If you’ve worked with batteries, you’ve probably heard the term “thermal runaway”. This phenomenon is a hot topic (pun intended) and has major implications for battery safety. So what exactly does thermal runaway mean, and what happens during it?
Understanding Lead-Acid Battery Thermal Runaway
Thermal runaway occurs when a battery generates heat due to uncontrolled self-discharge. Essentially, the battery produces more heat than it radiates to its surroundings. Sealed lead-acid (SLA) batteries’ natural self-discharge rate is typically very low, less than 3% per month. However, batteries can discharge during thermal runaway in as little as a few minutes! This rapid discharge can lead to overheating, leading to physical changes in the battery, such as expansion. In extreme cases, the battery casing can rupture, causing corrosive battery acid to leak, which can damage any non-metallic surfaces it comes in contact with.
Causes of Lead Acid Thermal Runaway.
The primary causative agent for runaway high-rate self-discharge is an internal short circuit. Internal short circuits are usually caused by improper battery use, such as dropping the battery, overcharging, over-discharging, exposure to high-vibration environments, or even poor manufacturing processes. Internal short circuits are particularly susceptible to thermal runaway, which occurs during charging when the charger is continuously powered, causing the battery to consume more energy than it can handle.
What happens when there is an internal short circuit?
An internal short circuit occurs when the lead plates inside the battery are too close together or even touching, usually due to a failure of the spacer material. The consequences of this situation are similar to an external short circuit. Just as touching the terminals of a car battery with a wrench creates sparks and overheating, an internal short circuit causes stored energy to surge to that point, generating a lot of heat. Other types of internal shorts, known as soft shorts, typically result in a lower self-discharge, causing the battery to drain without triggering a thermal runaway.
When an internal short circuit occurs, the heat generated can cause the acid in the battery to boil and produce a sulfuric odor similar to rotten eggs, which is an important warning sign of a potential thermal runaway event. If you notice the battery overheating or this odor, you must immediately disconnect the battery from any loads or chargers and keep a safe distance until the battery cools. Open a nearby window or door to allow the released gases to escape quickly.
Is there a fire risk from thermal runaway?
While thermal runaway can generate enough heat to boil acid, the temperature is usually not hot enough to start a fire or melt the battery case. The risk of battery acid leakage is much greater than any potential fire or explosion hazard.
Preventing Thermal Runaway in Lead-Acid Batteries
To minimize the risk of internal short circuits, careful manufacturing of SLA batteries is essential. ACE adheres to high standards of manufacturing processes to ensure quality and prevent defects that could lead to internal short circuits. Alignment of the separators is critical to prevent contact between the positive and negative plates, thereby reducing the risk of thermal runaway. For more information on battery types and their characteristics, check out our 18650 vs. 26650 battery characteristics comparison.
At Techhem, we understand the importance of ensuring that the welded strips connecting the battery slats are well assembled. This careful assembly prevents metal fragments from falling onto the battery slats, thus avoiding internal short circuits. As an end user, you can significantly reduce the risk of internal short circuits by taking good care of your batteries.
It is critical to avoid drop shocks and high-vibration environments as these conditions can lead to internal damage. Drop shocks can dislodge the lead and drop it into the plates. If a piece of lead connects two pole plates, this can cause a direct internal short circuit, which usually leads to thermal runaway. Similarly, high-vibration environments can accelerate spacer wear, bringing the positive and negative plates into contact. This contact can lead to an uncontrollable high discharge rate, resulting in thermal runaway.
Another important aspect of battery maintenance is avoiding deep discharges. While some batteries are designed for deep cycling, even deep cycling batteries should have a cycle depth of less than 100%. Deep discharging of SLA batteries can lead to accelerated vulcanization and, in rare cases, excessive heat buildup due to increased demand from poorly performing batteries, triggering thermal runaway.
Now that we’ve discussed thermal runaway in SLA batteries, you may be curious about thermal runaway in lithium batteries. Lithium batteries have a higher energy density compared to SLA batteries, so the potential exists for a high-temperature event to occur. Stay tuned for our upcoming blog on thermal runaway in Li-ion batteries for more information.