What is Thermal Runaway?

By 31st January 2019Constant learning

Thermal Runaway is defined as a critical condition arising during constant voltage charging in which the current and the temperature of the battery produces a cumulative, mutually reinforcing effect which further increases them, and may lead to the destruction of the battery.

The above can be expanded upon. A lead-acid battery that has been on float charge for some time, typically 3 months, will have a stable float current. If the temperature of the battery increases there is a natural reduction in the back EMF presented by the battery to the charger. The charger will see this and will automatically adjust the current to match the set charging voltage. In so doing, because the current increases a corresponding rise in battery temperature will result. Because the temperature has increased, the back EMF will reduce and again the charger will see this and the charger current will increase, causing additional heat and the cycle is repeated.

It should be noted that a ‘type’ of Thermal Runaway can also occur when using constant current chargers but there will be no mutually reinforcing effect and instead, the initial current applied will be sufficient to cause the destruction of the battery.

Typical examples of Thermal Runaway can be seen in the above image.


Thermal Runaway is explained above but what triggers the cycle of events? There are several reasons.

a. Poor charge voltage regulation. If there is a large swing in the D.C. voltage as a result of the A.C. input voltage variations.

b. Poor ventilation and inadequate cooling normally associated with restricted space in an enclosure or room.

c. Excessive A.C. ripple current from the charger

d. High operating temperatures as a result of the natural temperature such as climatic conditions or being located in a high-temperature environment as the result of an outside influence.

e. Poor air circulation in restricted areas which may cool only part of the battery.

f. Restricted air circulation.

If you spot any signs of thermal runaway in your installation, do not hesitate to contact Constant Power Services to carry out a survey of your install.

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