Are Your UPS Batteries At Risk From Thermal Runaway?

Power Control
17 Aug 2023

Whilst the majority of us relish the sizzling summer weather, it can have its drawbacks. Every year, as the temperature soars, reports arise of batteries starting to melt or in the worst cases, exploding in the extreme heat. This occurrence, known as thermal runaway can happen at any time of the year but is more common over the summer months as temperatures rise.

There is no doubt that the risk of thermal runaway is a serious issue. It can affect any business that utilises battery power – at best causing disruption through equipment malfunction, at worst resulting in severe damage to company premises or even posing a significant risk to life.

Thermal runaway is a topic we are often questioned about at Power Control, yet many UPS operators still remain unaware of the catastrophic consequences that can result should it occur within the battery chain. The good news is there are some basic practices that can be taken to mitigate the risk of thermal runaway happening. Before we go on to discuss these in the blog, let’s take a look at the circumstances that initiate this sequence of events and what happens to the battery during this process.


What is Thermal Runaway?

Thermal runaway is a chain reaction within a battery cell which can affect both VRLA and lithium-ion batteries. It will start when the heat generated within the battery exceeds the amount of heat that it is able to dissipate into the surrounding environment. At this point, a chemical reaction will start to occur within the battery, producing more heat and driving the battery temperature higher.

If left unchallenged, the internal battery temperature will continue to increase, causing further chemical reactions and creating further heat. This rise in temperature within a single battery can then begin to affect the other batteries that are in close proximity and so on, hence giving rise to the term ‘runaway’.

In thermal runaway situations, the battery cell temperature will rise in milliseconds causing the outer casing to bulge (100⁰C), melt or rupture (160⁰C), releasing all the energy that is stored within the battery at a rapid rate. The chemical reaction can reach extremely high temperatures of up to 400⁰C posing a serious risk of damage to equipment, sudden system failure, personal injury, fire or explosion.


What Causes Thermal Runaway in a UPS Battery?

  • Ambient Temperature

Lithium-ion batteries are generally less sensitive to wider ranges of temperature fluctuations than Valve-Regulated Lead-Acid (VRLA) ones, however both will degrade at excessively high temperatures.

Battery life becomes compromised when the ambient temperature reaches a continual level of 25⁰C or above, the heat of the surrounding environment reducing the battery’s ability to shed heat and resulting in the chain of chemical reactions as described above.

Short-term, temporary fluctuations in temperature or ‘spikes’, will not create the same effect. These may cause a faster rate of overall deterioration in a VRLA battery (under certain conditions) but will have little or no impact on the overall battery life of a lithium-ion one.   

  • Battery Age

All batteries, whether lithium-ion or VRLA, have a designated design life. Under ideal conditions, a battery with a design life of 10 years will last for 10 years however due to the complexity and make-up of each battery cell, its unique performance level will gradually deteriorate over time. This will be influenced by a number of factors including storage temperatures, humidity, the number of times the battery has been called into action and the rate of discharge. The older the battery gets, the greater the level of deterioration. All batteries are therefore considered to be at the end of their service life when they reach 80% of their rated capacity (or design life).

Batteries that are approaching ‘end of life’ have therefore been degrading over time and may have been subjected to higher currents and internal heat generation, giving rise to conditions within the cells that may initiate thermal runaway.

  • Overcharging

Overcharging will damage the internal workings of any battery, shortening its overall life-span. It can also cause an increase in cell voltage, raising the battery’s temperature and resulting in the accumulation of heat and gases within the cells.

Ageing batteries, that have degraded over time, will have reduced battery capacity and therefore take longer to charge. It is important however to ensure overcharging does not occur.  

  • Float Charging Voltage

The battery manufacturer will provide the appropriate float voltage, which should be routinely monitored and adjusted as needed.

An excessive float charging voltage can cause an increase in float charging current thereby increasing the internal battery temperature and onset of chemical reactions.

  • Physical Damage to the Battery

Thermal runaway can occur due to an internal short circuit caused by physical damage to the battery or through poor battery maintenance. This style of scenario could also cause an external short circuit to occur, starting the chain of thermal runaway events. 

 Preventing Thermal Runaway in USP Batteries

As mentioned earlier, there are a number of ways to reduce the risk of thermal runaway occurring within a UPS battery system.  Here are some of the best practices that can be undertaken:

  • Storing Batteries at the Correct Temperature

The key practice of storing and housing batteries at the correct temperature is one of the simplest ways to prevent runaway occurring. Ideally this temperature should fall within a range of between 5⁰C and 20⁰C.

As battery enclosures tend to trap heat, additional cooling or ventilation to the area where the UPS is housed may be required. This will ensure the ideal ambient temperature can be maintained at all times.  

  • Replace Ageing Batteries

Batteries that have reached 80% of their design life (and therefore the end of their service life) should be replaced as soon as possible to safeguard against further deterioration of the battery cells.

  • Avoid Overcharging

At all times, follow manufacturer guidelines on the appropriate charging requirements for your individual battery/batteries.

  • Use A Battery Monitoring Systems (BMS)

A BMS is an electronic system capable of managing a single cell or entire battery pack. It monitors the overall health of the battery; managing cell voltage, current, temperature, charge balancing and charge control, whilst also helping to detect any internal short-circuits that may arise. 

If the BMS detects that the environment is too warm it can activate cooling to the area to balance and stabilise the battery environment. This regulating process ensures the battery remains in a constant condition reducing the potential for risks.

VRLA battery systems rely on remote monitoring whereas lithium-ion batteries have a BMS system embedded into each battery cell. This allows for monitoring at the cell, module and cabinet level providing a superior analysis of overall battery health.

Should temperature thresholds be breached the BMS system will shut the individual battery cell down, isolating it to allow other batteries in the string, where present, to carry the load.

  • Physical Checks, Impedance Tests and Load Bank Testing

Physical inspection, impedance tests and load bank testing will help to identify any signs of weakness, deterioration or general wear and tear in the battery.


Early detection will allow for replacement batteries to be installed and help to prevent any further degradation that could result in runaway issues.

The vast majority of thermal runaway cases are preventable by putting a few simple practices in place. The catastrophic circumstances that can arise as a result of not doing so however can have serious health and safety, financial and legal repercussions. By following the simple practices, as outlined above, it is possible to substantially minimise the risk of an unwanted incident occurring.

If you would like further advice on UPS battery health or protecting against thermal runaway, please get in touch with a member of the Power Control team on 01246 431431.

With nearly 30 years of experience working with businesses in niche sectors, Power Control has the knowledge and confidence to offer advice and guidance on the correct UPS solution for any size and type of farming business. Having partnered with renowned European three phase UPS manufacturers such as Legrand and Borri SPA, and single phase manufacturer, CertaUPS, Power Control has an all-encompassing product portfolio that covers all applications across all sectors.


Contact us for a free site survey by emailing or call 01246 431 431