How to Measure Capacity
The traditional charge/discharge/charge cycle still offers a dependable way to measure battery capacity. Alternative methods have been tried but none deliver reliable readings. Inaccuracies have led users to adhere to the proven discharge methods even if the process is time-consuming and removes the battery from service for the duration of the test.
While portable batteries can be discharged and recharged relatively quickly, a full discharge and recharge on large lead acid batteries gets quite involved, and service personnel continue to seek faster methods even if the readings are less accurate. This section explains what’s available in new technologies, but first we look at the discharge method more closely.
Discharge Method
One would assume that capacity measurement with discharge is accurate but this is not always the case, especially with lead acid batteries. In fact, there are large variations between identical tests, even when using highly accurate equipment and following established charge and discharge standards, with temperature control and mandated rest periods. This behavior is not fully understood except to consider that batteries exhibit human-like qualities. Our IQ levels also vary depending on the time of day and other conditions. Nickel- and lithium-based chemistries provide more consistent results than lead acid on discharge/charge tests.
To verify the capacity on repeat tests, Cadex checked 91 starter batteries with diverse performance levels and plotted the results in Figure 1. The horizontal x-axis shows the batteries from weak to strong, and the vertical y-axis reflects capacity. The batteries were prepared in the Cadex laboratories according to SAE J537 standards by giving them a full charge and a 24-hour rest. The capacity was then measured by applying a regulated 25A discharge to 10.50V (1.75V/cell) and the results plotted in diamonds (Test 1). The test was repeated under identical conditions and the resulting capacities added in squares (Test 2). The second reading exhibits differences in capacity of +/–15 percent across the battery population. Other laboratories that test lead acid batteries experience similar discrepancies.
Figure 1: Capacity fluctuations on two identical charge/discharge tests of 91 starter batteries. The capacities differ +/–15% between Test 1 and Test 2.
Courtesy of Cadex (2005)
Capacity vs. CCA
Starter batteries have two distinct values, CCAand capacity.These two readings are close to each other like lips and teeth, but the characteristics are uniquely different; one cannot predict the other. [BU-806, Changes in Capacity and Resistance]
Measuring the internal battery resistance, which relates to CCA on a starter battery, is relatively simple but the reading provides only a snapshot of the battery at time of measurement. Resistance alone cannot predict the end of life of a battery. For example, at a CCA of 560A and a capacity of 25 percent, a starter battery will still crank well but it can surprise the motorist with a sudden failure of not turning the engine (as I have experienced).
The leading health indicator of a battery is capacity,but this estimation is difficult to read. A capacity test by discharge is not practical with starter batteries; this would cause undue stress and take a day to complete. Most battery testers do not measure capacity but look at the internal resistance, which is an approximation of CCA. The term approximationis correct — laboratory tests at Cadex and at a German luxury car manufacturer reveal that the readings are only about 70 percent accurate. A full CCA test is seldom done; one battery can take a week to measure.
The SAE J537 CCA test mandates to cool a fully charged battery to -18°C (0°F) for 24 hours, and while at subfreezing temperature apply a high-current discharge that simulates the cranking of an engine. A 500 CCA battery would need to supply 500A for 30 seconds and stay above 7.2V (1.2V/cell) to pass. If it fails the test, the battery has a CCA rating of less than 500A. To find the CCA rating, the test must be repeated several times with different current settings to find the triggering point when the battery passes through 7.2V line. Between each test, the battery must be brought to ambient temperature for recharging and cooled again for testing. (For CCA DIN and IEC norms, please refer to “Test Method” on this essay.)
To examine the relationship between CCA and capacity, Cadex measured CCA and capacity of 175 starter batteries at various performance levels. Figure 2 shows the CCA on the vertical y-axis and reserve capacity* readings on the horizontal x-axis. The batteries are arranged from low to high, and the values are given as a percentage of the original ratings.
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Figure 2: CCA and reserve capacity (RC) of 175 aging starter batteries
The CCA of aging starter batteries gravitates above the diagonal reference line. (Few batteries have low CCA and Courtesy of Cadex |
Test method: The CCA and RC readings were obtained according to SAE J537 standards (BCI). CCA (BCI) loads a fully charged battery at –18°C (0°F) for 30s at the CCA-rated current of the battery. The voltage must stay above 7.2V to pass. CCA DIN and IEC are similar with these differences: DIN discharges for 30s to 9V, and 150s to 6V; IEC discharges for 60s to 8.4V. RC applies a 25A discharge to 1.75V/cell and measures the elapsed time in minutes.
The table shows noticeable discrepancies between CCA and capacity, and there is little correlation between these readings. Rather than converging along the diagonal reference line, CCA and RC wander off in both directions and resemble the stars in a clear sky. A closer look reveals that CCA gravitates above the reference line, leaving the lower right vacant. High CCA with low capacity is common, however, low CCA with high capacity is rare. In our table, one battery has 90 percent CCA and produces a low 38 percent capacity; another delivers 71 percent CCA and delivers a whopping 112 percent capacity (these are indicated by the dotted lines).
As discussed earlier, a battery check must include several test points. An analogy can be made with a medical doctor who examines a patient with several instruments to find the diagnosis. A serious illness could escape the doctor’s watchful eyes if only blood pressure or temperature was taken. While medical staff are well trained to evaluate multiple data points, most battery personnel do not have the knowledge to read a Nyquist plot and other data on a battery scan. Nor are test devices available that give reliable diagnosis of all battery ills.
* North America marks the reserve capacity (RC) of starter batteries in minutes; RC applies a 25A discharge to 1.75V/cell and measures the elapsed time in minutes. Europe and other parts of the world use ampere/hours (Ah). The RC to Ah conversion formula is as follows: RC divided by 2 plus 16.
Comments
can i get tables or graphs or info on depth of discharge vs voltage .. i have 12 x 2v 1550 amp hour batteries and want to be able to monitor them if possible with a volt meter
figure 1 doesn’t make sense. If capacity readings are +-15% inaccurate, how could the first test results draw such a perfect line?
Plan is to install an 89 amphour agm for a bilge pump which draws 7 amps. Recharge will be with a solar panel. Runtime is estimated/desired to be 2 hours total, (intermittent 15 minute cycles) service. Will the battery be sufficient? What size solar output is needed to recharge in three days and still remain connected to float the battery?
The pump flowrate varies in direct proportion the speed. Speed varies to the square of the voltage. Its pressure varies to the cube of the speed. What is the expected fall-off of the voltage versus time? With this I can calculate the real-time flow and pressure and obtain a useful life of the pump without destrying the battery.
Anybody care to submit their ideas on this
sir
how can we calculate a 12volt/ 80ah battery efficiency is 90%(lead acid battery)