Predictive Test Methods for Starter Batteries

Starter batteries have two end-of-life modes: Heat failure and capacity fade. Heat failure is caused by corrosion that appears early in life and manifests itself in poor cranking caused by high internal resistance. This deficiency can easily be measured with a CCA tester. A skilled mechanic can also estimate CCA performance by cranking response.

Capacity is more difficult to estimate and specifies the energy a battery holds. While CCA tends to stay high during the life of a battery, capacity fades gradually without the user noticing until the vehicle won’t start one day for lack of capacity. An analogy is a bridge that holds up well and then suddenly collapses with little warning.

New batteries are overrated to allow for performance drop; 25–30% is the low-end capacity cutoff of a starter battery. Motorists also get stranded when cold temperatures lower the already low capacity further. Most service garages replace the battery when dropping to a capacity of 40%.

Battery life has decreased from the typically 5 year to 4 years in new cars even though the battery is said to have improved. Auxiliary loads such as heating elements, mechanical gates and start-stop function lowers the longevity because strenuous loading hastens capacity fade. In most cases capacity governs the end of battery life. This hints to the importance of capacity checking, but the measurement is difficult to attain. Measuring CCA alone is insufficient as CCA and capacity do not correlate well.

Each battery system manifests capacity loss differently. Lead acid loses active material, also known as softening or shedding. A deep-cycle battery contains thick plates to endure repeat cycling, but the starter battery has thin, sponge-like plates to provide a large surface area and achieve high cranking power but the cycle life is limited to 12–15 full cycles. A third variety is the standby battery built for longevity that is filled with low-gravity electrolyte to reduce corrosion. The resulting lower specific energy and resulting larger size is less critical with stationary than mobile batteries. Since UPS batteries are seldom cycled, the plates have a moderate thickness.

Acid stratification is another battery failure in starter batteries that is caused by a fixed regime of charging and partially discharging. Charged electrolyte is heavier than water and gravitates to the bottom. High concentration hastens plate corrosion from the bottom up as illustrated in Figure 2.

Another commons cause of battery failure is sulfation. This occurs when the lead acid battery dwells in a partially charged state and seldom receives a full charge. Batteries in cars driven in city traffic with accessories engaged typically suffer from sulfation because of under-charge. A motor in idle or at low driving speed may not charge the battery sufficiently.

If serviced in time, sulfation can be reversed by applying a slow charge with a regulated current of about 2 amps for 72 hours. Once the sulfate crystals turn into large crystalline structures, restoration is no longer possible. These large crystals are said to block the electrolyte from entering the pores of the plates, rendering the battery unserviceable.

Rapid battery testing does not involve “measuring” a condition but evaluating symptoms that change with state-of-charge (SoC) and temperature. Agitation after loading and charging and prolonged idle times also alters these symptoms. The challenge is distinguishing a good battery with low charge from a poor battery with full charge. The performance of both batteries is similar but the condition differs.

Figure 3 compares CCA and capacity of a battery that suffered heat damage. The capacity is still high but the energy cannot be delivered because high resistance prevents power delivery.

Symptoms: Poor cranking due to high internal resistance. Failure mode is not sudden but progressive.
Test Method: AC conductance tester or equivalent. Shows low CCA reading.

Figure 4 demonstrates an aged pack with capacity fade. CCA may still be good but the battery lacks sufficient energy to crank the engine.

Symptoms: Failure mode is sudden as capacity fade goes unnoticed. Capacity should be checked as part of preventative service.
Test Method: Spectro™

Figure 5 demonstrates CCA and capacity fade as a function of aging. In the lengthy test, a German luxury car maker examines 175 starter batteries to evaluate CCA and capacity behavior. Batteries in the green “pass” field are well within specifications but those in the narrow amber “fringe” field are of interest because they still crank well and are subject to failure because of low capacity. The batteries in the red “fail” area no longer function due to low capacity or other defects. Very few batteries examined failed due to low CCA; most go through the capacity line.

Most batteries pass through the Capacity Line; few fail because of low CCA. The test batteries were trunk mounted and driven in a moderate climate.

Note: Test was conducted by a German luxury car manufacturer.
Capacity and CCA were done according to DIN and IEC standards. Heat damaged batteries were eliminated prior.

Battery Test Instruments

Testing a battery resembles a medical doctor examining a patient. A serious illness could go unnoticed if only blood pressure or temperature were taken. A false assessment would also occur with a battery if only voltage and internal resistance readings were taken. While medical staff is well trained to evaluate tests taken, battery analysis does not receive the same care, nor have test devices advanced to the same level as medical instruments.

An example of advancements in medical devices is the x-ray. Wilhelm Röntgen invented the machine in 1895 to check broken bones. Healthcare meanwhile progressed to CAT scan and MRI to also reveal soft tissues.

Similar advancements are being made in battery testing. Cadex Electronics has been pioneering with electrochemical impedance spectroscopy (EIS) to check the state-of-health of lead acid batteries non-invasively with frequency scanning. EIS also evaluates the condition of lithium-ion batteries.

Research laboratories have been using EIS for many years to evaluate battery characteristics but high equipment cost, slow test times and the need for trained personnel to decipher large volume of data has limited this technology to laboratories. Battery scientists predict that future battery diagnostics will be based on EIS technologies.

The battery rapid-tester based on EIS developed by Cadex is ruggedized to serve on the garage floor. Based on multi-model electrochemical impedance spectroscopy, also known as Spectro™, the device injects sinusoidal signals into the battery at a few millivolts. After digital filtering, the extracted signal forms a Nyquist plot onto which various electro-chemical models are superimposed. Spectro™ selects the best matching models; non-fitting replicas are rejected. Data fusion then correlates the values of the key parameters to derive capacity and CCA estimations.

The Nyquist plot was invented by Harry Nyquist (1889–1976) while at Bell Laboratories in the USA. The parametric plot presents the frequency response of a linear system displaying both amplitude and phase angle on a single scheme using frequency as parameter. The horizontal x-axis of a Nyquist plot reveals the real ohm impedance while the vertical y-axis represents the imaginary impedance.

Spectro™ scans the battery with a frequency spectrum as if to capture the topography of a landscape and then compares the imprint with stored matrices to estimate battery capacity, CCA and SoC. Typical applications are warranty verification on new vehicle batteries and predicting battery end-of-life by estimating capacity as part of after-service. Figure 7 illustrates the Spectro A+ with printer.

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 fourth 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.