One of the most urgent requirements for battery-powered devices is the development of a reliable and economical way to monitor battery state-of-function (SoF). This is a demanding task when considering that there is still no dependable method to read state-of-charge, the most basic characteristic of a battery. Even if SoC were displayed accurately, charge information alone has limited benefits without knowing the capacity. The objective is to identify battery readiness, which describes what the battery can deliver at a given moment. SoF includes capacity (the amount of energy the battery can hold), internal resistance (the delivery of power), and state-of-charge (the amount of energy the battery holds at that moment).
Stationary batteries were among the first to include monitoring systems, and the most common form of supervision is voltage measurement of individual cells. Some systems also include cell temperature and current measurement. Knowing the voltage drop of each cell at a given load reveals cell resistance. Cell failure caused by rising resistance through plate separation, corrosion and other malfunctions can thus be identified. Battery monitoring also serves in medical, defense and communication devices, as well as wheeled mobility and electric vehicle applications.
In many ways, present battery monitoring falls short of meeting the basic requirements. Besides assuring readiness, batterymonitoring should also keep track of aging and offer end-of-life predictions so that the user knows when to replace a fading battery. This is currently not being done in a satisfactory manner. Most monitoring systems are tailored for new batteries and adjust poorly to aging ones. As a result, battery management systems (BMS) tend to lose accuracy gradually until the information obtained gets so far off that it becomes a nuisance. This is not an oversight by the manufacturers; engineers know about this shortcoming. The problem lies in technology, or lack thereof.
Another limitation of current monitoring systems is the bandwidth in which battery conditions can be read. Most systems only reveal anomalies once the battery performance has dropped below 70 percent and the performance is being affected. Assessment in the all-important 80–100 percent operating range is currently impossible, and systems give the batteries a good bill of health. This complicates end-of-life predictions, and the user needs to wait until the battery has sufficiently deteriorated to make an assessment. Measuring a battery once the performance has dropped or the battery has died is ineffective, and this complicates battery exchange systems proposed for the electric vehicle market. One maker of a battery tester proudly states in a brochure that their instrument “Detects any faulty battery.” So, eventually, does the user.
Some medical devices use date stamp or cycle count to determine the end of service life of a battery. This does not work well either, because batteries that are used little are not exposed to the same stresses as those in daily operation. To reduce the risk of failure, authorities may mandate an earlier replacement of all batteries. This causes the replacement of many packs that are still in good working condition. Old habits are hard to break, and it is often easier to leave the procedure as written rather than to revolt. This satisfies the battery vendor but increases operating costs and creates environmental burdens.
Portable devices such as laptops use coulomb counting that keeps track of the in- and out flowing currents. Such a monitoring device should be flawless, but as mentioned earlier, the method is not ideal either. Internal losses and inaccuracies in capturing current flow add to an unwanted error that must be corrected with periodic calibrations.
Over-expectation with monitoring methods is common, and the user is stunned when suddenly stranded without battery power. Let’s look at how current systems work and examine up-and-coming technologies that may change the way batteries are monitored.
The Volkswagen Beetle in simpler days had minimal battery problems. The only management system was ensuring that the battery was being charged while driving. Onboard electronics for safety, convenience, comfort and pleasure have greatly added to the demands on the battery in modern cars since then. For the accessories to function reliably, the state-of-charge of the battery must be known at all times. This is especially critical with start-stop technologies, a mandated requirement on new European cars to improve fuel economy.
When the engine stops at a red light, the battery draws 25–50 amperes of current to feed the lights, ventilators, windshield wipers and other accessories. When the light changes, the battery must have enough charge to crank the engine, which requires an additional 350A. With the engine started again and accelerating to the posted speed limit, the battery begins charging after a 10-second delay.
Realizing the importance of battery monitoring, car manufacturers have added battery sensors that measure voltage, current and temperature. Packaged in a small housing that forms part of the positive clamp, the electronic battery monitor(EBM)provides useful information about the battery and provides an accuracy of about +/–15 percent when the battery is new. As the battery ages, the EBM begins drifting and the accuracy drops to 20-30 percent. The model used for monitoring the battery is simply not able to adjust. To solve this problem, EBM would need to know the state-of-health of the battery, and that includes the all-important capacity. No method exists today that is fully satisfactory, and some mechanics disconnect the battery management system to stop the false warning messages.
A typical start-stop vehicle goes through about 2,000 micro cycles per year. Test data obtained from automakers and the Cadex laboratories indicate that with normal usage in a start-stop configuration, the battery capacity drops to approximately 60 percent in two years. Field use reveals that the standard flooded lead acid lacks robustness, and carmakers are reverting to a modified version lead acid battery. Read about Environmental Concerns.
Automakers want to ensure that no driver gets stuck in traffic with a dead battery. To conserve energy, modern cars automatically turn off unnecessary accessories when the battery is low and the motor stays running at a stoplight. Even with this measure, state-of-charge can remain low if commuting in gridlock conditions because motor idling does not provide much charge to the battery, and with essential accessories like lights and windshield wipers on, the net effect could be a small discharge.
Battery monitoring is also important on hybrid vehicles to optimize charge levels. Intelligent charge management prevents stressful overcharge and avoids deep discharges. When the charge level is low, the internal combustion (IC) engine engages earlier than normal and is left running longer for additional charge. On a fully charged battery, the IC engine turns off and the car moves on the electrical motor in slow traffic.
Improved battery management is of special interest to the manufacturers of the electric vehicle. In terms of state-of-charge, a discerning driver expects similar accuracies in energy reserve as are possible with a fuel-powered vehicle, and current technologies do not yet allow this. Furthermore, the driver of an EV anticipates a fully charged battery will power the vehicle for the same distance as the car ages. This is not the case and the drivable distance will get shorter with each passing year. Distances will also be shorter when driving in cold temperatures because of reduced battery performance.
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