Uninterruptible Power Supply: External Battery Testing

Batteries provide the lifeblood of most uninterruptible power supplies (UPS) and power protection solutions.

They provide protected loads with continuous energy in the event of a mains failure, which will support loads whilst a generator or alternative power source starts up or provide enough time to gracefully shut down equipment or ride through short duration power cuts.

Often, external battery 'strings' or 'sets' constitute a large part of an uninterruptible power supply. A whole battery string, however, is only as good as its weakest battery.

It is, therefore, imperative to regularly test UPS batteries and battery strings for 'weak links', which should be immediately replaced - even if they have not quite fulfilled their design life, which for UPS batteries is five to ten years.

There are various methods for electrically testing UPS batteries:

• Automatic battery set testing.
• Load bank discharge testing or battery set, string and individual cell testing.
• Electro-chemical testing.

Many uninterruptible power supplies are designed to test their batteries every 24 hours and will sound an audible alarm if a battery fault or condition is detected. How it works is that the test places a load on the battery set and the discharge performance is monitored.

This is a basic type of test, however, and cannot be relied upon to give detail - other than an overall indication of the state of a battery set. Battery sets usually age uniformly but individual blocks of batteries can fail earlier than others and be rendered unable to sustain a reasonable level of charge, which can bring the whole set down under load conditions.

A more accurate and reliable way to test batteries or battery sets is to use a load bank, which places a load on the set from which measurements can be taken intermittently to assess its discharge performance. But even this has its limitations.

The disadvantages of load bank testing are that it reduces resilience whilst the system is being tested, which for critical applications like hospitals or financial processing is unacceptable.

For large battery sets, individual block testing is much more reliable and need not take up additional man-hours as it can be incorporated into a routine maintenance visit.

It should include a visual inspection to check that battery casings are not damaged or showing signs of aging such as buckling and emission of electrolyte discharge, which can sometimes be seen around terminals and valves.

Sulphating Batteries:

Because UPS batteries employ an electro-chemical process, they can deteriorate over time. When batteries sulphate, Lead Sulphate crystals form within their electrolyte and at plate terminals, which will reduce charging performance and prevent normal operation.

They can be recovered from this state by being charged at a higher current for around twelve hours. But care must the taken as this process itself can generate heat.

If the battery does not recover through this means, it must be immediately removed and disposed of and replaced.

Impedance Testing:

Impedance testing can identify battery blocks that are about the fail. As batteries age, internal resistance increases. Impedance testing consists of applying an ac voltage, suitable for the Ah size of battery, through probes attached to the battery block terminals.

Impedance is measured and recorded and compared to published data from manufacturers and/or previous recordings.

Electro-Chemical Testing:

Electro-chemical testing relies on measuring the frequency response to voltage and current signals passed into the battery using probes placed on the positive and negative terminals.

Electro-chemical testing is a non-invasive method that uses the principle of Frequency Response Analysis to compare measured data to algorithms of known battery conditions, specifically sulphate and electrolyte dry-out.

This problem is the principle cause of UPS battery failure and poor performance. Although originally only available in laboratories, electro-chemical testing can now be applied to uninterruptible power supply batteries using a specially designed, portable hand-held testing unit.