available.