Sony has provided greater detail about a battery manufacturing problem that is expected to see the replacement of up to 9.6 million laptop computer battery packs.

The problem was first acknowledged in August when Dell issued a recall for 4.1 million batteries and until now had been explained as metallic particles that got into the battery during the manufacturing progress. On Tuesday, Sony expanded on this and said the particles, believed to be nickel, likely got into the battery during two stages in production: when a groove was created in the battery case and when the electrolyte was poured into the cell.

But that alone wouldn't be enough to cause the fires that have been reported by laptop owners. For that to happen Sony believes that the particles would have to fall into a small triangular gap in the cell body right at the point where the cathode ends between two layers of spacer material. Then, depending on system configuration, the conditions could be right for a fire to start in the battery.

"The probability of this occurrence very much depends on system configuration," said Takashi Enami, senior general manager of the energy business group at Sony. He said size and shape of the battery pack and the charging configuration could all increase the risk but he wouldn't offer any specific information citing confidentiality agreements between Sony and its customers.

As a result of the problems 6.1 million batteries have been recalled by Dell and Apple. An additional 3.5 million batteries are covered by a Sony-led replacement program that offers new batteries to laptop users who are worried about the safety of their system. Last week Sony said it anticipates costs of ¥51 billion (£228 million) as a result of the battery problems.

Sony considered the battery replacement after a Lenovo-made laptop caught fire at Los Angeles International Airport, said Yutaka Nakagawa, executive deputy president of Sony and head of its semiconductor and component group. That incident is still under investigation and as only two of the six cells inside the battery pack remain it's difficult to pinpoint the exact cause, he said.

"In preparing for the program we have been in discussion with the CPSC [the US Consumer Product Safety Commission] and the PC manufacturers, which required time, and that is why the announcement is being made today," said Nakagawa.

The replacement batteries won't all come from Sony because it doesn't have the manufacturing capacity to produce them all in the time required, said Nakagawa. As a result Sony will source some cells from rival companies. Nakagawa said as a result there is a general possibility that Sony might not win back all the business it had before the battery problems occurred. Its success or failure in keeping business very much depends on how well Sony does to persuade customers that the problems are behind it, he said.

Additionally Sony also increased the number of its own battery packs eligible for the replacement program. Last week it said 60,000 batteries used in PCs in Japan and China would be replaced and on Tuesday increased this to 250,000 batteries worldwide.