Resistance Heating
Lead/Acid Batteries

Lead/acid batteries have served as a mainstay in motor vehicles for many decades, supplying electricity for starting the engine and running accessories. The electrochemical system inside the battery consists of lead, lead dioxide and sulfuric acid. Over time, many design improvements have been instituted to improve reliability, service life, and durability.

For the purpose of fire investigation, there are several circumstances in which batteries may be involved as an ignition source.

1)         Battery displacement or physical damage may result in a short circuit. A loose battery may shift due to vibration or road bumps causing terminals to contact conductive surfaces. If the contact is maintained, heating and fire may result. Battery energy is high; it is generally expected that any short involving a battery terminal or cable to ground will leave an easily visible mark. Collision damage may cause displacement of the battery, shorting of cables or damage to the battery case that can result in fire.

An example of a fire caused by collision damage to a battery can be found in a General Motors crash test report [1]. This test was conducted with standard fluids and a charged battery. During the test, deformation caused a sharp screw to penetrate the case of the battery, shorting against the internal plates, and resulting in a fire.

2)         Batteries release hydrogen gas while charging. There have been cases of the hydrogen gas igniting when subjected to sparks from static electricity or other sources. This topic will be discussed in the Fuels Section.

3)         Batteries cases may also heat to the point of ignition. This does not appear to be a frequent occurrence, but at least 2 failure modes have been documented:

a)    Conductive contamination on the surface of the battery may create a high-resistance short between battery terminals. For example, leakage of electrolyte on the surface of the battery will create a circuit between terminals [4]. Most of the time one would expect the heating itself to change the local conditions and thus interrupt the circuit without heating to the point of ignition. However, in the rare occasion when the short circuit is maintained, the battery may overheat and ignite [4]. Vehicle operation with loose or missing hold downs may allow battery vibration and leakage of electrolyte through vents.

b)    Battery abuse can also lead to overheating [2-6]. When batteries are overcharged, heat is generated, hydrogen gas is released, and electrolyte levels are reduced. In old, abused, or overcharged batteries, internal damage to the battery may compound with use. This failure mode is expected to be accompanied by a smell of sulfur (rotten eggs). A history of charging system complaints or battery problems indicates increased likelihood that a fire was caused by battery overheating.

c)    In recent tests of of battery abuse (in accordance with SAE J2464, "Electrical Vehicle Battery Abuse Testing") 12 and 36 volt batteries were subjected to dead shorts across positive and negative terminals. Shorts inside the battery stopped the current before significant heating [7] Overheating would still be possible if the short across terminals had enough resistance to heat but insufficient to cause an early failure in the battery.

As in other factors of electrical causation, burn damage to batteries observed after the fire may be a result of the fire and not because the battery was the source of ignition. When investigating a battery fire in a fleet vehicle, battery conditions in non-fire fleet vehicles of similar design, usage or maintenance may give information about the probability of battery fire in the subject vehicle. This technique of examining non-fire vehicles in a fleet can be applied to fire investigation in general.

To view references for this section before continuing, click here.