Apple Japan has reportedly been ordered by the Japanese ministry of trade to investigate the cause of an iPod nano sparking while charging.

About 420,000 units of the first-generation device, model MA099J/A, were sold in Japan between September 2005 and September 2006. This latest incident reportedly happened in January outside Tokyo but Apple informed the trade ministry only in March.

In addition, a number of people commenting on these articles reported problems with iPod nano batteries melting, bulging, or leaking. Apple reportedly advises users to take iPods out of their cases while charging to prevent overheating.

The typical cause of lithium ion battery fires is believed to be the presence of metal fragments, left over from the manufacturing process, floating around in the liquid casings. That liquid is supposed to contain lithium, though manufacturers have maintained that it is impossible to completely remove these fragments. Since that liquid is necessary to conduct a charge, the odds increase that one of those fragments could spark a fire.

Hardware analysis firm iSuppli estimates Apple's average cost of an iPod nano's lithium battery is just above $3. Apple's Asian office has not yet responded to press inquiries, and it is not yet known which company manufactured the battery. A report from the Japanese news service Nikkei alleges the batteries in question come from China, but without much else to go on with regard to the batteries' identity, it's difficult to see at present how that conclusion is reached.