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Capacity Page: 080428
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Capacity, the Key to Battery Runtime
A look at emerging rapid-test technologies for deep-cycle lead-acid batteries
By Isidor Buchmann, Cadex Electronics Inc. ([email protected])
December 2007The secret of battery runtime lies in the capacity. Capacity defines the energy a battery can hold. The definition for capacity is usually given in ampere-hours (Ah); it specifies the elapsed time when discharging a battery at a calibrated current to the end-of-discharge voltage. Portable batteries commonly use a one-hour discharge; larger batteries are rated at either a 5 or 20-hour discharge.
Lead acid batteries come in two basic architectures: deep cycle and starter types. The deep cycle battery is designed for maximum capacity and high cycle count. This is achieved by installing thick lead plates. Typical applications are golf carts, wheelchairs, people movers, scissor lifts and RVs. Starter batteries, in comparison, are made for maximum CCA (cold cranking amp). The battery maker obtains this by adding extra plates to get a large surface area for maximum conductivity. Capacity and deep cycling are less important for automotive because the battery is being recharged while driving. If continuously cycled, the thin lead plates of the starter battery would wear-down rather quickly. As a rule of thumb, the heavier the battery, the more lead it contains and the longer it will last.
What is the difference between Capacity and CCA?
The characteristics of the lead acid battery can best be explained by making capacity responsible for energy and CCA for delivery. Capacity and CCA do not age at the same pace. The CCA tends to stay high through most of the battery's life, and then drops quickly towards the end. This often leaves us stranded when all of a sudden the car won't start in the morning. In comparison, capacity decreases gradually. A new battery is designed to deliver 100% of its rated capacity. As the battery ages, the capacity steadily drops and it should be replaced when the reading falls below 70%. The reader will soon realize that capacity measurement is a more reliable state-of-health indicator than CCA.
Let's look at the aging mechanism of capacity and CCA with graphic illustrations. Figure 1 shows two lead acid batteries, one with high capacity and one that has aged. The build-up of so-called “rock content” as part of aging robs the battery of usable energy although it may still provide good cranking power.
Figure 2 illustrates a battery with high and low CCA by simulating free-flowing and restricted taps.
CCA is responsible for starting an engine. Cranking requires 200A;
a golf cart typically draws 56A.
The third criterion of battery runtime is state-of-charge (SoC). The battery capacity is always measured on a fully charged battery and the most simplistic method of estimating SoC is reading the open terminal voltage (OTV). This approach is accurate if the battery has rested for at least four hours after charge or after applying a load. The rather long rest period is the required recovery time to pacify a battery when disturbed. The reader should also be aware that different plates composition alter the OTV reading. Calcium raises the voltage by 5-8%, affecting SoC estimation. Calcium is an additive that helps in making the battery maintenance-free.
Battery rapid-test methods
Battery capacity is commonly measured by applying a full discharge. While this method provides accurate readings, it is cumbersome, time consuming and wears the battery down unnecessarily. During the last 15 years, several rapid-test methods have emerged that eliminate the need for discharge, so the manufacturers claim. Introduced in 1992, AC conductance became popular in measuring conductance, from which CCA is estimated. This non-invasive method was hailed as a major breakthrough because the test only takes a few seconds and the instrument stays cool. Unfortunately, AC conductance is unable to read capacity and is of limited use for deep cycle batteries.
During the last five years, critical progress has been made towards capacity estimations. Cadex has developed a battery rapid-tester based on multi-model electrochemical impedance spectroscopy (Spectro™). The Spectro CA-12 injects 24 frequencies ranging from 20-2,000 Hertz. The signals are regulated at 10mV to stay within the thermal battery voltage of lead acid. The 24 slices from the frequency excitations are compared and the minute nuances analyzed. The instrument completes 40 million transactions during the short 15-second test.
Electrochemical impedance spectroscopy (EIS) is not new. Equipment using this technology has been in use for decades. A full-fledged EIS requires dedicated instruments and a computer to analyze the data. The set-up is expensive, requires trained staff for analysis and is so large that the machinery is moved on wheels. Furthermore, long calculation times make the system unsuitable for commercial use. The Spectro CA-12 has solved these problems by using powerful digital signal processors, but the heart of the engine lies in the patented algorithm.
What are typical battery problems?
Let's look at the most common battery problems and evaluate how modern battery rapid-testers can detect these deficiencies. One can immediately see the benefit of knowing the capacity.
Low charge. A low charge reduces the drive power and the battery appears weak. Checking a low-charge battery with a discharge unit will show low capacity. Rapid-testers such as the Spectro CA-12 are able to measure the capacity with a SoC as low as 40%. If lower, the instrument will prompt to charge and retest.
Low capacity. This low capacity battery will likely have good conductivity and strong torque. The voltage checks out fine and everything appears normal except the short runtime. Knowing the capacity on an aging deep cycle battery is very important because it's the best indication when a battery should be replaced.
Mismatched set. Batteries do not age at an equal pace. Like the links of a chain, the battery with the lowest capacity will govern the runtime. Battery testers reading capacity can identify low performers and allow a timely replacement. The high performers can be regrouped for continued service.
As encouraging as battery rapid testing may be, the reader needs to be reminded that rapid-testers, such as the Spectro CA-12, are not universal instruments capable of measuring the capacity of any battery that will come along; they need a battery-specific matrix as a reference. On purchase of such a unit, the instrument includes one or several matrices that are automatically matched with the selected battery. Cadex is in the process of expanding the matrix library to eventually include all major battery types.
In time, measuring battery performance through non-invasive means will become the acceptable standard, making discharge methods redundant. Typical applications are: checking batteries to reduce false warranty returns, preventing unexpected downtime by assessing battery state-of-health before a breakdown occurs, and improving the reliability of battery operated rental equipment.
Designers of battery rapid-test methods tend to be overly optimists and create targets that may not be achievable outside the laboratory. However, multi-model electrochemical impedance spectroscopy represents a great leap forward and opens the door to an entirely new way of battery testing.
About the Author
Isidor Buchmann, founder and CEO of Cadex Electronics Inc., has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. As an award-winning author of many articles and books on the subject, Mr. Buchmann has delivered battery-related technical papers around the world.
Cadex Electronics Inc. is a Canadian company specializing in the design and manufacturing of advanced battery testing instruments. For product information please visit www.cadex.com
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