Understanding the HTM: Section 11.4 UPS Batteries

Power Control
13 Apr 2021

Following the immense strain placed on the NHS due to the recent COVID-19 pandemic, addressing ageing infrastructures will feature highly on agendas. While there have been positive signals that the UK is on the road to recovery, it would be foolish not to be prepared for any similar future scenario.

Healthcare facilities have many layers to consider – from the core aesthetics, pharmaceuticals, IT, telecoms, and medical equipment, through to managing ‘hard’ facilities such as lighting, lifts, fire safety, and the overall electrical infrastructure. It is the latter that is often wrongly neglected. The electrical infrastructure of any healthcare estate – be it a hospital, GP’s surgery, or auxiliary care facility, requires a resilient power supply. With such a broad spectrum of electrical applications to protect, establishing this can be an extremely complex process.

The Department of Health and Social Care developed a series of documents under the Health Technical Memoranda (HTM) to provide standardised advice, policies and best practice guidance in an attempt to alleviate some complexities. The HTM 06-01 focuses on the electrical services supply and distribution within any healthcare estate and provides a thorough outline of the critical power infrastructure set up.

In the first of a four-part series of blogs, where we will be evaluating specific aspects of section 11 of the HTM 06-01 relating to the delivery of tertiary power supplies, we look at subsections falling under ‘Batteries for UPS systems, inverter units and generators.’

As well as being a key component in a UPS, batteries are also a concern for being a significant point of failure if not specified or maintained adequately and will determine the performance and backup time of the power supply.

A lot rides on the correct specification and sizing of the batteries to ensure continuity and patient and staff safety and there are a number of considerations that must be addressed when specifying batterie for tertiary power supplies to meet HTM guidelines.

Battery Type

Valve regulated lead acid (VRLA) batteries are the most common battery type to be used in healthcare facilities. They work well in standby applications and have an array of safety benefits. Specifically referred to in the HTM, these batteries can be easily installed in plant rooms on racks, and are easily maintained with near zero gassing.

VRLA batteries suitable for use in UPS solutions are available in a range of lifespans which should be considered during the design phase. Starting with a 3-5 year design life, these VRLA batteries are most appropriate for typical standby applications including IT and server racks or hospital comms rooms within risk grade E and grade D where the UPS is not being used to support life safety systems including escape lighting, Fire alarm systems, etc. They may also be used to support risk to business grades IV and III where there are no life safety systems present.

Batteries with a design life of 6-9 years tend to be for ‘general use’. They are likely to be smaller batteries, without flame retardant casing and have spade terminal connections. This makes them inappropriate for HTM applications where batteries are required to have flame retardant casing and screw down terminals as per BS EN 60896 parts 21 and 22.

Batteries with these features are generally 10-12 year, high performance batteries that are suitable for risk grades in both the business continuity scale and patient safety scale. Although they have higher initial investment costs than standard five-year life batteries, they offer significant long-term benefits in terms of security of function and reduced lifetime costs.

Batteries with a design life of 12 years and over, are available but are uncommon due to their high cost.

Although not mentioned in the HTM 06-01, battery technology is evolving. Over recent years, lithium-ion has been one of the most anticipated developments in the UPS industry. When paired with a UPS, lithium-ion batteries present not only an efficient and reliable tertiary power supply but also present numerous benefits to the healthcare industry.

When compared to VRLA batteries, the benefits of lithium-ion include:

  • Higher depth of discharge – the amount of overall capacity that can be used before recharging the battery. A lithium-ion battery can handle a depth of 80%. 20% more than a VRLA battery.
  • More battery cycles – Number of times a battery can be charged and discharged before the battery starts to degrade. A lithium-ion battery can handle on average 10x the number of cycles as a VRLA battery.
  • Recharge time – Where VRLA batteries typically take 6-12 hours, a lithium-ion battery takes 30 minutes to an hour to recharge.
  • Temperature tolerance – Due to its molecular structure, a lithium-ion battery can operate reliably at wider temperature ranges than other rechargeable batteries.
  • Energy storage – By utilising the energy storage benefits of lithium-ion, more advanced medical technology can be installed into existing older hospital infrastructure. For example, if the maximum input of a hospital is 100KW, where a lithium-ion UPS is installed, the input supply can be limited to the maximum 100KW and any additional power needed to support the medical imaging device will be drawn from energy stored in the batteries of the UPS system.
  • Power to size/weight ratio – Lithium-ion batteries have a 50% smaller footprint for the same power outage and are 50% lighter.

It is not only the energy storage capabilities of lithium-ion that make them an ideal choice for powering sensitive applications. With a longer design life, the technology also improves the reliability, efficiency and flexibility of the facility’s overall backup power infrastructure. The HTM 06-01 recommends that batteries used for tertiary power supplies, such as those for a UPS, should have a design life of 10 years. Whilst specialist VRLA batteries do meet these guidelines, a standard lithium-ion battery has an average lifespan of 15 years, with no battery replacement necessary.

Despite the benefits presented by this technology, it is still significantly more expensive than traditional VRLA batteries which is why uptake of lithium-ion UPS systems across the healthcare industry is still low. Runtime and power configurations are still limited within lithium technology and due to the complect battery management, not all UPS are currently compatible.

However, over the past decade, the UPS industry has seen a dramatic reduction in price and the as the production of this technology increases exponentially, the cost of a battery is dropping quickly. We anticipate in another decade, lithium-ion batteries will become more commonplace across a plethora of industries, including healthcare.

Battery Runtime

Battery runtime refers to the length of time that a tertiary power supply, such as the UPS, is designed to support the critical load in the event of a complete mains power outage. Section 11.11 of the HTM 06-01 signifies three runtime lengths.

If the secondary supply were to kick in within 15 seconds of a mains failure, then only 1-hour battery autonomy is required from the UPS.

If it’s over 15 seconds, then a 3-hour battery autonomy is required. The secondary power supply could be in the form of a diesel generator or a combined heat and power system (CHP).

The third option considered is when the UPS is providing tertiary power to other applications in operating theatres. In this scenario the battery runtime should be designed to take into account the requirements of the operating theatre staff, giving them enough time to “facilitate patient closure for all theatre cases.”

In most cases, 1 hour or 3 hours of backup time is implemented. It is important to ensure the UPS system is capable of supplying the design load as well as able to recharge the batteries within the required timescale. In some cases, it may be necessary to oversize the UPS in order to offset spare capacity to charge the batteries.


Batteries are made up of chemical elements and so are susceptible to the conditions of the environment in which they are placed. In order for a battery to achieve its design life, it needs to be placed in optimum environmental conditions. The longevity can be affected by how they have been used over the years, the number of cycles and the quality of charging from the UPS or CPSS. However, the biggest factor affecting battery performance and lifespan is the temperature. Battery manufacturers will typically recommend that batteries operate within an environmental temperature of between 20-25C. Beyond this and the lifespan begins to deteriorate considerably. For example, the battery lifespan would be 50% if the environmental temperature was 30C and at 40C the battery life would be reduced by 75%.

With continued operation at high temperatures, these batteries can plummet in terms of performance and potentially become a fire hazard. Therefore, it’s of utmost importance that the batteries are in a suitable location with an optimum environment, adequate ventilation and temperature control to ensure peak performance and maximise battery life.


The configuration of battery strings in a UPS installation is critical to eliminating single points of failure. Batteries can be arranged in a single string or dual parallel strings. For simplicity and to keep costs to a minimum, UPS battery subsystems are typically configured as single strings. However, during a mains failure, this leaves the system vulnerable by creating single points of failure.

In line with section 11.9 of the HTM, the use of parallel or split battery banks should instead be specified. This configuration allows the UPS to remain online even when half the battery system is being serviced.

Power Control has been specifying, installing, and maintaining tertiary power systems in healthcare estates for over 27 years. Every UPS system, both monolithic and modular, suitable for healthcare applications are available with either VRLA or lithium-ion batteries and meet all relevant medical regulations including the IEC 60601, HTM 06-01, BS 7671 and BS 6290-4. Contact us to find out more, 01246 431 431, power@powercontrol.co.uk