Apple Support:
iPhone 18 Pro Max offers a huge increase in battery life, in part
due to increasing the battery capacity to over 20 watt-hours (Wh).
Devices with a single-cell battery over 20 Wh have additional
shipping requirements in most countries and regions. To address
these requirements, iPhone 18 Pro Max uses an innovative
battery-firmware solution that limits battery capacity to below 20
Wh from the time it’s manufactured through its transport from the
factory. When you activate your iPhone, the limit is removed and
the battery’s full charging capacity is available.After activation, if you need to ship your iPhone 18 Pro Max for a
repair, trade-in, return, or as a gift for example, use the
Prepare to Ship feature in Settings. This feature discharges your
battery to below 20 Wh and sets a charge limit so your iPhone
stays below this limit while shipping. This can be reversed and
you can reset your charge limit after shipping.The battery in iPhone 18 Pro Max is safe, including for shipping
and air travel. This process is needed only to meet shipping
requirements and isn’t required while traveling or for any other
iPhone model.