What is lead acid battery thermal runaway?
If you’ve been around batteries for any amount of time, you have likely heard the term, “thermal runaway.” Besides being a ‘hot topic’ (pun intended), what does this actually mean and what happens when a thermal runaway event occurs?
What is lead acid battery thermal runaway?
First, what is thermal runaway? A battery is considered to be experiencing a thermal even when the battery begins to generate heat from uncontrolled self-discharge. Essentially, the battery is generating more heat than there is the possibility for it to transfer the heat into its environment.
Sealed Lead Acid (SLA) batteries all have a small amount of natural self-discharge simply from the behavior of the chemistry. This phenomenon is described in greater detail in our technical manual for SLA batteries. Natural self-discharge occurs at an extremely low rate – usually less than 3% per month. During a thermal runaway event, the battery will self-discharge its entire capacity in a matter of minutes! The by-product of discharging so fast is an excessive amount of heat – and all of that energy has to go somewhere. Most commonly, this presents itself as a swelled battery – the battery will bulge from all sides. In rare situations, the battery case can fail and spill battery acid. This acid is corrosive and will likely damage any non-metal that it meets.
What causes lead acid thermal runaway?
Uncontrolled high-rate self-discharge usually stems from an internal short, often caused by misuse—dropping, overcharging, over-discharging, vibration, or poor manufacturing. Thermal runaway typically occurs during charging, when the short dissipates more energy while the charger adds power.
What happens during an internal short?
An internal short occurs when two lead plates touch or come very close, often due to a failed separator. This is like bridging the battery’s terminals—energy rushes to the contact point, releasing heat and potentially sparks. Some internal or “soft” shorts cause only gradual self-discharge, leading to a dead battery without triggering thermal runaway.
When a short circuit condition occurs inside the battery, enough heat is generated to boil the acid in the battery. The sulfur odor – rotten egg smell – is an immediate way to detect if a battery is possibly experiencing a thermal runaway event. If you ever notice excessive heat or this smell, immediately disconnect the battery from any load or charger and stay away from it until the battery has cooled. If possible, open any doors or windows in the area to ensure quick dispersion of the released gasses.
Will thermal runaway cause a fire?
While enough heat is generated to boil the acid, this temperature is far below any flash point that may cause fire. The temperatures are generally not even high enough to melt the case. The dangers of battery acid spillage are far higher than any fire or explosion risk.
How to prevent lead acid battery thermal runaway
Internal shorts can be best avoided through careful SLA battery construction. Power Sonic goes to great lengths of putting in the effort required to ensure high manufacturing quality. These high standards are in place to prevent manufacturing defects that can lead to internal shorts. The separator plates are well aligned to prevent the positive and negative plates from touching and the welded bars that connect the plates are carefully assembled to ensure no pieces of metal fall off, down into the plates.
You, the end user, can best avoid internal shorts through caring for your battery! Things like drop shocks or high vibration environments can cause internal damage that can lead to internal shorts.
A drop shock can knock pieces of lead loose which will fall into the plates. When this piece of lead connects the two plates, a direct internal short is created which will often lead to a thermal runaway. High vibration environments can cause accelerated wear of the separator plates allowing the two plates it keeps separated to touch. Positive and negative plates touching, as described earlier, can also lead to uncontrolled high-rate discharge that causes a thermal runaway.
Avoid deeply discharging your battery. Even deep-cycle batteries benefit from less than 100% DoD, as high DoD in SLA batteries can accelerate sulfation and, in rare cases, cause thermal runaway from excess heat.
Now that we have covered thermal runaway in SLA, you may be wondering about thermal runaway in lithium batteries. With lithium, you can expect a higher heat event since the energy density of lithium is much higher than SLA. Look for the Lithium Thermal Runaway blog coming soon.
