A battery resembles a living organism that cannot be measured; only estimated by diagnostics similar to a doctor examining a patient. The accuracy of rapid-testing varies according to symptoms. These indicators change with state-of-charge (SoC), agitation after charge and discharge, temperature and length of storage. A rapid-test must have the ability to distinguish between a good battery that is partially charged and a weak pack that is fully charged. Both variants deliver similar runtimes in the hands of the user but have different performance levels.
A widely used performance analysis is coulomb counting in which in-and-out-flowing energies are measured. Coulomb counting goes back 250 years when Charles-Augustin de Coulomb first established the “Coulomb Rule.” Elegant in concept, but coulomb counting has its own problem in that it loses accuracy when the battery is randomly charged and discharged. State-of-health (SoH) estimation by a digital solution is incomplete without also including the chemical battery.
The leading health indicator of a battery is capacity. Capacity represents energy storage, a quality that gradually and permanently fades with use. Other characteristics responsible for SoH are internal resistance that governs load current and self-discharge that examines mechanical integrity. All three characteristics must be met to give a battery a clean bill of health.
Estimating the capacity of the chemical battery on the fly is most complex. This involves algorithms and matrices that serve as lookup tables similar to letter or face recognition. Modern rapid-test methods move towards advanced machine learning in capturing the many moods of a battery.
Here is a summary of simple to complex test methods to examine batteries.
No single test can capture all health indicators of a battery. Many rapid-test devices look only at voltage and internal resistance. While capacity loss of a fading NiCd or NiMH may correlate with rising internal resistance, this relationship is less evident with lithium- and lead-based batteries. Advertising capacity estimation with a tester that only measures voltage and internal resistance can be misleading. It confuses the industry into believing that complex results are attainable with simplistic methods. Resistance-based instruments will indeed identify a dying or dead battery; but so does the user.
A battery is a reactive device and the method by which resistance measurements are taken matters. A DC measurement looks at pure resistive values while AC includes reactive components that provide additional information. Figure 1 represents the impedance of a good and faded Li-ion battery when scanned with AC from 0.1Hz to 1kHz. The strongest variances in impedance (-Imp -Z) are observed on the low frequency scale ranging between 1Hz and 10Hz.
Figure 1: Frequency scans of a good and weak mobile phone battery.
Impedance variances are most visible below 10Hz. The horizontal scale is logarithmic to condense the frequency range.
Source: Cadex Electronics
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