• The right way to specify a UPS battery system

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Block diagram of a UPS and battery system

A properly designed UPS battery solution is essential for safe and reliable operation. The right design not only ensures functionality, but also helps to minimisze the risk of system damage or fire.

‘Many people assume that battery systems are simply groups of connected batteries’, said Pearl Hu, senior research specialist at Schneider Electric’s Data Center Science Center. ‘But an effective system is actually quite complex and requires careful planning and proper execution’.

What’s difficult about choosing a battery solution? One inherent characteristic of battery power is variable short-circuit current, which can present a challenge when selecting and rating protection devices. Another factor is the need to balance the trade-off between performance and cost, where a focus on cost generally prevails.

However, by following proven best practices, system designers can collaborate with battery providers to ensure the most productive and cost-effective solution.

Key design steps for protecting a battery system

An engineered and validated battery system must consider and accommodate multiple aspects: UPS performance, charge and discharge characteristics, battery service life, types of potential faults, system maintenance plan or schedule, environmental factors, and more.

Whether using a standard or custom battery solution, the key steps for designing large UPS battery systems are:

  • Step 1: Select and size the battery based on detailed load profiles, specified backup time, UPS characteristics, battery specifications, parallel battery strings, and battery redundancy requirements.
  • Step 2: Calculate discharging current, correlated with the battery charge state, the load, and the UPS inverter efficiency.
  • Step 3: Select conductor size in accordance with accepted safety standards.
  • Step 4: Calculate short-circuit current by following industry practices for safe operation, allowing for likely charging/discharging scenarios.
  • Step 5: Select protection devices based on battery configuration, number of devices needed, device ratings, and duration of short-circuit currents.
  • Step 6: Implement the whole battery system assembly to best monitor the status of each battery unit, to ensure normal operation during charging/discharging modes, and to alert operators of abnormal conditions.

Executing a system design

In addition to selecting equipment and determining parameters for a battery system, designers and specifiers need the right approach to execution.
‘We view a standard, pre-engineered, modular battery system as the top choice for system design’, said Hu. ‘The big advantage of standardised solutions is that they have been analysed, designed, tested, qualified, and proven out over many systems’. She added that this option will eliminate one-time engineering costs.
With this approach, UPS vendors specify product performance criteria such as runtime, voltage, and discharging current characteristics. A pre-engineered solution makes the process easier for designers — they need only select standardised modular battery systems for a specified backup time.
However, when an end user’s requirements or preferences call for ‘engineered to order’, a vendor-engineered battery system is the next-best choice. For this approach, UPS vendors follow common best design practices based on validated testing for improved electrical safety.
But when neither of these approaches can support an end user’s requirements for a large UPS battery system, a third-party, custom-engineered solution may be the only option. The following checklist for custom battery systems is a guide to ensuring reliable system design:
  • Follow the previously listed “’ey design steps’.
  • Analyse and validate the safety of the entire UPS system under normal and abnormal operation, as well as at full charge and end-of-discharge.
  • Perform strict quality control of critical devices including UPS, batteries, protective devices, etc., and continue doing so during system assembly.
  • Ensure that enough spare parts are kept in inventory.
  • Set up a periodic maintenance plan and ensure the implementation of the defined plan.
Regardless of evolving technology or the preferred design approach, a safe and reliable battery system for large UPS is essential for minimising risks while increasing critical load reliability.

For expert advice on battery systems, download the white paper Design and Specification for Safe and Reliable Battery Systems for Large UPS
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