Tracking Battery Capacity and Resistance as part of Aging

Let’s examine the aging mechanism of batteries in terms of fading capacity and increasing internal resistance. Figure 1 shows a battery with high capacity and another that has aged. The capacity loss is illustrated with growing “rock content;” the rocks mark the unusable part of the battery. Figure 2 looks at resistance and illustrates a good battery with a free-flowing tap and a high-resistance one with restricted flow.

New battery has high capacity

Aged battery has low capacity

Figure 1: Battery capacity illustrated as liquid content. Both batteries are fully charged, but the “rock-content” limits the amount of energy being stored.

Battery with high CCA

Battery with low CCA

Figure 2: Free-flowing and restricted taps representing CCA performance.The cranking current is about 300A. (A golf cart typically draws 56A.)

Automotive technicians are most familiar with CCA, but this reading reflects engine cranking only. Capacity, the energy storage component, remains mostly unknown. Figure 3 illustrates the relationship between CCA and capacity on hand of a fluid-filled container. The liquid represents capacity and the taps symbolize CCA at different loading capabilities.

Figure 3: Relationship of CCA and capacity of a starter battery Capacity represents energy content and CCA is power delivery. A battery with 40% capacity can still have a healthy crank but the low capacity indicates end-of-life.

Most rechargeable batteries maintain low internal resistance during the service life, and this reflects in a high CCA (cold cranking amps) on starter batteries. Capacity, on the other hand, begins to drop gradually as the battery ages. To study these changes, Cadex measured the capacity and CCA of 20 aging starter batteries. The results are laid out in Figure 4, sorted according to capacity levels in percentage.

Figure 4: Capacity and CCA readings of 20 aging batteries. Batteries 1 to 9 have good CCA and high capacity; the CCA of batteries 10 to 20 remains reasonably strong but suffers from capacity loss. CCA tends to remain high while the capacity drops steadily as part of aging.

Test method: CCA was estimated with the Spectro CA-12 and the capacity was measured with an Agilent load bank by applying full discharges according to BCI standards.
Courtesy of Cadex
Batteries 1 to 9 perform well on capacity and CCA, but batteries 10 to 20 show notable capacity loss while the CCA remains strong. The motorist is unaware of the fading capacity until the car won’t start one morning. This is especially critical during a cold spell, which further reduces the capacity.
Capacity is the leading health indicator of a battery, and car manufacturers often use 65 percent as the pass/fail threshold for warranty replacement. This magic level forms a natural bend, a cliff between a high performing battery and one that is beginning to age. Service garages usually take 40 percent as an end-of-life indication.  Even though a starter battery with 40 percent capacity may still crank well and have 6 to 12 months of service left before it will finally quit, the battery should be replaced. Thrifty drivers, (including this author) prefer to wait, but invariably get caught with a dead battery at the worst possible moment.
 Evaluating the capacity of a starter battery gives the most accurate end-of-life prediction. Capacity sets the floor upon which CCA and other readings are compared. Without knowing capacity, other measurements mean little.