Reliability- Centred Maintenance in Batteries
Reliability-centred maintenance (RCM) is a service strategy that provides improved system reliability with a reduced level of required maintenance.
RCM began in the 1960s; was adopted by the airline industry in the 1970s and the military in the 1980s. In the 1990s, it spilled into civil users (such as nuclear power plants, oil and gas, subways, and hospitals). Defined by the technical standard SAE JA1011, RCM provides risk awareness that improves reliability while reducing the need for invasive maintenance that lowers operational costs.
“Fix it when broken” worked with old machinery. With air travel, this method no longer applied, and United Airlines was the first company to adopt RCM for safety reasons. Modern aircraft and new machinery now harmonize with RCM to reduce maintenance requirements. As an example of cost savings, the DC-8 airplane built in a pre-RCM era needed four million man-hours of structural inspections; the Boeing 747 built on RCM standards required 66,000 man-hours – 60 times less than the DC-8.
RCM is also suitable to monitor batteries in systems. The battery often needs replacement before other parts because of capacity fade. However, before battery monitoring can be implemented effectively, better diagnostic technologies must be developed. Most test systems only provide voltage, current and temperature. Without knowing the capacity of the battery, the end of battery life cannot be predicted.
Battery diagnostics tests have not advanced as quickly as other technologies and this is due to complexity and volatility. A battery resembles a living organism that changes symptoms when fully charged, empty, agitated, or in storage. A pack looks the same when fully charged or empty, new or in need of replacement. In comparison, a car tire distorts when low on air or indicates end-of-life when the treads are worn.
Batteries are often installed and forgotten. A new battery starts with a capacity of 100 per cent and fading goes unnoticed at first. Similar to a mechanical part, batteries can also fail prematurely, especially if stressed. Considering their growing importance, batteries should receive the same treatment as a critical part in an aircraft or machine in which wear and tear falls under strict maintenance guidelines.
The Association for the Advancement of Medical Instrumentation (AAMI) rates battery management as one of the top 10 challenges. An FDA survey reveals that up to 50 per cent of issues in hospitals are battery related. FDA points to deficiency in battery quality assurance by device manufacturers, lack of understanding in battery systems integration, and not knowing the end of battery life.
Commonly asked battery questions
At a recent workshop with biomed technicians, they were asked the following 10 questions.
Are batteries a problem in the devices you service? Do you trust them?
There is a general distrust in batteries, and technicians agree that up to 50 per cent of system failures are battery related.
Who is responsible for the battery in a device?
Devices with removable batteries have an advantage in that the user can replace an empty pack with a fully charged one. The operator can also check the pack with a battery analyzer and retire it when faded. With built-in packs, the responsibility to test and replace the battery falls on the service technician.
When should a battery be replaced? What percentage denotes end-of-life?
Unless checked with a battery analyzer, the capacity is not known. “Fix it when broken” applies. Batteries come oversized to allow for some fading; end-of-life is commonly at 80 per cent. Battery capacity should be verified when servicing a device. Batteries should also be tested before replacement.
Are there regulatory procedures regarding battery testing?
In the absence of a battery analyzer, device manufacturers recommend replacing batteries on a date stamp. The time allotment is commonly two years and a battery can often be one year old when entering service. This leads to discarding good batteries. Research has shown that the capacity of a defibrillator battery is still above 90 per cent after two years in service.
As a general guideline, lead- and nickel-based batteries are good for about three years, but Li-ion can last for more than five years. In contrast, batteries for the electric vehicle are guaranteed for eight years. This longevity is also attainable with quality Li-ion properly fitted. To demonstrate battery endurance, the organizer of this workshop reuses spent batteries from patient heart pumps to cut the grass with his modified electric lawn mower.
Do you follow the date stamp or common sense when replacing a battery?
Here the philosophy differed. One gentleman said he replaces the battery according to the mandated date stamp, but most others use the common sense approach, provided the evidence can be proven. Device manufacturers are aware that date stamping leads to underutilization and high replacement costs.
How do you check battery capacity?
Many batteries and portable devices include a fuel gauge displaying battery state-of-charge (SoC). While this is helpful, the readout does not guarantee runtime. A serious error occurs if an aged battery shows 100 per cent SoC while the capacity has dropped to 50 per cent. In this case, the runtime is cut in half.
Unless checked with a battery analyzer, the capacity remains unknown. Capacity is the leading health indicator that also governs end-of-life. When I ask battery users “At what capacity do you replace the battery?” most reply in confusion, “I beg your pardon?”
What are the benefits of the smart battery?
Modern devices equipped with smart batteries offer state-offunction (SoF) that is instantly readable. full charge capacity (FCC) in a smart battery represents the “digital capacity” that correlates to the “chemical capacity.”
Smart battery applications have room for improvement. Few portable devices show SoF in an easy-to-read format. In fear of high warranty claims with consumer products, SoF is often only accessible by an access code. A technician said that the “digital battery” causes more problems than the chemical battery. Many chargers are hybrid in that they switch to regular charging when digital communications fail.
Would an SoF icon work?
As the presenter, I proposed the Fishbowl, an icon that provides SoF graphically.
It became apparent that not all technicians are fully familiar with SMBus. It is unfortunate that device manufacturers hesitate to use SMBus data for the benefit of the user. Valuable diagnostic information is locked up and the battery remains a black box.
How beneficial would a database be?
Online databases that share test results are gaining popularity with battery users, technicians, and fleet supervisors. With the Cadex Cloud, battery packs that drop below a user-set target capacity are identified and replaced. The Cadex Cloud also records the energy consumed by reading SoC before charge. Batteries returning from the field with low spare charge can be tracked as part of risk management. An analogy is an aircraft carrying enough fuel to anticipate headwinds and a second landing approach.
How well are batteries documented?
It came as a surprise that healthcare requires minimal documentation for batteries in service. There are no set standards, and little is done to track performance history. Tools to assist in-service and online documentation will likely come from the private sector.
RCM of Batteries
Improvements in battery technology do not rest in higher capacity alone, but in supplying dependable and safe energy over the entire service life of the battery. Batteries follow different criteria than the wear and tear of a mechanical part. Key performance indicators of a battery that need monitoring are capacity, internal resistance, and self-discharge.
Capacity is the leading health indicator of a battery that determines the end of battery life. Capacity is difficult to measure on the fly and requires a battery analyzer or rapid-test methods. The capacity of Li-ion can also be measured by coulomb tracking during use.
Internal resistance governs current flow. Ohmic values are easy to get, but they do not correlate with capacity. The resistance of Li-ion and lead acid batteries tend to stay low with good power loading capability while the capacity drops gradually and predictably.
Self-discharge reflects the mechanical integrity of a cell. Li-ion has low self-discharge; elevated levels hint to damage that can compromise safety. Self-discharge of lead acid is moderate but rises with age due to contamination by grid shedding; NiMH has high self-discharge even when new.
Advancements in Diagnostic Battery Management will lead to an effective use of RCM to make the battery transparent. Accurate diagnostics keeps batteries in service longer and reduces operational costs. Equally important is reducing environmental harm associated with fabricating and disposing of batteries.
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Isidor has studied the behaviour of rechargeable batteries in practical, everyday applications, and has written award-winning articles and a best-selling book, Batteries in a Portable World.