Although uninterruptible power supply (UPS) systems perform many important power protection functions, their key role is to maintain a secure electrical supply if the mains fails or transgresses acceptable limits. To achieve this, they need an energy source which is safely stored and available for instant use on demand, and easily replenished when the mains power returns.
The long-established lead-acid battery remains the technology of choice in most applications, although suitable alternatives are available in some circumstances and are likely to become more commercially viable in the future. Accordingly, these alternatives are summarised below, followed by a review of the factors essential to an effective, reliable UPS lead-acid battery installation.
Alternative UPS energy sources
Flywheels mechanically store kinetic energy. During normal operation, mains power drives a motor-generator which rotates the flywheel, establishing it as an energy store. In a power outage, this energy is converted back to electrical energy that powers the UPS DC bus. The major problem with flywheels is they only provide 30 – 45 seconds of back?up time, which is insufficient for most organisations. They are also significantly more costly than a lead-acid battery equivalent.
Hydrogen fuel cells generate electricity through a chemical reaction, effectively converting hydrogen gas into electrical power. They are environmentally friendly in that their only waste product is water, however the energy required to manufacture hydrogen calls fuel cells’ environmental credentials into question. Fuel cells are significantly more expensive than batteries and because hydrogen is an explosive gas, great care has to be taken with its handling.
Lithium-ion batteries have future potential for UPS backup. They are smaller and lighter than lead-acid batteries while offering improved backup times. Against this, they are considerably more expensive than comparable lead?acid products, and may explode if overheated or overcharged.
However, their small size and high power density is attractive to electric vehicle designers, so much effort is being devoted to developing safer, more compact and more affordable models. These batteries may become commercially viable for UPS applications as a result.
Lead-acid batteries comprise a number of cells connected together to deliver the required voltage and capacity. Cells have a nominal voltage of typically 2V and usually six are connected in series to provide a convenient 12V block. As a rule, UPS use either open-vented or valve-regulated lead?acid (VRLA) battery types. Of these, VRLA has become the favoured choice because it is more environmentally friendly and has lower routine maintenance requirements. Also, VRLA’s safe and self-contained construction allows more flexibility in storage and use; battery location within server rooms and office environments as well as separate battery rooms becomes possible.
Ensuring that a VRLA battery installation successfully fulfils its role as an alternative energy source has three aspects. Firstly, battery capacity requirements must be calculated and catered for by considering the load size and the specified back-up or autonomy time.
Secondly, users must understand how poor site conditions or high environmental temperatures can limit or terminate battery life, and take preventative action accordingly. Finally, a maintenance plan must be designed and implemented for each UPS installation. This will provide early warning of any issues which can then be remedied before they cause failures.
Load size in kW seen by the battery is based on the critical kVA load, the load’s power factor and the efficiency of the UPS. The autonomy time is the time for which the load must be supported plus, if appropriate, generator start-up and stabilisation time.
In choosing the load support time, it is worth considering that statistically 95% of all mains disturbances last for less than 5 minutes, while anything longer usually lasts several hours. A battery autonomy time of 10–30 minutes, depending on user requirements, is therefore standard.
If possible, the UPS supplier will accommodate batteries within the UPS cabinet. Larger capacities can be handled by additional matching cabinets, or by open or cladded racks. These are normally located in a dedicated battery room with controlled access to meet health and safety requirements.
The battery installation will consist of at least one serial string, where the sum of all the string battery voltages equals the UPS’s float voltage setting. Additionally, two or more serial strings may be paralleled, mainly to increase the Ah capacity of the battery bank. However, paralleling also increases battery bank resilience; a single battery failure will not deprive the UPS of all backup power.
Threats to battery life
All battery manufacturers quote a finite design life. This presumes ideal conditions of charging, temperature and charge/discharge cycles that are unlikely to be met in real use for several reasons. If a battery is left discharged for an extended period, lead sulphate crystals form which prevent recharging.
Recovering a battery from this state may be possible by constant current charging at a high voltage. If the battery becomes deeply discharged so that its on-load voltage drops below a predetermined level, its capacity and working life will be reduced. Although careful recharging may be possible, this may not be practical for critical load applications. In extreme cases, the battery will not recharge and must be replaced.
Overcharging caused by various interrelated voltage, current and temperature factors can be equally problematic. It can corrode the positive plate material, reducing the battery’s lifetime. This can also be reduced by any AC waveform, known as AC ripple, superimposed on the DC charging voltage. Environmental temperatures exceeding the battery ideal of 20ºC can shorten a battery’s operating life too, with the severity in life reduction being directly related to the extent of temperature increase. In extreme cases, where higher ambient temperatures, life-expired batteries and poor maintenance can all be factors, thermal runaway conditions can arise. The battery will be destroyed and a significant health and safety issue will be created. Low temperatures have little effect on a battery’s service life, but will reduce its performance.
Effective battery maintenance starts with managing ambient temperature. Although good UPS design can protect batteries from the other threats described, the UPS supplier cannot control the operating environment. Visual checks of the batteries, their terminals and connections for corrosion, cracks, leaks, swellings and dirt should be performed, and connections tightened if necessary.
More detailed assessment of battery condition and future life is currently gained using battery impedance testing. As a battery ages, its impedance increases marginally due to normal internal corrosion. This should occur at a similar rate for all batteries in a string, so any battery showing a significant deviation is suspect. Impedance testing is relatively easy, does not discharge the batteries, and if regularly conducted can track battery condition and accurately predict end of operational life.
Load bank testing is another battery maintenance tool, as it indicates available capacity. However it involves discharging, which can reduce battery life as well leaving the batteries in a discharge state for a period of time. Also, arrangements for supporting the critical load must be made, and load banks are large and expensive to handle. Therefore this method should be used sparingly. In any case all batteries, as consumable items, eventually need replacing, however, careful management of the batteries’ environment together with planned maintenance will maximise available lifetime.