The saying goes: “So you can start the car, but will the battery crank the engine again tomorrow?” With rapid electrification in vehicles, concerns also arise with steering and breaking drawing draw 100 amperes from the battery.
The battery is the most failure-prone part in a system. Most battery testers and the Battery Management Systems (BMS) read the open battery circuit voltage and calculated the internal resistance (Ri) on voltage-drop under load. This method will find battery anomalies but cannot assess capacity, the leading health indicator. Capacity of a battery declines unnoticed until cranking fails. To provide predictive maintenance in the absence of capacity assessment, some BMS advise the driver to replace the battery on a fixed number of starts. A new technology is gaining acceptance that assesses battery capacity by Electrochemical Impedance Spectroscopy (EIS). The technology is non-invasive and is available in commercial products known as Spectro™. Figure 1 illustrates the compact Spectro Modular. |
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EIS applies a sinusoidal signal from 2,000Hz to 100mHz to create a Nyquist plot. Modelling refers to matrices that had been derived from same-type batteries but different performance levels. Data fusion correlates the values to derive capacity, CCA, SoC and other battery readings. Figure 2 illustrates complex modeling a simplified flowchart.
U.S. PATENTS: 6,778,913; 7,072,871; 10,302,709
Capacity governing end of battery life is demonstrated again and again. ADAC, a German Auto Club, reports that 46% of car breakdown are caused by the battery, up from 42% in 2010. A study by Johnson Control (JCI) involving 800 AGM starter batteries revealed that 48% of batteries became unusable due to low capacity while only 12% failed by low CCA. Failure by poor cranking gives a warning, capacity fade goes unnoticed. Capacity assessment is valuable as the correlation between capacity and CCA is 55% on average. The JCI test pool in Figure 3 also shows 23% in low charge. This is often caused by city driving with auxiliaries engaged. A shorted cell may also be the reason lowering the battery voltage when fully charged. This is a common in large trucks because of vibration. Batteries are often replaced for no apparent reason, and this amounts to 15% in this JCI test. This misjudgment can be resolved with better testing technologies. |
 Source: Johnson Controls Power Solutions EMEA; AABC Europe 2017, Mainz, Germany |
Secret of Battery Capacity
The capacity of a starter battery may drop below 30% and still crank the engine. The pass/fail threshold of most Spectro™ battery testers is set to 40%. This gives a grace period on pass to next service; however, fail calls for a replacement even if cranking is still strong. Capacity determines the end-of-life.
Test time is 30 seconds; longer for large batteries because of an extended frequency range. Spectro™ battery testers can be configured for most batteries, including lithium based systems. Starter batteries use a generic matrix with pass/fail classification; model-specific batteries require a designated matrix. The dexterity of designated matrix hinges on the Matrix Integrity Level (MIL) which can be developed to provide numeric readings. Figure 4 demonstrates predictive capacity fade with a 40% pass/fail threshold.
CCA Measurement is not enough
The CCA on a starter battery tends to stay solid while the capacity gradually decreases. Figure 5 illustrates 20 aging starter batteries that deliver acceptable CCA performance while the capacity on half the test pool dropped below working level.
A German luxury car manufacturer studied capacity and CCA fade of 175 AGM batteries. Figure 6 shows a trend by the arrow that is passing through the Capacity End-of-life gate set at 40%.
A capacity threshold of 40% works well for consumer auto; truck and bus batteries may require a reserve capacity of 50% and higher to accommodate hotel loads. Reading capacity by EIS brings us closer to predicting the remaining useful life that includes large batteries with capacity up to 300Ah.
The test batteries were trunk-mounted AGM driven in a moderate climate.
Test Method: Capacity and CCA were tested according to DIN and IEC standards.
U.S. patent 7,072,871 U.S. patent 6,778,913
ConnectivityModern battery test devices enable storing test results in a cloud database to enable analytics. The Spectro Modular shown in Figure 7 displays test results on a computing device that also connects to the cloud. Advanced battery test systems include lithium based batteries of up to 48V. These need dedicated matrices to perform quality assurance in manufacturing and check incoming batteries in supply chain. Test accuracies rest on MIL that can be improved on continuous bases. |
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Conclusion
EIS opens a non-invasive way to test batteries. Battery characterization using EIS has been in use for decades to study aging effects of batteries but the interpretation was manual. Multi-model EIS automates state-of-health (SoH) assessment. Scientists predict that battery diagnostics is gravitating to EIS, a technology that has not yet explored its potential fully.
Cadex was first-to-market testing batteries with EIS under Spectro™. Being non-invasive and small in footprint will enable including this technology into BMS to assess battery capacity in real life.
Last Updated: 26-Apr-2024
Batteries In A Portable World