BU-905a: Testing Starter Batteries in Vehicles

Learn more how to test a battery, a poorly understood subject

Research firms and trade associations report increased failure rates of starter batteries in modern cars. The reason for the shorter than expected service life is blamed on in-car technology systems that draw high current and prevent a fully charged battery. Other failures are heat related stresses that occur when driving in hot climates. Fewer than 7% of reported problems are caused by factory defects. (See BU-901: Fundamentals in Battery Testing.)

Why Starter Batteries fail

Battery failures are in part influenced by driving habits, the liberal use of auxiliary loads, hot climate conditions, start-stop and battery mount. Even under the best conditions, a quality battery ages and this manifests itself in a gradual drop of capacity. Common failures of starter batteries include:

Heat failure Heat promotes corrosion that reflects in rising internal resistance, resulting in labored cranking. Batteries installed in engine compartments and driving in hot climates result in higher heat failures than those operating in temperate climates. A heat failed battery can be identified with a CCA tester.
Low charge Idling and driving in gridlock with auxiliary loads engaged does not produce sufficient charge and the battery may dwell at 70% state-of-charge (SoC), a level that induces sulfation. A prolonged low-charge condition makes the battery inoperative but this condition can often be improved with long-distance driving or external charging.
Capacity fade Auxiliary loads such as heating elements and mechanical gates hasten capacity fade. The capacity loss goes unnoticed until the battery stops cranking for lack of energy. Replace the battery when the capacity drops to 40%. A CCA test cannot detect a battery with low capacity.
Factory defect Factory defects are roughly 7% or less. Improved manufacturing practises have decreased factory defects while stress related faults are increasing.


What does CCA, RC (Ah) and SoC mean?

CCA Capacity SoC
Figure 1: Graphic presentation of CCA, capacity and SoC.
Capacity loss that occurs naturally with usage is illustrated  as a build-up of rocks. Capacity and CCA do not correlate.
Source: Cadex Electronics

The most common specification of a starter battery is CCA representing cold cranking amps. Batteries also specify capacity, the electrical storage capability that is marked in ampere-hours (Ah) or Reserve Capacity (RC) in minutes. Europe uses Ah while North America goes for RC. (See BU-904: How to Measure Capacity.)

CCA Cold cranking amps is responsible for cranking the engine, a reading that correlates to internal battery resistance. Figure 1 illustrates CCA in the form of a free-flowing tap pouring liquid.
Capacity Represents energy storage measured Ah or RC. Figure 1 shows capacity as liquid storage. Capacity loss is shown as rock buildup that reduces volume.
SoC State-of-charge is demonstrated with liquid levels that can be replenished if low.

What goes wrong?

Figure 2 illustrates a battery with low CCA that is dying after 1–2 years because of heat failure. A typical cause is corrosion that raises the internal battery resistance and lowers the CCA. The capacity remains steady but it cannot be delivered due to low CCA. Replace a battery when CCA drops to 50%.

Heat Failed Battery
Figure 2: Heat failed battery.
Poor cranking due to high internal resistance. Failure mode is gradual and gives early warning.
Test Method: AC conductance or impedance reader. Replace battery when CCA drops to 50%.
Source: Cadex Electronics

Figure 3 shows the natural decrease of CCA and capacity on a well maintained battery. The pack will eventually fail at a ripe old age due to low capacity while CCA is still in working range. A ‘no-start’ occurs when the capacity drops below a required capacity level to crank the engine. A starter battery should be replaced when the capacity drops to 40 percent.

Full Service Battery
Figure 3. Full 4–5 year life.
Symptoms: Capacity fade goes unnoticed and the failure appears sudden. Check capacity as part of preventative service.
Test Method: Rapid-test with Spectro™. Full discharge with load bank is not recommended.
Source: Cadex Electronics

A starter battery must have low internal resistance and sufficient capacity to enable cranking. CCA and capacity can be presented on a two-dimensional table by plotting CCA on the vertical and capacity on the horizontal axis. Figure 4 demonstrates three batteries in various conditions.

Battery Evaluation
Figure 4: Battery evaluation based on CCA and Capacity.
A starter battery can fail by heat failure or capacity fade. Both faults are permanent and cannot be reversed. CCA relates to internal resistance that is easy to measure; capacity reflects energy storage that is more complex to estimate.


Capacity, the Leading Health Indicator

A battery tester should examine CCA and capacity. CCA and capacity do not correlate. Each reading is unique, of which capacity is the more complex to estimate. (See also BU-806: Tracking Battery Capacity and Resistance as part of Aging.)

To study failure modes, a German luxury car maker tested 175 starter batteries. Figure 5 plots capacity and CCA of this extensive test that lasted six-months. Heat failed batteries were excluded.

Reserve Capacity
Figure 5: Capacity and CCA of 175 aging starter batteries.
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 done by a German luxury car manufacturer. Heat damaged batteries were excluded.  
Test Method: Capacity and CCA were tested according to DIN and IEC standards.

As the graph illustrates, most batteries pass through the Capacity Line on the left of the green field. Very few fail by dropping through the CCA Line. Without ability to estimate capacity, batteries with high CCA pass as good, only to fail on the road because of low capacity. Meanwhile, good batteries are being replaced in error. CCA alone cannot predict the end of battery life. CCA tends to stay high while the capacity drops predictably. The Figure 6 illustrates this phenomenon on 20 aging batteries.

Starter Batteries CCA Capacity
Figure 6: Comparing CCA and capacity of 20 aging batteries.
Starter batteries cannot be tested by CCA alone as the reading of a normally aging battery tends to stay high while capacity drops predictably with age.
Test Method: CCA was taken with Spectro CA-12; capacity was measured with an Agilent load bank by applying full discharges according to BCI standards.


Spectro CA-12 Battery Rapid-tester

Spectro CA-12 is a battery rapid-tester that is capable of estimating CCA, capacity and SoC of 12V starter batteries. The CCA range is 100–1,200A, capacity range is 30–105Ah or 60–210 minutes in RC.

To test a starter battery, select between flooded and AGM, and then enter CCA and capacity ratings in ampere hours (Ah) or reserve capacity (RC). The battery should have a minimal SoC of 50%. The test time is 15 seconds.

For more information on battery test methods, see BU-902: How to Measure Internals ResistanceNote:  The Spectro CA-12 is currently only available to Original Equipment Manufacturers (OEM)

Cadex CA-12

Figure 7. Spectro CA-12. Battery rapid-tester estimates CCA, capacity and state-of-charge.
U.S. patent 7,072,871


How does the Spectro™ Rapid-tester work?

Spectro uses multi-model electro-chemical impedance spectroscopy (Spectro). This is a further development of electro-chemical impedance spectroscopy (EIS), a technology that has been in use for many years. The non-invasive test scans the battery with multiple frequencies to generate a Nyquist plot. The Nyquist information is filtered and superimposed onto electrochemical models to derive battery state-of-health (SoH). Nyquist analysis is suitable to test lead acid and Li-ion batteries.

This is the first commercial application using “multi-model EIS” technology to estimate the state-of-health of a battery. The Spectro CA-12 is also the only instrument on the market capable of estimating capacity on the fly.

In medical terms, Spectro™ can be compared to a modern CAT scan. While the traditional X-ray machine only reflects shadowy outlines of a skeleton, the Spectro™ technology applied to batteries can see what doctors would refer to as soft tissues. Battery scientists predict that advanced battery diagnostics is gravitating towards EIS technologies using complex modeling.

Last updated 2017-10-16

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