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Fundamentals in Battery Testing

Fundamentals in Battery Testing

Discover what causes batteries to fail and why testing is still in its infancy.

No practical method exists to quantify all conditions of a battery in a short, comprehensive test. State-of-health (SoH) cannot be measured per se, it can only be estimated to various degrees of accuracy based on available symptoms. If the symptoms are vague or not present, a reliable measurement is not possible. When testing a battery, three SoH indicators must be evaluated:

1.Capacity, the ability to store energy
2.Internal resistance, the capability to deliver current, and
3.Self-discharge, reflecting mechanical integrity and stress-related conditions

Batteries come in many conditions and a charge can easily mask a symptom allowing a weak battery to perform well. Likewise, a strong battery with low charge shares similarities with a pack that exhibits capacity loss. Battery characteristics are also swayed by a recent charge, discharge or long storage. These mood swings must be clearly identified when testing batteries.

Figure 9-1 demonstrates the usable battery capacity in volume that can be filled with a liquid, permanent capacity loss in the form of “rock content” that reduces the volume, and internal resistance in tap size symbolizing current flow

Figure 1: Conceptual battery symbolizing the usable capacity, the empty portion that can be refilled, permanent capacity loss as “rock content” and the tap symbolizing power delivery as part of internal resistance.

The leading health indicator of a battery is capacity, a measurement that represents energy storage. A new battery should deliver 100 percent of the rated capacity. This means a 5Ah pack should deliver five amperes for 1 hour. If the battery quits after 30 minutes, then the capacity is only 50 percent. Capacity also supports warranty obligations with a replacement due when falling below 80 percent. Most importantly, capacity defines end of battery life.

Lead acid starts at about 85 percent and increases in capacity through use before the long and gradual decrease begins. Lithium-ion starts at peak and begins its decline immediately, albeit very slowly. Nickel-based batteries need priming to reach full capacity when new or after a long storage.

Manufacturers base device specifications on a new battery. This state is temporary and does not represent a battery in real-life situations because fading begins from the day it is made. The decrease in performance only becomes visible once the shine of a new device has worn off and daily routines are being taken for granted. An analogy is an aging man whose endurance begins to wear off after the most productive years (Figure 2).

 

Figure 2: Battery can be likened to a man growing old. Few people know when to replace a battery; some are replaced too early but most are kept too long.

 

Knowing when to replace a battery is a blur for many battery users. When asked, “At what capacity do you replace the battery?” most reply in confusion, “I beg your pardon?” Few are familiar with the term capacity as a measurement of runtime, and fewer know that capacity is used as a threshold for retiring batteries. In many organizations, battery problems only become apparent with increased breakdowns, which may be caused by a lack of battery maintenance.

Battery retirement depends on the application. Organizations using battery analyzers typically set the replacement threshold at 80 percent. Some industries can keep the battery longer than others and a toss arises between “what if” and economics. Scanning devices in warehouses may go as low as 60 percent and still provide a full day’s work. A starter battery in a car still cranks well at 40 percent, but that is cutting it thin.

Any battery-operated mission must plan for a worst-case scenario. Although manufacturers include some reserve when specifying runtime, the amount is seldom clearly defined. Critical missions demand tighter tolerances and the battery must be replaced sooner than when a sudden failure can be tolerated.

Medical and military devices are considered critical and batteries are often replaced too soon. Rather than testing them, device manufacturers prefer to use a cycle count or a date stamp to mandate retirement. To cover all eventualities, the service duration on a date stamp is often limited to 2 or 3 years.

Medical technicians have discovered that many batteries for defibrillators have more than 90 percent capacity left when the mandatory 2-year date-stamp expires, replacing perfectly good medical batteries prematurely. In spite of this apparent waste, a US FDA survey says that “up to 50 percent of service calls in hospitals surveyed relate to battery issues.” Healthcare professionals at AAMI (Association for the Advancement of Medical Instruments) say further that “battery management emerged as a top 10 medical device challenge.”

Another application where battery capacity is important is in a drone. With a good battery, the device may be specified to fly for 60 minutes, but if unknown to mission control, the capacity has dropped from 100 to 75 percent, the flying time is reduced to 45 minutes. This could crash the $25,000 vehicle when required to negotiate a second landing approach. By marking the capacity on each pack as part of battery maintenance, batteries delivering close to 100 percent capacity can be assigned for long hauls while older packs may be sent for shorter errands. This allows the full use of each battery and establishes a sound retirement policy.

Many batteries and portable devices include a fuel gauge that shows the remaining energy. A full charge always shows 100 percent, whether the battery is new or faded. This creates a false sense of security by anticipating that a faded battery showing fully charge will deliver the same runtime as a new one. Batteries with fuel gauges only indicate SoC and not the capacity.

Battery failure is not only limited to portable devices. Starter batteries in vehicles have also become failure-prone. In 2008, ADAC (Allgemeiner Deutscher Automobil-Club e.V.) stated that 40 percent of all roadside automotive failures are battery-related. A 2013 ADAC report says that battery problems have quadrupled between 1996 and 2010.

ADAC, Europe’s largest automotive club, says further that each third breakdown involves either a discharged or defective battery. The report, published by German Motorwelt in May 2013, also mentions that only a few starter batteries reach the average age of five years, and this applies to all cars. The statistic was derived from the more than four million breakdowns that the ADAC car club typically receives in a year. The study only included newer cars; service-prone vehicles more than 6 years old were excluded.

BCI (Battery Council International) reports similar results. A 2010 study by the BCI technical subcommittee revealed that grid-related failures had increased by 9 percent from 5 years earlier. Experts suspect that higher electrical demands in modern vehicles lead to higher failure rates.

Battery failure in Japan is the largest single complaint among new car owners. The average car is driven 13km (8 miles) per day and mostly in congested cities. The most common reason for battery failure is undercharge, developing sulfation. Battery performance is key; problems during the warranty period are recorded as component failure and tarnish customer satisfaction.

A German manufacturer of luxury cars reported that one in two starter batteries returned under warranty had no problem. A German manufacturer of high-quality starter batteries stated that factory defects account for only 5 to 7 percent of all warranty claims. Battery failure during the warranty period is seldom a factory defect; driving habits are the main culprits. A careful assessment with advanced battery test instruments capable of looking at various failure symptoms can greatly reduce warranty claims.

The mobile phone industry experiences similar battery warranty issues. Nine out of ten batteries returned are said to have no problems. Rather than trouble-shooting a customer complaint because of lower than expected runtime, the clerk simply replaces the battery. This burdens the vendor without solving the problem; it may also lead to repeat complaints.

Dilemma of Battery Testing

Part of the problem lies in the difficulty of testing batteries, and this applies to storefronts, hospitals, combat fields and service garages. Battery rapid-test methods seem to dwell in medieval times, and this is especially evident when comparing advancements on other fronts. We don’t even have a reliable method to estimate state-of-charge, which is based mostly on voltage and coulomb counting. Assessing capacity, the leading health indicator of a battery, dwells further behind. Measuring the open circuit voltage and checking the internal resistance do not provide conclusive evidence of battery state-of-health.

The battery user may ask, “Why is the industry lagging so far behind?” The answer is simple: “Battery diagnostics are complex.” As there is no single analytical device to assess the health of a person, nor are instruments available that can quickly and reliably check the state-of-health of a battery. Like the human body, batteries can have multiple hidden deficiencies that no singular test method can identify with certainly.

A dead battery is easy to check and all testers are 100 percent accurate. The challenge comes in evaluating a battery in the 80–100 percent performance range while on duty. Regulators struggle to introduce battery test procedures. This is mostly due to the unavailability of suitable technology that can assess a battery on the fly.  The battery is labeled “uncontrollable” for good reason; this gives it immunity.

The battery world devotes much effort on the super battery, but this improved battery is incomplete without being able to check performance while in service. Improving performance and reliability does not rest in a better battery alone, but in tracking the performance as it ages.

Professor Mark Orazem compares the complexity of testing batteries with the Indian tale in which blind men touch an elephant to learn what it is (Figure 3). Because each man only feels a part of the body, disagreements arise among them. Battery testing is complex even for the sighted man but progress is being made. Better technologies will eventually immerge.

 

 

Figure 3: Indian tale reflecting the complexity of estimating battery state-of-health.

Story of blind men trying to figure out an elephant through touch. The tale provides insight into the relativism and opaqueness of a subject matter, such as a battery.

 

 

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