Secondary batteries play an essential role, usually as a source of standby power, in many areas of business and commerce, such as telecom and data centers. To ensure those batteries deliver their full performance when called upon to do so, you must rely on a regular testing regime. But the big question is: What type of testing should that be?
Let’s start by making it clear that no one testing method tells the entire story — each has its own set of benefits and tells us something about the battery string. This is why a regular broad-based maintenance schedule is required to ensure standby batteries (when required) operate as expected.
Typical methods of battery testing include:
- Visual inspection — A visual inspection locates cracks, leaks, and corrosion. You can find these problems before they become catastrophic failures; however, visual inspection says nothing about the string’s state of charge (SOC), capacity, or state of health (SOH).
- Voltage testing — A low float voltage shows a cell that is not fully charging and cannot supply full capacity, which may be an indication of plate sulfation. A high float voltage is an indication of overcharging. This can lead to premature grid corrosion and higher temperatures in the battery. High temperatures may lead to valve-regulated lead-acid (VRLA) batteries drying out or even thermal runaway. Incorrect float voltage shows something is wrong. However, if the float voltage is correct, it says nothing about SOC, capacity, or SOH.
- Float current — A high float current could be a precursor to thermal runaway. It could also indicate a short circuit/ground fault or be a sign of a high float voltage. Incorrect float current, once again, shows that something is wrong. But if the float current is correct, it says nothing about capacity or SOH.
- Ripple current — Excessive ripple will cause internal heating of batteries. Ideally, ripple current should be less than 5A for every 100Ah. However, ripple current says more about the state of the charger than the battery. Ripple current does not provide any information about SOC, capacity, or SOH.
- Temperature testing — Temperature is critical for batteries, as high temperatures reduce battery life. Temperature testing provides no information about SOC, capacity, or SOH.
- Specific gravity measurement — A specific gravity measurement reads the ratio of the density of a liquid to the density of water. Specific gravity measurements in batteries indicate how much sulfate is in the electrolyte, providing information about the SOC, but not capacity or SOH.
- Impedance testing — An impedance test does not measure the capacity of the battery, but it is an indicator of the SOH of the battery.
- Discharge testing — Discharge testing is the only form of test that will determine the actual capacity of the string, but not necessarily the SOH.
IEEE standards (see IEEE Standards At a Glance) recommend that discharge testing be performed at the time of a battery strings installation and then every two to five years after that, depending on the age and capacity of the string. Specially designed test sets are available to make this process as easy and convenient as possible. For complete discharge testing, the battery must be taken out of service for the duration of the test. This can easily last as long as two days in order to allow a full discharge/charge cycle to be completed. Although it offers the most accurate results possible, this test method is clearly costly, time consuming, and often inconvenient. How many users do you know that would be okay without standby power for two full days?
One way of addressing this problem is to carry out limited discharge testing, which involves discharging the batteries by up to 80% without taking them out of service. This yields results almost as accurate as those provided by carrying out a 100% discharge test.
Partial discharge testing is a very useful way of assessing battery condition, but it is not ideal in every application. For one thing, the test is still time consuming. Although the batteries remain in service — should they be called upon to supply power at the point of deepest discharge during the test — they will only have 20% of their full capacity available.
It is also recommended that impedance testing be performed quarterly. Although impedance testing does not directly provide information about battery capacity, it is the only method that reveals the SOH of the battery. Impedance testing can be performed without taking the battery out of service.
As a battery ages, it may corrode, sulfate, dry out, or deteriorate in many other ways, depending on maintenance, chemistry, and usage. All of these effects cause a chemical change in the battery, which, in turn, causes a change in the battery’s internal impedance/resistance. By looking at the change in the battery’s internal impedance, it is possible to ascertain the amount of chemical change in the battery, which is an indicator of its SOH.
Battery test sets work by applying an AC voltage to the battery and measuring the resulting current flow. Because the applied voltage is known, the impedance of the battery can be calculated using Ohm’s law. Extensive research has confirmed that this impedance is, to a good approximation, inversely proportional to the capacity of the battery.
The benefits of impedance testing are easy to see. It is completely noninvasive, and it is fast, taking approximately 30 min., for example, to test a typical substation battery bank. In addition, the charge held by the battery is not affected at any stage of the test. As mentioned previously, there is no need to take the battery off-line. Despite these advantages, it’s still important to remember that no single battery testing method tells you the whole story. What then, is the best way to monitor the condition of important battery banks? A complete maintenance schedule is the recommended method to maintain critical battery backup strings.
Full load cycle testing carried out once per year will yield accurate information and provide an excellent baseline for impedance testing. This type of testing, carried out on a much more regular basis, will provide reassurance throughout the year that the condition of the battery has not deteriorated significantly.
Other factors deserving careful attention include float current, ripple voltage, and operating temperature. Float current is the current delivered by the charger when the battery is in its fully charged state. Normally this current is small, but if it starts to increase for any reason, the temperature of the battery will rise. The increase in temperature allows more current to flow, which further increases the battery temperature, allowing even more current to flow. The result is thermal runaway. In extreme cases, this has been known to lead to battery meltdown, particularly in VRLA batteries, which have no free electrolyte that can evaporate to help keep the batteries cool. Fortunately, the onset of thermal runaway is usually relatively slow — typically several months — so regular monitoring of float current can prevent problems of this kind.
A high ripple current is often indicative of a fault in the battery’s charging circuitry — possibly the failure of a diode in a bridge rectifier. Excess ripple current again increases the heating of the battery, thereby shortening its life. Again, it is recommended to check this on a regular basis.
Regular measurement of the battery’s operating temperature is invaluable, because high temperatures invariably lead to premature failure. As a rule of thumb, battery life is halved for every 10°C increase in temperature. This means that a battery with a rated life of 20 years, which is operated at 30°C rather than 20°C, will only have a life of 10 years.
There are, of course, other problems that can affect the performance of a battery bank. It has been stated, for example, that loose inter-cell connectors cause around 50% of the failures in battery banks. The connectors loosen because of the heating and cooling that takes place during charging and discharging — the cell terminal posts expand and contract. Because the lead from which they are made is very malleable, they cold flow with every cycle. Fortunately, you can detect these types of problems relatively easily by testing with a low-resistance ohmmeter.
Throughout this article emphasis has been placed on the desirability of regular testing for ensuring batteries are in good condition (see Battery Test Interval Guidelines). The reasons are simple. Regular tests for problems like ground faults and loose interconnecting straps allow these issues to be identified and corrected at an early stage.
When it comes to capacity testing, routine testing is even more desirable. One test in isolation may well provide useful information, but a series of tests carried out over a period of time will not only provide much more information, but will also make that information vastly easier to interpret.
A sudden change in a test result that has previously remained almost constant, for example, immediately suggests that further investigation is desirable, even if both the old and new values in isolation would be considered as falling within the acceptable range.
At first sight, monitoring the condition of storage batteries may appear to be a complicated undertaking. As this article has shown, however, if the testing regime is properly planned and split into simple tasks that can readily be carried out with modern instruments, the overall effort and inconvenience are small.
Sagl is product manager for power quality and battery products at Megger, Norristown, Pa. He can be reached at: [email protected].
SIDEBAR: Battery Test Interval Guidelines
- Make a capacity test when the battery is new as part of the acceptance test.
- Make an impedance test at the same time to establish baseline values for the battery.
- Repeat the above within two years for warranty purposes.
- Make an impedance test every year on flooded cells and quarterly on VRLA cells.
- Make capacity tests at least for every 25% of expected service life.
- Make capacity test annually when the battery has reached 85% of expected service life or if the capacity has dropped more than 10% since the previous test — or is below 90% of the manufacturer’s rating.
- Make a capacity test if the impedance value has changed significantly.
- Follow a given practice (preferably from the IEEE standard) for all temperature, voltage, gravity measurements, etc., and fill in a report. This will be a great help for trending and fault tracing.
SIDEBAR: IEEE Standards at a Glance
The three well-known IEEE maintenance and testing standards include:
- IEEE 450, “Recommended Practice for Maintenance, Testing and Replacement of Vented Lead-Acid Batteries for Stationary Applications”
- IEEE 1188, “Recommended Practice for Maintenance, Testing and Replacement of Valve-Regulated Lead-Acid Batteries for Stationary Applications”
- IEEE 1106, “Recommended Practice for Installation, Maintenance, Testing and Replacement of Vented Nickel-Cadmium Batteries for Stationary Applications”