Power problems are a reality in South Africa for the foreseeable future, and many organisations have implemented solutions to minimise downtime and ensure business continuity. One of the most common solutions that people turn to is the use of an Uninterrupted Power Supply (UPS) with battery backup.
UPS systems can provide enough time to safely shut down equipment that may be damaged by sudden power cuts, they can provide continued power supply to the load while a transfer from utility supply to standby generator power takes place, or they can supply the extended backup time required for the duration of the power failure itself. Essential to the success or failure of a UPS system, however, is the battery bank.
Unfortunately, many organisations fail to take into account the specific planning, installation and maintenance required by batteries in standby mode, to ensure that they deliver the required power to the load, and don’t fail when they are needed the most.
Battery systems need to be engineered to the type of application they are being utilised for, for example, you cannot use just any battery off the shelf for UPS applications. Batteries need to meet the demanding requirements of a standby power environment.
Typical standby batteries, called Deep Cycle batteries, need to be charged according to the manufacturer’s specifications. As most modern-day battery systems are of the maintenance-free type, your typical quickest typical recharge time is approximately 10hrs. This is usually the fastest recharge time one can expect, and most average UPS systems have a slower recharge time. One of the design requirements for a battery system is to establish how much backup time the customer requires and how much time they need the battery to recharge back to full capacity, based on the type of UPS being specified. Once this has been established, there are three fundamental conditions that will directly affect the life span of a battery system:
Most standby batteries are performance rated at 20˚C. For every 10˚C that the battery operates at above the 20˚C mark, a half-life aging takes place. For example, the most common standby battery used in UPS applications has a specified lifespan of 3-5 years, however if it is operated at an average temperature of 30˚C, the life-span reduces to between 18 months and 2.5 years. The temperature of the environment in which the batteries operate in is very critical to the expected life-span and performance of the battery bank. The battery banks must be installed in a clean, cool or temperature controlled environment.
One full discharge down to 80% depth of discharge (20% battery capacity remaining) and a full recharge back to 100% charged state is considered a cycle. Typical UPS deep cycle batteries only have a cycle limit of between 200 and 500 cycles. In a situation where a power failure occurs once per month, that is 12 cycles per year, a 200-cycle rated battery would last its intended 3-5 years without any problem. However, where daily load shedding occurs, a battery would not last a year. Monitoring the number of charge and discharge cycles the battery is subjected to over a period of time is essential to understanding the expected life-span of a battery and when it should be replaced.
By the very nature of its application, Deep Cycle UPS batteries are usually float charged, awaiting the next discharge to take place. There could be months between discharges, which means that there is very little activity taking place within the individual battery cell chemistry. This can result in cell impedances differing slightly between each other and, as a result, cells accepting a different charge current from each other. Or, over the multiple number of battery blocks that make up a UPS battery bank, some battery blocks will be at a higher voltage and some at a lower voltage, which, if left unattended, could result in some battery blocks over charging and some under charging. In both cases, if left un-managed, the battery block will fail during discharge, and the UPS will be unable to provide continuity of supply during critical power failure periods.
On the opposite scale, if the battery bank is floated at too high a voltage for too long a period, the battery blocks will start to gas and overheat, resulting in permanent damage to the battery blocks. As most Deep Cycle batteries are typically “maintenance-free”, as they are either sealed or semi sealed, there is nowhere for the gasses to vent. This causes a gas build-up, distorting the battery housing and permanently damaging the battery.
If the battery is floated on too low a voltage, the individual cells will not be chemically stimulated correctly, can cause a reduction of capacity and internal sulphation of the cells. Ultimately the electrolyte fluid becomes less able to conduct the charge current, and eventually one will be unable to recharge the battery block.
Proper UPS system design is critical to ensuring maximum functionality is achieved. In fact, a poorly designed system could destroy the batteries in as little as a few months, and since a battery is around half of the total cost of a UPS solution, this can be a costly error.
In addition to correctly designed systems, it is also essential to ensure that batteries are proactively monitored and maintained in order to address any issues before they can become problematic. For applications where uptime is mission critical, such as for hospitals, financial institutions and security organisations, it is recommended that a permanent online monitoring solution be installed on all batteries. These solutions are self-contained, meaning they will not be influenced by other equipment, and will manage and measure all relevant aspects of the battery, proactively sending out alerts to possible failures or issues.
Permanent, proactive battery monitoring solutions make use of historical data that is continuously monitored, to assess the status of the quality of the batteries in the field. By applying this methodology, one can immediately determine the risk associated with the possibility of batteries failing. This ensures that battery backup is continuously maintained at optimal levels, and will never fail without warning, preventing the potentially catastrophic consequences of such an event.
By Kevin Norris, Consulting Solutions Architect, The Jasco Group