The likely cause for the explosion was a build-up of gases emitted from the battery within the sealed unit containing the internal battery of the jump starter unit, which ignited when the unit was switched on. The most likely cause of gas build-up was when the internal battery was abnormally charged using a charger that was defective, or one not supplied with the unit. A large number of these and similar units have been sold and used without incident. The supplier of the unit wants to investigate this incident further and will try to develop ways to minimise the risk of any similar accidents in the future.