With the launch of the iPhone 12, Apple introduced a new "ecosystem of accessories" through the MagSafe magnetic back. So far we've been offered accessories such as the MagSafe Charger and snap-on leather wallet, but Apple is also working on a MagSafe battery pack.
Apple has sold battery pack accessories for iPhones before in the form of a Smart Battery Case, but MagSafe offers an opportunity to ditch the case and sell a snap-on battery instead. As Bloomberg reports, Apple has been working on the battery accessory for at least a year, but there's been problems. Top of the list is the iPhone 12 thinking the battery is overheating when attached. Another is an issue where the battery pack isn't guaranteed to work when attached to an iPhone that's inside a MagSafe-friendly case.
If Apple can't solve the overheating detection problem then it will likely have to scrap the idea, but it's sure to prove extremely popular if it ever sees the light of day. A spare full battery charge you can just throw in a bag and then snap-on to your iPhone when necessary is convenient and therefore potentially a very profitable accessory for Apple to offer. On the flip side, the bad PR over a MagSafe battery overheating and damaging an iPhone must be avoided at all costs, hence the extended development time.
The iPhone 11 Smart Battery Case costs $129, so if a MagSafe battery pack does eventually get released, it seems likely Apple will price it in the same range, or opt for a $99 price tag considering it doesn't double as a protective case.