UPS batteries are vital to the reliable operation of the UPS and ultimately of the critical system they support. Many standby battery systems, however, are not designed or managed in the best way and therefore the aims of reliability and longevity are not achieved.
The choice of manufacturer and type of battery should be considered carefully but is beyond the scope of this article. In my experience you tend to get what you pay for, but in any case, the following advice will achieve the best results for whichever battery is used.
Design and installation
Use experienced battery installers with professionally insulated tools and ensure that terminals are tightened to manufacturer’s recommended torque setting. Ensure a 1cm space between cells or monoblocs for cooling and to minimise the risk of thermal runaway in the future.
Think about maintenance requirements. Obviously there needs to be room around the battery to work on it but also consider installing fused isolating transition boxes on multiple string batteries so that individual strings can be isolated and maintained without having to disconnect the whole battery from the UPS. For safety, consider installing splitter boxes to break down strings to 120V DC to comply with electrical regulations during maintenance.
Wherever possible, control room temperature to 20°C as temperature is the major factor in achieving full battery life. Every 10°C above 20°C halves the battery life; cold temperatures also have a detrimental effect.
Have a realistic idea of what battery life can be achieved. The manufacturer may state a ten-year design life but what does this really mean? It means that the battery is constructed to last for ten years in ideal conditions. Internal corrosion is the main end-of-life failure mechanism and because batteries are electrochemical devices there will be a spread in the point at which any battery will fail.
There are large numbers of cells in a normal UPS battery, a few of which may fail earlier than the rest, around eight to nine years, the vast majority should last ten years, some may go on for 12 years. However, this means that after eight years the battery as a whole becomes less reliable and as the load on the system is critical, say, a data centre, then maybe eight years should be assumed as the anticipated reliable operating life of the battery.
Getting back to temperature, it is the constant average temperature that is critical; the temperature must stay close to 20°C at all times and must not vary greatly. Consider an extreme case to make the point: If the battery was kept at 10°C for six months then at 30°C for six months the average would be 20°C, but the conditions for the battery would be very poor.
Above 20°C the battery corrodes faster, below 20°C the negative plates sulphate due to under charging. Below 20°C an increase in charge voltage can solve the problem; above 20°C lowering the float voltage can help, but not significantly. If it is not possible to closely control the room temperature, check whether it is possible to have a temperature compensated charger, with the sensor(s) at the battery, not in the charger cabinet.
Commissioning the battery
To achieve maximum life it is very important that the parameters of the cells in the new battery are closely matched and of course that all cells are in a good condition. The batteries should come from the same batch. For instance, it would not be good if some of the batteries had been in the warehouse for a few months and the rest had been recently manufactured.
The best procedure is to test the battery before connecting to the charger using an electrochemical analyser. This will record the voltage and impedance of the cells and also indicate whether any are sulphated, which can occur in batteries that have been in stock for some time. Sulphated cells will need to be replaced, or restored by removal of the sulphation, prior to final commissioning of the battery system.
The next stage is a discharge test. This test is to prove that the battery is of sufficient capacity and also to check the integrity of the multiple connections and associated switch gear and cables. It is a good idea to scan the battery with a thermal imaging device during the load test to locate any hot spots which could be caused by loose connections or faulty cells.
Remember that during the battery’s life the capacity will reduce and end-of-life is defined as the point where the cell’s capacity has dropped to 80% of its original capacity. If the battery is to be required to provide full autonomy until the end of its life then a 125% factor should apply to the initial battery capacity. This should be proved by the initial discharge test.
Ongoing maintenance
The battery needs to be monitored regularly during its life. Individual cells do fail and because the cells are always part of a long series of cells which make up battery strings individual failures can cause significant problems if not dealt with promptly.
Electronic battery monitoring systems
A decision has to be made whether to use an electronic battery monitoring system, preferably before the battery is installed so that monitoring tags can be fitted to terminals during installation.
Battery monitoring systems used to have a poor reputation as many of the original versions were complex, expensive and unreliable. However, technology has moved on and reliable, sensibly priced, simple to install and use systems are available, indeed they are rapidly becoming the norm for data centres. These systems can be networked such that every cell or monobloc in a large data centre can be viewed in detail from a single central computer and any change outside of preset parameters flagged up.
Modern monitors will record cell or monobloc voltage, impedance and temperature as well as string voltages and currents. They are also particularly valuable in tracking cell performance during discharges. A battery monitor can provide a swift return on investment as the time spent on site by battery maintenance engineers is much reduced, although service visits will still be required.
Manual maintenance procedures
Where no monitoring system is installed the battery will need regular monitoring by trained service engineers. It is best to use one of the specialist battery maintenance companies for this as they should have the necessary experience, training and correct equipment available to complete the task safely and effectively. Often the UPS supplier will include the batteries within their maintenance contract sometimes using their own service engineers to maintain the battery and often, on larger systems, subcontracting the work to the specialist organisations.
Three things are important; the frequency of inspections, the quality of the data provided and, above all, safety. In regard to the first point where there are no site personnel available to monitor the batteries, the frequency of inspections is dependent on how critical is the continued operation of the load and how expensive and disruptive are the regular maintenance procedures. In general six months is the most commonly used interval.
There are two basic battery tests; discharge testing – which determines the capacity of the battery at the time of the test, but is not necessarily predictive of future performance – and condition monitoring which determines the physical state of the battery and is predictive if used to monitor trends but does not confirm capacity. Both of the methods should be used at specific times.
Load testing should be used sparingly for valve regulated lead acid (VRLA) batteries. There is still a commonly held view that discharging the battery is good for it. This is a hangover from the days of flooded cells where a process called stratification caused variation in acid strength within the cell. The hydrogen bubbles caused by charging stir the acid but this does not apply to VRLA batteries.
Load testing is necessary to establish the capacity of the battery and integrity of connections at commissioning but subsequently should be applied sparingly and perhaps only to provide improved data when issues are raised by other test methods. When load testing batteries near to end of life, the batteries should first be tested with an electrochemical analyser to determine weak cells which would be in danger of being damaged during the load test, i.e. by venting heavily or by internal pressure cracking their cases.
Load testing has the disadvantage of discharging the battery so that it has to be recharged before being capable of supporting the UPS sufficiently and of needing space and ventilation for the load banks. The safety of cables used and avoiding actuating smoke/heat detectors etc. is also essential.
The quality of any data provided will much depend on the test equipment available to the maintenance engineers and, at the time of writing this article, this is very relevant as new and much improved battery test equipment is now available.
For the last 20 years impedance testing has been the staple method of condition monitoring for VRLA batteries. The author can claim to be somewhat of an expert in regard to impedance testing having co?designed an impedance tester in 1994 which my company manufactured for some 16 years and which we have used to test many thousands of batteries. Advanced Battery Care has since ceased production of impedance testers as we now view them as obsolete.
Impedance testing evolved following the migration from flooded cells to VRLA batteries for the support of commercial UPS systems. The problem for battery maintainers was that many of the traditional ways of maintaining batteries were no longer possible for VRLA batteries.
Common tests for a flooded cell included a visual inspection of the plates to assess state of charge by the colour and the amount of sediment due to plate disintegration (the jars were of glass or transparent plastic), also sampling the electrolyte using a hydrometer to measure specific gravity. These methods were no longer possible and impedance testing, which is simply a way to determine internal resistance, was developed as a substitute.
Initially impedance testing was very effective because the major cause of battery failure was corrosion of the lead components which was easily detected. However, battery manufacturing techniques have improved and grid bar corrosion of VRLA batteries has virtually been consigned to an end of life condition. Current battery failure mechanisms are more complex and less suited to identification by impedance testing which has become less effective as a result.
This is not a problem as over the last ten years technology, previously only available in laboratories, has been developed in a hand held electrochemical analyser for onsite testing. The technology is based on frequency response analysis which is used worldwide in laboratory equipment manufactured by companies such as Solatron in the UK and Maccor in the USA in the testing of batteries, fuel cells, super-capacitors etc.
California-based company Global Energy Innovations has condensed this technology into a handheld device and produced complex algorithms which analyse the data to report on a variety of battery failure mechanisms. With this in mind it is obviously essential to ensure that whoever is maintaining the UPS battery system is using the most modern and effective test instruments available.
Always the most important issue during battery maintenance is that of safety. Batteries are always live, even when isolated from the charger. They have no internal protective device (fuse or circuit breaker) and are capable of delivering thousands of amps into a short circuit.
For UPS circuits the batteries are connected in series producing hundreds of volts. Add to this that they contain lead and produce explosive hydrogen gas and it is fairly obvious that anyone maintaining these batteries needs to know what they are doing!
Occasionally the question arises whether the battery terminals connection torque should be checked. This is necessary where the connection is made by bolting the connection lug to a lead post.
Being relatively soft, the lead can compress, causing the connection to become loose. Most modern VRLA batteries feature male or female brass inserts and these connections do not require the torque setting to be checked. Correct monitoring of the batteries during routine maintenance will detect any high resistance connections.
Finally, to achieve long life and reliability remember to act on any report of failing cells immediately as, like the bad apple, faulty cells can have a detrimental effect on other cells in the string.
