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conductance-battery-testerData centre UPS systems are inherently reliable. The chances of a catastrophic failure and load crash are extremely rare but they can occur and the cause of over 90% of them is a discharged battery set, writes Thamesgate Group director Robin Koffler.

UPS systems can be thought of as two components: the UPS electronics and the DC fuel source. In most UPS systems, the DC fuel source is a Valve Regulated Lead-Acid (VRLA) battery. Greener sources can include Lithium-ion batteries, DC flywheels and fuel cells.

The battery is there to provide immediate power to the inverter on mains power failure. It may be sized to provide power for only a short runtime allowing a standby generator to power up to full cycle (1-2minutes) or last for longer periods reaching several hours.

Disruptive individual

Many data managers assume that their UPS system ‘manages’ its connected battery set. This is partly true but typical monitoring solutions used by a UPS are firmware and therefore algorithm-based. A UPS can monitor or test the battery set as a whole but cannot identify individual batteries that may be failing. This is important because a failing battery can leak acid as its internal impedance rises and heat cracks the case.

Each individual UPS battery block has a DC voltage (typically 12V DC) and Ampere-hour rating (e.g. 100Ah). The UPS battery set comprises several individual batteries. These may be connected in series into a string (40x12V=480V DC, 100Ah) and then several strings connected in parallel, to form the set (2x100Ah=200Ah). If the total capacity of a battery set is 200Ah, the runtime available is dependent upon the actual UPS load (kVA/kW), with 480V being a sufficiently high DC voltage to power the inverter.


As previously noted, a UPS is inherently reliable – under normal conditions. If there is an issue or alarm condition, the UPS will report this. If the condition is critical, an automatic static switch may trigger. This is a bypass arrangement configured to instantaneously transfer the inverter output’s critical load to a secondary supply (normally the mains power supply). This occurs if the inverter output voltage is seen to collapse in any way. Reasons for this may include:

  • UPS overload. Most UPS inverters will take up to 150% overload for several milliseconds before the inverter starts to shut down.
  • Faults with the inverter, rectifier, cooling fans or any other internal assembly that could lead the UPS to shut down and protect itself.
  • The battery set being unable to deliver sufficient power to the inverter, either because of a failed battery or lack of battery charge

Worst nightmare

This last condition can be a data centre manager’s worst case scenario. If the battery set cannot deliver power to the inverter, the inverter output voltage waveform will collapse. If there is no mains power supply available, the static switch cannot transfer the load to any other source of AC power. The data centre load then crashes.

Separate battery monitoring systems are available but these may only be economical for very large UPS installations (>500kVA) due to capital costs. Some suppliers will rent them but again the cost is high. Battery monitoring systems are expensive, do not benefit greatly from scale economies and require an individual sensor to be connected to each battery block. These sensors are then wired to a central monitoring system that monitors battery block health over time, reporting and/or alerting when performance fails to meet acceptable limits.

Everything in hand

Though a fixed battery monitoring system will provide data centre managers with greater confidence in the UPS batteries, there is a simpler, more cost-effective solution. This is a hand-held battery conductance tester that a UPS engineer can use during routine preventative maintenance visits. With such instruments it is possible to test up to 40 battery blocks per hour.

Each battery block is given its own unique identifier, as is the UPS and site. The results can be stored locally on a network or Cloud for later analysis and comparison. While on-site, UPS engineers can also look for signs of heat damage (buckled plastic case) and electrolyte deposits around terminal connections.

Hand-held conductance testers can be used for a wide range of batteries and help data centre operators plan battery replacements, ensuring the integrity of their UPS systems and, more critically, battery sets.

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