Michael Tay Ming Kiong*, Wong S.M
5th European Academy of Forensic Science conference
Glasgow, United Kingdom, 8-11 Sep 2009 (poster presentation).
Background of case
After overnight charging, a large lead acid battery which provided standby power for emergency lighting and other equipment in a civil defence vehicle was being topped up with distilled water, when a long collapsible iron tripod accidentally fell onto the battery. This triggered an explosion which shattered the plastic casing of the battery and splashed sulphuric acid on the officer and the surroundings. The damaged battery and ejected broken parts were submitted for forensic examination to determine the likely cause of the explosion.
The exhibit was found to be a 12V, 6-cell flooded (“wet”) battery with removable caps for replenishing water. The lead and lead oxide plates and internal structures were found to be largely intact. The relative density of the remaining sulphuric acid was measured, allowing the battery voltage and state of charge to be determined from charts.
Gassing of hydrogen
Lead acid batteries start to gas when they are overcharged or charged faster than the electrochemical kinetics at the positive and negative plates allow. Continued charging of a fully charged battery results in heating and electrolysis of water into oxygen and hydrogen. Hydrogen is highly flammable and explosive, with flammability limits of 4% and 74%.
Lead acid batteries need to be charged in a well ventilated area in the absence of sparks, arcs, flames and other sources of ignition. A 12V battery that is “shorted” across its terminals can deliver an extremely high short circuit current. Arcs can also occur when a battery is engaged, igniting any accumulated hydrogen. Heating due to charging abuse (extended overcharging or too rapid charging) can lead to a dangerous thermal runaway and destruction of the battery.
The battery explosion was likely to have occurred due to the following contributing factors:
- Overcharging at the end of the charging process resulted in “gassing” of electrolyte and production of hydrogen gas.
- Poor ventilation in the charging area (vehicle compartment) did not adequately dissipate the hydrogen, resulting in accumulation of hydrogen.
- When the iron tripod fell onto the battery, it likely caused a short circuit by either coming into contact with both positive and negative terminals of the battery; or contact with the positive terminal and the metal vehicle frame bonded to the negative terminal.
- The short circuit created an arc which ignited the hydrogen, triggering the explosion of the battery.