New iPhone 16 Pro battery draining quickly

It's very unlikely that your iPhone has a defective battery. Instead after you setup your new iPhone, certain related tasks including indexing continue in the background, in which you can experience an increased battery drain. When the update is fully complete, your iPhone will return to normal conditions.

Also read my User Tip addressing this further.

Addressing the Increased Battery Drain a… - Apple Community

All rechargeable batteries are consumable components that become less effective as they chemically age.

Recent software updates can accelerate that process by adding features that consume more power.

As lithium-ion batteries chemically age, the amount of charge they can hold diminishes, resulting in shorter amounts of time before a device needs to be recharged.

Apple won’t replace the battery until it reaches 80% capacity.

Li Ion (Lithium Ion) batteries have recharging capabilities; however there are limits.

I recommend to slow charge your iPhone to maximize the charge capacity, battery health and efficiency. ⚡️🔋

NOTE:

Capacity loss in standby applications is largely determined by temperature.

Charge above -30°C and discharge below 55°C for best results.

Battery Performance Tips:

• iPhone Battery and Performance.
• Check battery health and usage.
• Optimized Battery Charging.
• Maximizing Battery Life.
• Use Low Power Mode.
• About battery usage.
• Show battery percentage.

Click ▶︎ iPhone User Guide. 📖

Good luck! 👋🏼😉