Apple has admitted it does intentionally slow down older model iPhones, but says it’s to make them work more reliably rather than to push people into upgrading.

The admission comes despite recent benchmarking that suggested the idea older iPhones suddenly get slower when a new model is released is a myth. Benchmarking company Futuremark said it couldn’t find any consistent or significant pattern of changes in the phones it studied, let alone one that mirrored Apple’s release schedule.

In contrast, a study posted this week suggested a very clear pattern of some iPhone processors having a dramatic slowdown when the handset went from iOS 10.2.0 to 10.2.1, implying a deliberate modification of behavior. Writer John Poole noted this drop could be overturned by replacing a battery and speculated Apple was trying to combat a problem of degraded batteries causing phones to shut down unexpectedly.

Apple has now confirmed that this is the case. It says the update to the iPhone 6 range was designed to “smooth out the instantaneous peaks” of processor demand that might cause the shut down in phones with aging batteries. It said a similar change was made for the iPhone 7 in iOS 11.2 and that it plans to use the technique for other products.

While the admission does appear to squash the theory Apple is trying to promote upgrades by intentionally slowing down the superceded models, keeping quiet about it until now has hardly helped its case. If nothing else, Apple’s silence has meant many people may have wrongly concluded the best fix was to buy an expensive new phone rather than simply replacing the battery.