Battery failure is common, but avoidable, if you know how.
A battery is a corrosive device that begins to fade the moment it leaves the assembly line. Its stubborn and unpredictable behaviour has left many users in awkward situations. Battery failure is common and up to 50% of system breakdowns are attributed to a failing battery. Much of this is avoidable, but even with the best of care, some batteries die early and scientists don’t know why. Batteries exhibit human-like characteristics and the health rests on the genetic makeup, environmental conditions, and user pattern.
The manufacturer specifies the run-time of a device on a battery performing at 100% capacity, but most operate at less. As time goes on, the performance declines further, and the battery gets smaller in terms of storing energy. Most batteries deliver 300 to 500 full discharge/charge cycles, more on a partial discharge.
In the first year, most batteries work well, but the confidence fades in the second and third year. As batteries begin to lose capacity, new packs are added and in time, the battery fleet becomes a jumble of good and failing batteries. This is when the headache begins. Unless date stamped or other quality controls are in place, the user has no clue about the history of a battery, much less its performance.
The energy in a battery can be divided into three segments: available energy, the empty zone that can be refilled, and the unusable part or rock content that has become dormant. Figure 1 illustrates these three sections graphically.
1. The run time on a device is measured on a 100% battery; most packs deliver less.
The “ready” light on a charger can’t verify a battery’s “health.” Ready only reveals that the battery is fully charged. As the active space of a battery declines with age, charge times also decrease. This can be compared to topping a water jug that’s filled with rocks. Many battery users are unaware that weak batteries charge faster than good ones. The low performers gravitate to the top and become disguised to the unsuspecting user who trusts the “green light.” A short charge time is not only reserved to a poor battery, a pack with a partial charge also charges quickly because there is little to add.
A battery needs constant care and feeding. Even if fully charged, self-discharge consumes valuable energy. This is not a manufacturing defect but a battery characteristic, although poor manufacturing and improper handling can elevate the problem. Table 1 shows the self-discharge of common batteries.
The amount of self-discharge varies with battery type and primary cells retain the energy longer than rechargeable systems. The energy loss is asymptotical, meaning that the self-discharge is highest right after charge and then tapers off. The self-discharge on all battery chemistries increases at higher temperatures and the rate typically doubles with every 10°C (18°F). High cycle count and aging also increase the self-discharge.
The care and feeding of a battery begins with the acceptance from the supplier and continues to its rightful retirement. Battery service includes the following:
Incoming inspection: All batteries should be checked before field deployment. Packs that fail to meet performance criteria should be returned. The open circuit voltage of a lead-acid battery should be at least 2.10 V/cell. Nickel-based batteries may need priming to get the full capacity, especially if they had been in storage for a while. After charging, lithium-based batteries out-of-box should be close to 100%.
Periodic capacity check: Batteries must be treated like any other medical device. While date stamping offers an alternative to analyzing batteries, this method does not guarantee reliable performance because some packs fail before the expiration date, but most last longer. Properly maintained modern batteries tend to outlive the date stamp. Longevity is a function of cycle count and depth of discharge. Capacity, and not the manufacturing date, is a battery’s leading health indicator.
Retirement: The battery capacity decreases with use and time. Medical staff may be unaware of capacity fade and continue using the battery. A battery should be replaced when the capacity drops to 80% (70% in some cases), and restoration isn’t possible. Do not retire batteries too soon. Discarding good batteries increases operational cost and raises environmental concerns. Battery analyzers can save money by predicting the correct replacement time.
Some batteries are in daily use, others are mainly on standby. Table 2 shows the recoverable capacity after one year of storage. Recovered capacity defines the remaining full-charge capacity after storage. Batteries deteriorate faster at higher temperatures and with a full charge; this phenomenon is especially apparent with Li-ion.
Battery analyzers have made critical inroads into the medical industry. For example, the Cadex C7400ER illustrated in Figure 2 services four batteries independently. Battery adapters permit plug-and-play operation, automated programs provide hands-off service, and manual operation enables the setting of unique parameters.
2. Each station is programmable to 30 V and 6 A; batteries connect by battery adapters or the programmable Smart Cable.
A speaker at a battery conference once said, “The battery is a wild animal and artificial intelligence domesticates it.” He wanted to make the battery intelligent. While adding an SMBus assists in battery management, it comes with baggage. “Smart batteries” are not standardized and the fuel gauge shows state-of-charge without reference to the actual capacity. In addition, a battery equipped with a fuel gauge needs periodic calibration to correct the tracking errors that occur with time. The error is about 1% with each cycle. To calibrate, apply a full charge/discharge and repeat the service every three months or after 40 partial cycles. A non-calibrated battery will still work, but will provide false state-of-charge readings. Figure 3 shows the calibration process.
3. Calibration occurs by applying a full charge, discharge and charge in the equipment or with a battery analyzer as part of maintenance.
A technical expert at a hospital commented on his personal experience regarding smart batteries, saying, “I have more problems dealing with the smart part of the battery than the actual cells. Many medical batteries have logic problems, memory errors, glitches, or low-voltage recovery issues.” Smart batteries for specialty applications have more anomalies than packs in laptops and other consumer products.
To eliminate system failures, authorities have implemented strict maintenance and calibration guidelines, but the battery enjoys immunity and escapes the inspector’s scrutiny as being “uncontrollable.” To satisfy regulatory requirements, the inspectors may validate logistic issues such as the model number and service dates and ignore the capacity, which is the leading health indicator.
Battery performance is difficult to estimate. Measuring capacity with discharge is time-consuming and rapid-test methods aren’t always dependable. Battery testing often appears to be stuck in medieval times. There are no simple solutions but progress is being made.
The user is at the mercy of the battery. Charge-and-run without maintenance doesn’t guarantee sufficient reliability. To avoid unnecessary risks, many hospitals and paramedics are taking a proactive approach toward battery maintenance. There’s also a strong interest in cutting costs to keep each pack in service for the full duration of the useful life, and modern battery analyzers make this possible. Advanced battery analyzers also provide rapid-test methods that sort batteries into good, suspect, and poor buckets in a matter of seconds.
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, and has written articles and books, including “Batteries in a Portable World.” 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.