When Apple launched its $29 battery replacement program at the end of 2017, it had no idea what it was getting in to. The company normally replaces the batteries of about one to two million iPhones every year, but with the revised replacement program, that number skyrocketed to 11 million.

The number comes from noted Apple analyst John Gruber. Apple CEO Tim Cook supposedly revealed the number at an all-hands meeting, which an unnamed source relayed to Gruber. The 11 million battery replacement number is a huge increase that Apple likely did not expect. It also played a role in slumping iPhone sales, which Cook highlighted during its revised guidance.

Back in December 2017, Apple was found to be throttling the performance of older iPhone models that had a degraded battery. It was either throttle the performance or have customers deal with the unexpected shutdown of their phone.

Once the news became public, Apple issued a public apology and launched the new $29 battery replacement program. However, it also raised awareness of how users could breathe new life into their iPhones by just replacing the battery instead of buying a new iPhone.

Judging by the numbers, customers chose the battery replacement program over upgrading their iPhone.

Moving forward, Apple may have to deal with customers choosing to replace their phone’s battery, even at the normal $79 price, as opposed to buying a new device. This could continue to negatively affect iPhone sales as Apple figures out a way to allure people to upgrade.