The factors that contribute to the uncertainty of battery life in a standby or stationary application are worsening as power networks try to handle new data and communication demands. Even in applications where the battery is the last line of power, real-time information about its ability to reliably deliver power is still not readily available.
Sophisticated hand-held test equipment and battery monitoring systems have improved industry-wide understanding of battery conditions and prevented major power failures, but few systems are intertwined with the rest of the charging and power systems. As a result, rectifiers and chargers continue to operate independently of the battery diagnostics, and the elements causing battery deterioration will likely reappear. Battery failure may be only a symptom of a system or environmental issue that is causing battery deterioration. By continuing to treat the symptoms to ensure power availability, you may have to prematurely replace batteries and incur added costs of installation, servicing, scrap processing, and continued technician resources.
A more elegant solution would be to incorporate a battery monitoring and management system into the charger/rectifier system. What if you continuously monitored the battery and environmental parameters to produce an accurate measurement of the battery state-of-charge (SOC) and state-of-health (SOH)? In addition to having advanced warning of an impending battery failure, you could use these SOC and SOH diagnostics to control the rectifier and optimize the charge supplied to the battery string — effectively eliminating float charging.
While an exact definition of SOC is debatable, for our purposes here think of it as fuel in a fuel tank. If SOC is the amount of fuel in the tank, then the SOH is the condition of the tank itself. You need both measurements to accurately estimate the amount of energy in the battery and to estimate the ability of the battery to hold and deliver that energy. You can define relative battery SOC in terms of the measured specific gravity compared to the design specific gravity. More simply, a battery that has lost 50% of its full charge capacity can still achieve an SOC of 100%, but only because that 100% SOC battery is capable of delivering only a diminished capacity. Even though your battery has achieved 100% SOC, it may no longer have the capacity to deliver the energy it was designed to deliver.
SOC is an accurate indicator of the amount of energy available in a particular battery. As the battery cycles, SOC indicates the relative amount of energy removed or added. Fig. 1 on page XX represents SOC calculations obtained during field-testing in a vehicle application in May, 2001.
State of Health (SOH)
As the battery ages or its environment changes, SOC can become less important because it will often not reveal a faulty battery on its own. To complete the diagnostics, you must calculate a measurement of the battery's SOH. For standby and stationary battery applications, the SOH diagnostic relates to the ampere-hour (Ah) capacity of the battery at a given discharge rate. SOH can be expressed from 0% to 100%, based on the real time Ah capacity of the battery measured against the minimum requirements for the application at 25 degrees C. You must compensate the SOH calculation for temperature and SOC. SOH also takes into account the predicted Ah capacity of a new battery at 100% SOC.
An accurate SOH measurement equates to the ability of the battery to deliver its energy. As a battery develops faults or is subject to adverse conditions, its SOH will decline. Continuous monitoring of this parameter and its elements will help you diagnose failure modes such as sulfating, grid corrosion, low-electrolyte level, loss of active material, terminal and connection fault, and single or multiple faulty cells. In addition, by always having the current SOH measurement, a power management system can alert you to a problem before it becomes an emergency and enable you to correct it before the battery is stressed or fails.
Fig. 2 on page XX displays SOH measurements compared to the actual Ah capacity of the battery. Each event is a revised calculation for an SOH measurement based on a change in another measurement or parameter, including SOC. Event No. 1 is equivalent to an assumption in the algorithm of 100% SOH at battery installation. Combined, the measurements of SOC and SOH allow you to determine, at any moment, the energy level of the battery and its ability to deliver that energy upon demand.
Using diagnostics for optimized charging
SOC and SOH measurements can form the backbone of real-time information, which you can use to control the charging system output to the battery. Such a system could vastly decrease battery failure and extend battery life by optimally charging the batteries according to the SOC, SOH and other system measurements.
Since few methods for determining an accurate SOC have been feasible or practical, experts have proposed other charging profiles, such as alternating constant voltage-constant current (CICVCICV), overcharging, hysteresis charging, and individual cell equalization.
Real-time, continuous monitoring can produce accurate SOC and SOH calculations. When supplied to the charging system — along with the measurements of battery conductance, voltage, current, and temperature — they enable the charging system to supply the appropriate charging current. These diagnostics are independent of the battery design, and can be calculated without performing repeated programmed discharge tests on the batteries.
The key to establishing the SOC and SOH diagnostics is measuring the battery's conductance. Similar conductance technology has already been proven with a less-sophisticated design to provide rapid, intelligently controlled charging for 6V to 36V lead-acid batteries. The most recent advances in conductance technology enable you to conduct the following diagnostics:
Real-time continuous monitoring while batteries are online.
Measurements at variable rates of charge and discharge. This represents a significant advance from previous technology, which could measure only during level float charge.
Multiple measurement use, or the ability to measure conductance at various loads.
Conductance technology offers many potential benefits that will drive its development for the stationary/standby power industry. It represents a significant opportunity for you to provide innovative battery monitoring diagnostics, and the potential to take advantage of accurate, instantaneous battery measurements. Expected benefits from this technology include improvements in the life and reliability of the battery, its charging economies and diagnostic charging intelligence, and its warning of impending failure.
Cox is director of the OEM Business Unit, Midtronics, Inc. in Willowbrook, Ill.
Essential parameters to monitor
To establish meaningful diagnostics, you must measure the following battery parameters:
Current (both charge and discharge)
The last item bears special mention. A battery's measured conductance correlates linearly with its ability to deliver current. As conductance declines, so does a battery's ability to meet its specified capacity and supply energy. A major benefit to using conductance is the ability to calculate a battery's capacity without performing an extensive discharge or load test. Instrumentation measures conductance by inserting a low-level AC signal, which does not cycle or age the battery.