The main problem of lack of predictability (premature capacity failure) and reliability in UPS systems came about through over-optimistic and hasty innovation. Prior to the 80s the only battery in wide usage for large stationary power systems was the pure-lead planté cell which needed considerable floor space, regular maintenance including specific gravity checks and watering, and ventilation. It was expensive to purchase, accommodate and run, but supremely reliable (as witnessed in telephone exchanges the world over). Then along came the battery industry with the sealed lead-acid, maintenance-free cell that didn't need so much floor space, maintenance or ventilation. It was also half the cost and the market, especially in the UK, embraced the new technology. Perversely, in the end, it proved to need regular inspection, testing, cleaning and, critically, it required ambient temperature control. The rest as they say is history.

The battery is the heart of all UPS systems and is as important to the overall design and availability of the critical power system as the UPS itself. Its prime function is to bridge the time delay between mains failure and mains return or an emergency diesel generator starting and taking over as the primary source of power. In some cases, where no emergency power source is available, it is intended to cover an orderly shutdown of the load but this is probably an urban myth.

As the UPS rises in kVA rating (say, up to 50kVA) the battery is often provided in matching battery cabinets with a restricted choice of options. However, even when the application is small or the choice of batteries is limited the customer should always be given the criteria for achieving the longest possible service life from the battery.

Many studies have shown that battery failure is the single largest contributory factor in reported UPS failures and this generally supports the market view that batteries are a problem.

However it is possible, with careful design, planned preventative maintenance and timely replacement cycles, to engineer-in both reliability and availability using a variety of strategies and techniques.

A battery is a collection of cells arranged in series or parallel to produce a particular combination of terminal voltage and storage capacity. Some small UPS modules use 24V-48V_DC whilst for the largest 3-phase UPS DC voltages of 480V_DC (240 cells in series) are common. In transformer-free UPS with IGBT converters two high-voltage (e.g. 400V) batteries can be connected in series with a centre tapped point such that it appears that the battery is 800V.

Cells are arranged into monoblocks of multiples of plate-pairs so that they are available in 2, 4, 6, 12 and 24V versions. The 12V package is typical of an automotive starter battery and often derives cost savings from the volume effect of the base components. Monoblocks can be connected in series and/or parallel to achieve the voltage and Ah (Ampere-hours) capacity requirements of the UPS.

The storage capacity of each monoblock is rated in Ah each having an upper limit of discharge power in Watts/Cell. The power limit is only reached when the required autonomy time is short, usually less than 10 minutes and most selection calculations focus on the Ah. Each cell has a nominal open-circuit voltage of 2V and a minimum discharge voltage of around 1.65V. Depending upon the state-of-charge, when a voltage higher than 2V/cell is applied the cell stores energy and charges and when connected to a load discharges with the terminal voltage decaying as the energy is drained.

From a UPS point of view the lead-acid battery has one prime feature - it changes state, from float to discharge with only a momentary reduction in voltage. This is easily taken care of by the inverter and enables the UPS to operate without any break in voltage and easily exceed the <20ms performance criteria of the ITC curve.

There are two basic types of lead-acid cells - valve-regulated and flooded.

Valve-regulated

VRLA is often erroneously referred to as sealed or maintenance-free. These cells are usually of the recombination type with a glass mat separator holding an acid-gel between pasted plates. They are not sealed and do discharge hydrogen gas during charging (although at a very much lower rate than flooded cells). Whilst no top-up of the acid is possible and that is where the maintenance-free tag line came from, they do require regular inspection, load-testing, cleaning and terminal re-torque/greasing.

Designs are available at several industry-standard steps of design life, typically 3, 5, 10 and 12 years but there are real and tangible differences between the different design points.

Flooded

Traditional vented or wet cells are rarely used in UPS systems in EMEA but are still popular in North America. As the name suggests the electrolyte is a free liquid rather than in suspension as a gel and can be topped-up with distilled water or acid if required. The ability to test the specific-gravity (SG) and replenish the electrolyte level avoids one of the common failure modes of VRLA, that of drying out. Automatic watering systems are available but not in wide use, partly because the SG has to be checked manually anyway.

In the purest form the plates are made from pure-lead (called a planté cell) and can result in a design life of over 25 years and an average service life of more than 20 years is not uncommon.

Design life v service life

Under no circumstances should the term ‘design life' be confused with the anticipated service life of a cell, monoblock or entire battery. The underlying failure mode of a lead-acid battery is corrosion, which is hardly surprising considering the internal conditions of a valve-regulated cell. Voltage applied corrosion is also accelerated by higher temperatures.

Standards

One historical benchmark standard, BS6290 Part 4, has stood the test of time and has formed the basis of the EN standards for classification of VRLA cells. This standard was, to all intents and purposes, written by and for British Telecom for VRLA cells in -48V_DC/4-hour telecommunication applications. The latest version defines performance groups such as flame-retardance and capacity on first discharge etc. A battery built to this standard with an established record of manufacturing quality is eminently suitable for reliable UPS duty.

Unfortunately many low quality products are on the market that are BS6290 compliant but without specifying the classification groups that apply.

Battery autonomy

A battery's autonomy is the time, usually measured in minutes, that the battery can support the design load from its stored energy when the primary power supply to the UPS has failed.

For small UPS's with integral batteries the choice of autonomy is limited to the standard ‘X' minutes and extendable by fixed-autonomy battery expansion packs. The same applies to matching battery cabinets in mid-range UPS's. Large UPS generally have custom-built battery racks which allow fully scalable autonomies to be achieved.

Maintenance-free VRLA

What has maintenance-free really turned out to mean? VRLA are just that, valve regulated. They were never sealed and excess pressure inside the cell is relieved by the simplest of valves. The production of hydrogen reduces the water content in the gel. However, no loss of electrolyte can be replenished through the valve. In this respect the maintenance is less because SG testing and watering is not possible. The VRLA battery needs the same level of cleaning, re-torque and re-greasing, plus the temperature control to ensure long life. Due to the relative unpredictability of point of failure, they require load capacity testing and, if the client is inclined, battery-monitoring.

Conclusion

A 100% reliable battery installation can be engineered but only if correct and timely maintenance is applied after installation. It needs an expertly designed replacement programme that, if followed, will guarantee availability and uptime.

Printer friendly version

Email this article to a friend