Design, maintain, test batteries in mission critical facilities
Engineering design, maintenance, and testing of batteries in mission critical facilities is imperative for proper operation and safety.
- Understand the need for reliable batteries in mission critical facilities.
- Explore the most common battery abnormalities that can be spotted via inspection.
- Review simple steps that can be taken to remedy battery problems.
When designing electrical distribution systems for mission critical facilities, the overriding factor is reliability. These systems are called mission critical for that very reason: It is critical that power remains uninterrupted. But this mission critical label does not apply to every single part of the facility. Some processes are more important to the business than others, and that is where the focus of the design lies. Certainly, designing for a mission critical facility can be challenging. There are many considerations in play. The system has to be very robust with no single point of failure.
When designing electrical systems, maintenance is an important factor, specifically in deciding what battery to implement. Maintenance includes not only the cost of replacement of a battery cell, but also the accessibility of the battery system, the ease of disassembly and reinstallation, the frequency of service, and the conditions needed in the space (temperature, humidity, etc.).
The goal of maintenance is to prolong the life of the batteries and to make sure the batteries perform as designed. Inspection is done to catch any abnormalities that could impact battery performance as well as long-term life expectancy. To this end, it is important to place the batteries in an environment that agrees with the manufacturer's recommendations. [subhead]
There are several types of batteries used for providing power to electrical systems. The most popular types of batteries being used nowadays are lead-acid and nickel-cadmium (NiCd). NFPA 110: Standard for Emergency and Standby Power Systems defines two types of lead-acid batteries:
Valve-regulated lead-acid (VRLA): A lead-acid battery consisting of sealed cells furnished with a valve that opens to vent the battery whenever the internal pressure of the battery exceeds the ambient pressure by a set amount.
Vented (or flooded): A lead-acid battery consisting of cells that have electrodes immersed in liquid electrolyte. Flooded lead-acid batteries may have a provision for the user to add water to the cell and are equipped with a flame-arresting vent, which permits the escape of hydrogen and oxygen gas from the cell in a diffused manner such that a spark, or another ignition source, outside the cell will not ignite the gases inside the cell.
Although NFPA 110-2010 recognizes the usage of NiCd batteries for emergency systems, no definition is provided for such batteries. A definition can be found in IEEE 1106-2015: Recommended Practice for Installation, Maintenance, Testing, and Replacement of Vented Nickel-Cadmium Batteries for Stationary Applications:
Partially recombinant nickel-cadmium cell: A vented NiCd cell providing an internal means for the recombination of internally generated oxygen and suppression of hydrogen gas evolution to limit water consumption, typically operating with a recombination efficiency of 90% or higher.
Mission critical facilities are not only the buildings that are critical to human life, such as hospitals, but also are facilities that have a great economic impact. In the critical systems (or processes) of these facilities, many methods are employed to mitigate the risk of power outages. But, almost always, the risk mitigation relies upon batteries. Uses include:
Uninterruptible power supply (UPS) systems: Batteries are used to back up power where UPSs are involved. While the power is processed and converted through the UPS to the load, the batteries are kept charged to provide the much-needed power to the critical system when normal power is out. Even though standby generators are most likely used as backup power, the batteries provide the critical power needed until the generators are able to receive load. Standby generators: Batteries are used to provide the initial crank to the generators as well as power the generator controls.
Controls: Batteries also are used to back up controls, especially when medium-voltage distribution is found throughout the facility. Nowadays, the operation of critical systems is more reliant on controls to avoid nuisance tripping and to enable fast-acting protection.
Emergency systems: Batteries are widely used to back up life safety systems, such as exit lights. Exit lights normally are backed up by a 90-minute battery to allow people to recognize the way out of the building in case of a life-endangering situation. All of the above-mentioned uses (UPS, generators, and controls) could be part of an emergency system as well.
The maintenance of the batteries in mission critical facilities is especially important. When maintained regularly, batteries will perform according to design to support the critical systems, ensuring the continuance of power. Maintaining the batteries also elongates their life, resulting in lower operation costs. But the first step of maintenance is to inspect. By first inspecting and then assessing, we can develop a plan for remediation.
Let's look at each battery type separately. IEEE has developed separate standards for just this reason.
Vented lead-acid: The most routine inspection for this type of battery is a visual inspection. IEEE has developed a standard, 450-2010, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications. This standard is a very good guide for the operations staff to create a maintenance procedure.
The monthly inspection, as recommended in the IEEE 450-2010 standard, includes the following:
A visual inspection of:
- The general condition of the area. The area (room) should be as clean as possible so that dirt or excessive dust does not cover the battery cells. Unclean equipment is harder to assess during a visual-only inspection.
- The battery cells for cracks. This step is important because a crack in the cell's outer shell could allow the electrolyte to leak and the battery to discharge.
- The electrolyte levels. A significant drop in the electrolyte level means that the specific gravity of the electrolyte has increased. A greater specific gravity would impact the life of the battery.
- The battery terminals for corrosion. Corrosion of the battery terminals will increase the resistance of the connection, thereby decreasing the amount of current supplied by the battery system.
A measurement of:
- The float voltage at the battery terminals. If the battery system performs at a float voltage outside the manufacturer's recommended range, the battery's life expectancy would be adversely affected.
- Room or area temperature and ventilation. The temperature is important because it impacts battery-life expectancy. Ventilation is important because proper movement of the air mitigates high concentrations of hydrogen, which is a byproduct of chemical processes in the battery cells. High concentrations of hydrogen in the air significantly increase the risk for explosions.
The IEEE 450-2010 recommendation for monthly visual inspection is just a general recommendation for any type of facility. NFPA 110 requires weekly visual inspections for battery systems employed in emergency and standby systems.
The recommended quarterly inspection includes the following:
A measurement of:
- The voltage of each cell. Lower-than-recommended (by the manufacturer) voltage levels could have an adverse affect on the life expectancy of the battery.
- The specific gravity of the cells' electrolyte. One needs to keep in mind that specific gravity of the electrolyte increases in a full charge. It would be best if three measurements are taken: one on top, one in the middle, and one on the bottom of the cell. The average of the three values is the value to be used. If taking three measurements is not possible, taking a reading as close to the middle as possible is best. Note that it's not necessary to measure the specific gravity of each cell in the battery string, rather only about 10% of the cells.
- The electrolyte temperature of a few cells. If we have a 125 V battery system, we'd have 60 cells. It would suffice to check the temperature of six of them. The desired temperature is the one recommended by the manufacturer. If the electrolyte is at a higher temperature, a higher float current is required to maintain the cell voltage. Too high of a charging current could adversely affect the electrolyte composition, as more of the hydrogen and oxygen is being gassed. On the other hand, the lower temperature causes a smaller floating current, which in turn, slows the charging process.
There is also a yearly inspection that applies the quarterly inspection to all the battery cells, which is, therefore, much more involved.
If abnormalities are observed during these inspections, there are corrective actions that can be taken. See Table 1 for some common corrective actions.
Of the abnormalities mentioned in Table 1, the room temperature and ventilation usually are set during the design. Total cost of ownership analysis is usually done by taking into consideration the optimal conditions so that the battery life is longest. The rule-of-thumb correlation between ambient temperature and lead-acid battery life, be it vented or VRLA, is as follows: battery life decreases by 50% for every 15°F above the normal temperature, which is 77°F. Figure 1 shows this correlation.