Battery types, sizes and hold-up time for Uninterrupted Power Supply (UPS) units

In the first part of this article on Uninterruptible Power Supplies (UPS), we looked at the two main types of units, rotary and static, along with what considerations need to be taken into account when selecting a suitable UPS system. Here, we continue our deep dive into UPSs, examining the run or hold-up time, battery types and sizing.

Hold-up time

Hold-up time is the amount of time that an electrical device can continue to run during an interruption of power without resetting or rebooting. This amount of time will vary depending on the industry, for example power supply units for IT equipment should be designed to allow enough energy to keep the device running when a power interruption of around 20ms. This allows the device to withstand brief power interruptions while the UPS transitions between modes of operation.

Typically the mode of operation transfer time to the UPS should be less than the hold-up time. This is because the longer the power supply unit has no power, the larger the in-rush current it will draw when power is connected again, which can result in the UPS shutting down if its current handling capacity is exceeded.

Typical transfer time differs slightly between the various types of UPS:

  • An Offline UPS has a transfer time of between 5-12ms, with 8ms being the average. As these units usually have a fast-acting mechanical relay as the power switch, the length of time before the battery transfer can take place is longer. While this is tolerated by most power supplies, a transfer time above 5ms does increase the chance of the in-rush current exceeding the capacity of the UPS inverter. This is a concern for highly critical servers if the output current the standby UPS dips below 10% of nominal current, the higher the chance of the in-rush current exceeding capacity and the power supply system shutting down, resulting in IT equipment resetting with potential data corruption
  • A Line interactive UPS has a shorter transfer time than an offline UPS, usually between 3-8ms (most typically 5ms) which is acceptable for most power supplies. If the transfer time is longer than 5ms, the power supply unit could experience in-rush currents exceeding 400% and the UPS inverter
  • A Double conversion UPS has the shortest transfer time, with there being negligible interruption in output power, therefore there is minimal risk of in-rush currents exceeding the capacity of the unit

Battery types, sizes and hold-up time for Uninterrupted Power Supply (UPS) units



There are three main types of batteries used in UPS units: valve regulated lead acid (VRLA), flooded cell and lithium ion.

Valve Regulated Lead Acid (VRLA) batteries

VRLA batteries, also referred to as sealed lead acid or maintenance free batteries, are the most common type found in UPS units today, these batteries are ‘sealed’ so no acid spillage will occur during normal handling and use, they can be mounted vertically or horizontally and are suitable for use within battery compartments, rackmount trays or external cabinets.

They are known as valve regulated due to the method of gas release they use, if the gas pressure builds up beyond a set value within the battery, a valve will vent the excess pressure without compromising the battery’s integrity.

VRLA batteries loss of acid liquid is minimised since the hydrogen formed during the charging cycle recombines with oxygen inside the battery. Under normal float conditions, virtually all the hydrogen and oxygen is recombined. As water cannot be added, its recombination is critical to the life and health of the battery and any environmental conditions that increase evaporation, such as higher ambient temperatures and heat from charging currents, as these reduce the battery life. VLRA batteries are best for applications where they can remain at a relatively stable temperature of around 25°C or lower.

While VRLA batteries typically have a lower upfront cost, are safer and require less maintenance than flooded cell batteries, they can suffer from a shorter lifespan, typically around three to seven years of ‘normal’ use.

Flooded cell vented lead acid (VLA) batteries

Flooded cell batteries, also known as wet cell or vented lead acid (VLA) batteries, are composed of thick, lead-based plates with electrolyte acid. This can give them a long lifespan provided the plates are continuously immersed in electrolyte, they’re not exposed to wide temperature ranges and their charging/discharging cycles are managed correctly.

Compared to VRLA batteries, flooded cell batteries typically have higher upfront costs and additional safety measures that require consideration, such as having a well ventilated area, minimising vibration and providing separation of the batteries from other equipment due to the potential chemical hazards involved. Flooded cell batteries also have additional maintenance requirements, such as only filling them with distilled ‘non-mineral’ water and regularly checking the electrolyte levels.

Lithium ion batteries

Lithium ion batteries are still relatively new for use with UPS units – originally used as battery cabinets in three-phase installations, but now also able to be used in single-phase installations.

This type of battery offers a range of advantages over other types of batteries including being safer and more stable, a longer lifespan, lightweight, smaller in size and expanded warranty coverage. A key advantage is that they have built-in battery management and monitoring, allowing charge current, voltage and cell voltage balance to be managed, as well as being able to adjust for issues of over temperature by disconnecting single batteries or strings if temperatures rise above safe levels.

Furthermore, they have high charge-discharge and recharge times, making them ideal for non-traditional UPS applications like grid sharing, peak shaving and industrial or process control support, lower operating costs as they don’t need to be frequently replaced and can operate in higher ambient temperatures.

However, they are also more expensive due to the technology and materials that go into the manufacturing process.

Battery types, sizes and hold-up time for Uninterrupted Power Supply (UPS) units


Correctly sizing a UPS for an application is critical to its performance, under-sizing will result in the potential for not having enough power when it’s required, while oversizing will involve greater upfront costs for no technical benefit.

There are a number of considerations that can be taken into account to work out what size UPS is needed, including:

  • Power outage history, particularly the duration of previous power failures
  • Critical load sizes
  • If there is a maintained standby generator on-site
  • The length of time required for load shedding and/or an orderly shutdown
  • The available environment, for example, space constraints, temperature and humidity
  • Cost versus space requirements for longer runtime systems that may require additional batteries

The key to sizing a UPS is that it needs to have enough capacity to support all the connected equipment, the higher the capacity, the more equipment and devices that it can support.

Battery types, sizes and hold-up time for Uninterrupted Power Supply (UPS) units

In order to find the capacity required, the load, which consists of the combined amount of power of all connected equipment and devices, needs to be calculated. This can be done in a simplistic way by adding up all the individual loads that will be connected to the UPS and determining the total ‘kW’ required.

Once the total load is calculated, the next step is to work out how long this load will need to run for, known as run-time or hold-up time, usually defined in minutes or hours at the specific load.

Another key consideration is future expansion, sizing should take into consideration not only the current equipment loads but the potential future load requirements if any additions or upgrades are due to be performed in the foreseeable future. Modular racking systems are easy to scale up and additional modules can be added to a vertical rack, however if horizontal scaling is required, additional racks will potentially need to be added as well as consideration for extra space and cabling.

In the next article in this series about UPSs, we’ll look at communications, centralised or distributed units for sites and maintenance.