Discovering Damage before Danger Arises with the Three-prong Battery Test
The media is quick to report mishaps with lithium-ion batteries. A failed Li-ion gets higher attention than an accident with an older battery chemistry. Electric hoverboards have been recalled because of battery fires; in 2006 Sony recalled laptop batteries, and in 2012/2013 the Boeing 787 Dreamliner was grounded to redesign the battery enclosure to withstand a fire. These calamities occurred after the battery had passed UL safety tests; instabilities surfaced in field use. With the proliferation of lithium-ion batteries and not knowing when and how they will die, safety must be addressed as part of work-force-to-retirement.
Lithium-ion is safe but with millions of industrial and consumer applications, failures will happen. The Sony recall was a one-in-200,000 breakdown caused by microscopic metal particles coming in contact with other parts in the battery cell, leading to a short circuit. Battery manufacturers strive to minimize the presence of such particles; however, assembly techniques make the elimination of dust a challenge. Modern cells with ultra-thin separators of 21µm (21-thousandth of an mm) are more susceptible to impurities than older designs with heavier separators and lower Ah ratings. While the classic 1,350mAh cell in the 18650 package could tolerate a nail penetration; the energy-dense 3,400mAh turns into a firework when performing the same test.
There are two basic types of battery failures. One is the one-in-10-million incident involving a manufacturing flaw that may lead to a recall. The more difficult failure is a random event like being hit by a meteor. Such a breakdown does not point to a design flaw but may be stress related, such as charging at sub-freezing temperature, exposure to heat or excess vibration. Some applications overstress the battery and hobby gadgets, hoverboards and drones are examples. An analogy is placing a souped-up sports car engine into a heavy truck. The small engine won’t last; neither will a wrongly chosen Li-ion battery.
Most battery failures leading to disintegration start with a mild electrical short that goes unnoticed. This can be an uneven separator with dry areas leading to poor conductivity and developing a heat spot. Fast charging at cold temperatures promotes dendrite formation, so does storing Li-ion below 1.5V/cell for more than a week. The stress events promote self-discharge that can lead to a destructive failure.
What is especially frustrating for Li-ion manufacturers and users is not being aware of a pending breakdown. The battery performs normally because the internal stresses are disguised. This is analogous to a steel beam that suddenly breaks under repeated heavy loads. Load bearing is well documented and the failure point is known, but random defects still occur due to metal fatigue or a structural fault. Once broken, the damage is finite and cannot be rewound for replay. This also happens with a destroyed battery; however there is an early prediction: elevated self-discharge.
As the condition of a battery deteriorates, the self-discharge increases, which can develop into a sizable current between the electrodes. Similar to a seemingly harmless water leak in a faulty hydro dam that can develop into a torrent and take a structure down, so can also high self-discharge build up heat and damage the separator, leading to an electrical short. The temperature will quickly reach 500°C (932°F), at which point the cell catches fire or explodes. The thermal runaway that occurs is known as “venting with flame; “rapid disassembly” is the preferred term by the battery industry.
The Technical University of München in Germany stressed Li-ion cells by applying a deep discharge and storing them in a shorted condition. Figure 1 demonstrates the self-discharge of a new Li-ion cell, one that underwent forced deep discharges and a cell that was fully discharged, shorted for 14 days, and then recharged. In normal use, BMS and protection circuits would prevent such conditions to occur.
Figure 1: Self-discharge of new and stressed Li-ion cells. Cells stressed by deep discharges and kept at 0V exhibit a higher self-discharge than a new cell.
Source: TU München
Battery research places most of its attention on birth-to-graduation. Equally important is workforce-to-retirement, a topic that tends to get ignored. While regulatory approvals for new batteries are tough, once accepted the officials wash their hands and place the responsibility of maintaining the battery to the user. That’s when battery problems begin. Rules on the usage of a battery are vague and the operator may ask, “How much capacity is sufficient for reliable operation? How often should I check the battery? At what capacity should I replace the pack?”
A bio-med whistle-blower in charge of medical instruments said, “Batteries are the most abused components. Staff care little about them and only do the bare minimum. References to battery maintenance are vague and hidden inside service manuals.” AAMI (Association for the Advancement of Medical Instruments) rates battery management as one of the top 10 challenges, and a US FDA survey says that up to 50% of issues in hospitals are battery related.
Batteries should receive the same treatment as a critical part in a machine or an aircraft where wear and tear falls under strict maintenance guidelines. Batteries are given special privileges and are labeled “uncontrollable.” This immunity excuses them from undergoing regular inspections, but this does not need to be so. Batteries can and should be checked and this is possible with the three-prong test. The three-prong battery test examines three most critical functions that include:
Figure 2: Three-prong battery test.
The three-prong battery test resembles a three-legged stool that represents capacity, internal resistance and self-discharge. Each characteristic is unique with no correlation between the properties.
Measuring capacity by applying a full discharge cycle provides the most reliable assessment. This method is most effective with portable batteries and a periodic analysis ensures that the batteries are kept within an acceptable performance range. With smart batteries, FCC (full charge capacity) can be used to estimate state-of-health (SoH). FCC is a coulomb count that remembers how much energy the battery received and delivered previously. Relying on digital reference readings reduces the need to cycle but any tracking mechanism loses accuracy over time and this can be restored with periodic calibration. Calibration consists of a full charge/discharge/charge cycle.
Larger batteries are normally not cycled because it is time-consuming and stresses the battery. That’s where non-invasive test methods come in. A number of rapid-test methods are available that assess the chemical battery by electrochemical impedance spectroscopy and other methods. No rapid-test is bang-on and the accuracy correlates to symptoms received and the sophistication of algorithms employed.
Battery analyzers became popular in the 1980s and 1990s to restore nickel-cadmium batteries that were affected by “memory.” Today these workhorses are employed to analyze a broad range of batteries as part of fleet management to assure system integrity. Battery analyzers act as gatekeepers to retire packs when the capacity falls below a set performance criteria; 80% is the typically accepted end-of-life threshold.
To satisfy the three-prong battery test, modern battery analyzers should also measure the internal resistance and self-discharge; functions that can be integrated into the test procedure without human intervention. The self-discharge test works as follows:
On program completion, a battery analyzer or charger measures the battery terminal voltage, known as OCV (open circuit voltage), as the Li-ion battery rests. Voltage neutralization after charge or discharge takes about two hours after which valid self-discharge readings can be taken. Monitoring continues as long as the Li-ion battery rests in the device. Self-discharge values are shown as a percentage of the rated Ah and should be about 5% per month for a good battery including protection circuit. More research will be needed to know what level of self-discharge is acceptable, what raises concern, and when a battery should be quarantined.
Monitoring self-discharge should also be included in a battery management system (BMS). This is especially critical for wheeled mobility where batteries undergo harsh environmental stresses caused by temperature extremes and vibration. Self-discharge should also be checked in devices that prepare Li-ion packs for air shipment by discharging them to 30% SoC. Excessively high self-discharge caused by internal damage leading to a possible fire during transport would clearly be identified beforehand.
The longer test times required to measure self-discharge is a drawback. Battery analyzers and chargers offering this feature will complete the capacity and internal resistance assessment in the anticipated time with the green light indicating readiness for service. If, however, the pack is left in the device for a few extra hours, such as an overnight service, the self-discharge readings will be added. No added logistics is needed to complete the three-prong battery test, just time.
The compounded growth for Li-ion batteries from 2014 to 2019 is said to be 22% per annum. Batteries are getting larger and the saying goes, “Small children, small problems; big children, big problems.” As mom and dad need to sharpen their parenting skills with teenagers in the house, so also do larger Li-ion batteries require better diagnostics and demand tighter safety regulations than smaller packs of the past.
“How will large Li-ion systems age under different environment conditions?” is an open question. Will they die peacefully or end life with a bang after years of hard work? Cadex is studying work-force-to-retirement by developing BMS, chargers and analyzers that include safety checks. Fundamental to this is the three-prong battery test revealing the key battery functions of capacity, internal resistance and self-discharge, which are responsible for runtime, power and safety.
Of large importance is also cell balance, a reading that is available with many SMBus batteries. The capacity of serially connected cells should be within +/-2.5%. Batteries with well-matched cells perform better and live longer than those that have become uneven. Identifying a battery with CAUTION should the cells get out of balance provides an additional safety check and hints to end-of-life because of reduced performance.
Battery service and test devices will soon communicate to the cloud by wireless connectivity to make battery state-of-health transparent to the user. Such a system will allow the operator to simply call up all packs with a low capacity for replacement at budget time. Gaining access to this information lowers operational risk as the state-of-health of each pack is known. This enables a full service life for each battery, lowering operational costs and protecting the environment because fewer packs are discarded prematurely. Knowing battery SoH also lowers repairs as the battery is the cause of many malfunctions.
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Buchmann has studied the behavior of rechargeable batteries in practical, everyday applications, has written award-winning articles including the best-selling book “Batteries in a Portable World,” now in its fourth edition. Cadex specializes in the design and manufacturing of battery chargers, analyzers and monitoring devices. For more information on batteries, visit www.batteryuniversity.com; product information is on www.cadex.com.
Last updated 2016-09-03
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