It pays to look after UPS batteries

08May

Temperature/life characteristics of a typical VRLA battery.

Batteries as an energy source are vital to the UPS’s role, yet they are also its weakest link, says Kenny Green, technical support manager, Uninterruptible Power Supplies. Here, he looks at the steps necessary to ensure maximum life and reliable performance from UPS lead acid batteries.

UPS operators can forestall battery failure and its consequences by understanding the factors that threaten capacity and working life, whilst taking steps to manage them.
When the mains supply fails, a UPS must call upon alternative power to continue supporting the critical load. The energy source for this power must be safely stored and instantly available for use whenever required. It must also be easily replenishable when mains power is restored. These constraints mean that options for energy storage in a UPS are limited.
Although other contenders, such as flywheels and hydrogen fuel cells, exist, these technologies are still in their infancy. Batteries remain the most popular energy storage choice for most UPS manufacturers. Of these, lead acid batteries are the most suitable for general UPS applications within the UK. However, they are vulnerable to poor monitoring, maintenance or operation during both storage and use.

Battery technology and topology
A lead acid battery (see Figure 1) comprises a number of interconnected cells, each with lead and lead oxide plates immersed in sulphuric acid electrolyte. Each cell develops a nominal 2V potential. Cells can be built with different plate sizes to vary current capacity, which is measured in Ampere hours (Ah). Cells can also be connected to increase the battery’s voltage. For convenience, most batteries are built as six cell blocks giving 12V output. Batteries for UPS applications are available as either open–vented or sealed Valve Regulated Lead Acid (VRLA) types. Open-vented is an older technology, whereas VRLA batteries are more environmentally friendly, require less maintenance and are favoured by UPS manufacturers today.
All UPSs are offered with standard batteries sufficient to support their full load, but with a choice of capacities for different back-up autonomy times. In many cases, the batteries can be accommodated within the UPS cabinet. However larger battery requirements may call for separate battery cabinets or even separate battery racks. These may be open or cladded, however open types usually require installation into a dedicated battery room with controlled access.

Configuration options
Different configurations of serial and parallel battery strings can be interconnected to achieve the application’s required DC voltage level, capacity and redundancy. A serial string, as the name suggests, comprises a single line of blocks connected in series. The overall battery voltage is the sum of the individual battery blocks and must match the float voltage setting of the UPS. The capacity of the serial string is the same as that of each individual block and all blocks must be identical. For example, 40 12V 10Ah blocks connected in series provide a battery in excess of 480V and 10Ah capacity.
Two or more serial strings can be combined into a parallel string. This is for two reasons - either to increase the resilience of the battery bank so that a single battery failure will not cause loss of all battery power to the UPS, or to increase the battery Ah capacity. For example, three strings of 40x 12V 10Ah capacity batteries can be paralleled for an overall battery of 480V and 30Ah capacity.
When multiple battery strings must be connected to a UPS, transition boxes are commonly used. Fused transition boxes allow individual battery strings and cables to be protected. They can also be isolated for repair or maintenance without completely disconnecting the UPS.

Potential threats
However they are configured and set up, batteries must always be considered as a vital link in any UPS application. Batteries have a finite useful working life and will eventually require replacement. Allowing for the factors that affect a battery’s health and performance will maximise its useful working life.
The first factor for VRLA batteries relates to their storage. This should be in a charged condition for not longer than three to six months - depending on manufacturer - at 20°C.
VRLA batteries should never be stored in a partially or entirely discharged state. Supplementary charging will be necessary for storage periods of more than six months. Once installed, it is important to realise that a battery’s real life will never reach its ‘design life’. This is because battery manufacturers base their design life forecast on assumptions about how the battery will be used and kept. Forecasts typically state that under certain charging conditions and certain ambient temperatures, with a set number of charge or discharge cycles, the battery will last X years. However, in a UPS application it can be hard to maintain an ideal temperature. It can also be difficult to control the frequency and depth of charge discharge cycles which are initiated by unpredictable mains events.
If a battery is left in a discharged state for a prolonged period, lead sulphate crystals begin to form. These prevent recharging and normal battery operation. The battery’s open-circuit output voltage will be less than its rated value. In limited cases it may be possible to recover the battery by constant-current charging at a higher than usual voltage. However this is not always fully effective, and the battery must be monitored for signs of over-heating while recharging.

Charging hazards
Optimum charging relies mainly on voltage, current and temperature factors. Incorrect application of these can cause over-charging. Excessive voltage charging will force a high over-charge current into the battery, possibly causing gas emission through the safety valve. This can rapidly corrode the positive plate material and accelerate the battery’s end-of-life, as well as a safety risk. Operating temperature is particularly important - most manufacturers recommend this to be 20°C.
As Figure 2 shows, high temperatures reduce the battery’s working life. In extreme cases this will cause a thermal runaway, resulting in possible oxygen/hydrogen gas production and battery swelling. VRLA batteries cannot recover from this condition and must be replaced. Lower temperatures will not have the same adverse effect on the battery’s service life, but will reduce its output capacity.
Deep discharge is another hazard for battery users. This is where the battery is discharged to the extent that its on-load voltage falls below a pre-determined limit. If this happens, both the battery’s capacity and its useful working life will be adversely affected.
Another factor to consider is that the battery is a DC power storage device. Its useful working life can be adversely affected by AC ripple superimposed onto its DC charging voltage.
These considerations are well understood by UPS designers, who can be expected to ensure that the UPS batteries are protected from under charging, over-charging or over discharging as well as AC ripple.
UPSs can also include temperature compensated charging to prevent over charging at higher than normal temperatures. However the ambient temperature in which the batteries are used is beyond the UPS designer’s control. The operator must ensure that this temperature is held within limits acceptable to the battery.

Monitoring and maintenance
Regular maintenance of the battery system is essential. This includes visual inspections to check for battery damage and connection integrity. It can also involve load bank testing, which is valuable in determining battery capacity. However it is expensive to conduct, disrupts support of the critical load, and can shorten the life of the batteries tested. It should therefore be used sparingly.
For VRLA batteries, battery impedance testing is the most important tool for tracking the battery’s condition and predicting the end of its reliable operating life. Impedance testing relies on the fact that batteries start life with a low impedance, which gradually increases with age, while almost any battery problem will cause impedance to rise more sharply. It is simple to perform with a hand held instrument or a fixed monitoring system like PowerNSURE.
PowerNSURE provides integrated battery monitoring and management over an Ethernet network. The system checks the impedance, temperature and voltage of each individual battery block. The system also corrects its charging voltage with equalisation as is required to obtain a balanced charging condition across the entire string. This constant monitoring and controlling of the individual voltages for each battery block ensure that they are kept in their optimal voltage operating range and guarantees the availability of the battery at all times.
In this way, over-charging and under-charging are avoided, while typical battery problems such as sulphation, corrosion, gassing, dry-out on thermal runaway are visible through a rise in impedance and temperature. The system helps to provide early warning to replace batteries. Service life can be extended up to 30% by maintaining all battery blocks constantly within their ideal voltage window.