Batteries back up every part of our digital society. They are complex electrochemical devices that take up a lot of expensive real estate in data centers, and their use presents environmental concerns.
Battery monitoring/testing is an excellent way of mitigating some of the risks associated with the use of this crucial product. The mission critical world uses UPS, engine start, and station batteries constantly, but we also find thousands of other battery applications in our cell phones, iPods, watches, etc.
During a discussion of the Deepwater Horizon oil spill, my colleague, Tom Leonard of BTECH Inc., reminded me that most failures are a result of a string of events, and no one event is typically responsible. Most reports point to the failure of a blow-out preventer as a major contributing factor leading to the oil spill, but few reports identify a dead battery as a possible reason that the blow-out preventer failed.
There are several compelling reasons to employ a battery monitoring system in the mission critical environment.
Many of the most experienced people are leaving the mission-critical workforce, either due to retirement or the economy. Either way, experienced talent will be hard to find in the future.
Some end users are not following maintenance guidelines from manufacturers and standards such as IEEE 450 and 1188 - probably due to cost and time.
Some battery failures can occur in weeks, so lengthening the time between maintenance intervals may allow a failing battery to go unnoticed.
- As the Smart Grid evolves, short-duration power anomalies will increase due to switching activity. That means that batteries will be used more frequently. In the case of UPS for example, power-quality set points are programmed into controls that tell the UPS when commercial power is unacceptable. When this happens, UPS units transition to battery operation and disconnect from the mains. When power returns, the UPS wants to make sure it is stable before reverting back to normal operation. Therefore, a very short outage may result in a longer run on battery power than the actual duration of the outage. Batteries are, after all, like a loaf of bread. Each time you use a battery, you take a slice out of the loaf. Eventually there are no slices remaining, the wrapper is empty, and the battery is dead. So staying on top of battery condition is essential.
Since the 1970s, efforts have been under way to find a better failure prediction mechanism. The term settled on, which best reflects the health of a battery is “ohmic value.”
In the simplest technical terms, ohmic value is based on Ohm’s law, which expresses the relationship of volts, amperes, and ohms in an electrical circuit. Ohm’s law can be expressed as follows: E (volts) = I (amperes) x R (ohms). If any two of the three values of are known, the third value can be calculated using Ohm’s law.
E = I x R
R = E / I
I = E / R
Thus, the ohmic value of a battery is determined by using voltage and current to determine the resistive characteristic of a battery. Higher resistance equates to a reduced ability to produce current. This characteristic is translated into a measurement of impedance (ohms).
Opinions of best method for determining the ohmic value of a battery vary by manufacturer. There is agreement that the common goal is to develop trending of quality data over time that can reliably predict a failure far enough in advance so that an orderly resolution can take place vs. an unplanned failure at a critical time. Three of the common methods for determining ohmic value are:
Internal impedance is the resistive component of a battery as influenced by the battery’s capacitance or ability to hold a charge. Ohmic value is derived by passing a constant current ac signal through the battery and then measuring the ac voltage drop across the cell.
Conductance involves the application of a constant ac voltage across a cell and measuring the amount of current flow passing through the cell.
- Resistance employs a constant dc load across the cell, partial discharge and measurement of the voltage reaction to removal of the load.
Since there is a direct relationship between ohmic value and a battery’s health, knowing a battery’s ohmic value answers the question “Is my battery good or bad?” To ensure valid results, it is essential to measure the battery at a constant state of charge and constant conditions. Within the battery industry, the importance of trending ohmic values against a single event measurement is gaining acceptance
Whichever technique is chosen, the importance of the software incorporated cannot be understated. Traditional battery maintenance is one of those jobs nobody really likes to do. It involves a visual inspection, hygrometer reading of each cell, testing interconnecting straps with a digital low resistance ohmmeter (DLRO), re-watering as needed, and annual battery run down tests. It is boring, hazardous work usually relegated to the new guy. The output, therefore, is highly subjective and depends on the skill and experience of the person doing the analysis. Highly sophisticated battery measurement/testing systems remove much of that subjectivity. The capabilities of the software make it possible for the user to efficiently manage large amounts of data and readily understand the condition of the battery.
It is to our benefit to extend the life of all critical systems; it’s the green thing to do. Extending useful life:
Stretches cap ex funds and reduces the energy required for recycling and replacement production
Lowers the risk of unexpected battery failure
Eliminates the risk of thermal runaway
Reduces personnel exposure to live dc systems and chemicals
- Helps you manage your mission-critical electrical environment on your terms
The biggest dividend is making sure your mission-critical system works when needed.
As demand for data processing capacity continues to grow, so does the pressure to reduce energy consumption. There are tremendous opportunities to gain efficiency in the data center. Reducing downtime is one. A comprehensive battery management/testing program can be a big contributor to savings while increasing reliability of your mission critical electrical infrastructure.
I want to acknowledge Tom Leonard and Mike Phillips of BTECH Inc. for their assistance with this article.