SAN FRANCISCO -- Apple may issue refunds to customers who paid full price to replace the batteries on older iPhones before the company offered a $50 discount as part of its apology for secretly slowing down the devices.

No details about the potential rebate were included in a five-page letter sent to Sen. John Thune, a South Dakota Republican who demanded last month that Apple provide more details about the iPhone slowdown.

Thune released Apple's Feb. 2 response on Tuesday. He said Apple would follow up with additional information about the rebates at a future date. Apple has been replacing batteries on older iPhones for $29 since late December, down from the usual $79.

The offer is good through this year. A new battery is supposed to prevent older iPhones from bogging down.